diff --git a/src/battle-scene.ts b/src/battle-scene.ts
index a8e8aa85121..74b44d324d7 100644
--- a/src/battle-scene.ts
+++ b/src/battle-scene.ts
@@ -138,7 +138,6 @@ import {
   formatMoney,
   getIvsFromId,
   isBetween,
-  isNullOrUndefined,
   NumberHolder,
   randomString,
   randSeedInt,
@@ -867,7 +866,7 @@ export class BattleScene extends SceneBase {
    * this is weird and causes a lot of random jank
    */
   getPokemonById(pokemonId: number | undefined): Pokemon | null {
-    if (isNullOrUndefined(pokemonId)) {
+    if (pokemonId == null) {
       return null;
     }
 
@@ -1319,7 +1318,7 @@ export class BattleScene extends SceneBase {
       if (
         !this.gameMode.hasTrainers
         || Overrides.BATTLE_TYPE_OVERRIDE === BattleType.WILD
-        || (Overrides.DISABLE_STANDARD_TRAINERS_OVERRIDE && isNullOrUndefined(trainerData))
+        || (Overrides.DISABLE_STANDARD_TRAINERS_OVERRIDE && trainerData == null)
       ) {
         newBattleType = BattleType.WILD;
       } else {
@@ -1383,7 +1382,7 @@ export class BattleScene extends SceneBase {
       newDouble = false;
     }
 
-    if (!isNullOrUndefined(Overrides.BATTLE_STYLE_OVERRIDE)) {
+    if (Overrides.BATTLE_STYLE_OVERRIDE != null) {
       let doubleOverrideForWave: "single" | "double" | null = null;
 
       switch (Overrides.BATTLE_STYLE_OVERRIDE) {
@@ -1572,7 +1571,7 @@ export class BattleScene extends SceneBase {
       // Give trainers with specialty types an appropriately-typed form for Wormadam, Rotom, Arceus, Oricorio, Silvally, or Paldean Tauros.
       !isEggPhase
       && this.currentBattle?.battleType === BattleType.TRAINER
-      && !isNullOrUndefined(this.currentBattle.trainer)
+      && this.currentBattle.trainer != null
       && this.currentBattle.trainer.config.hasSpecialtyType()
     ) {
       if (species.speciesId === SpeciesId.WORMADAM) {
@@ -2692,7 +2691,7 @@ export class BattleScene extends SceneBase {
             }
           } else if (modifier instanceof FusePokemonModifier) {
             args.push(this.getPokemonById(modifier.fusePokemonId) as PlayerPokemon);
-          } else if (modifier instanceof RememberMoveModifier && !isNullOrUndefined(cost)) {
+          } else if (modifier instanceof RememberMoveModifier && cost != null) {
             args.push(cost);
           }
 
@@ -3007,7 +3006,7 @@ export class BattleScene extends SceneBase {
       }
       if (
         modifier instanceof PokemonHeldItemModifier
-        && !isNullOrUndefined(modifier.getSpecies())
+        && modifier.getSpecies() != null
         && !this.getPokemonById(modifier.pokemonId)?.hasSpecies(modifier.getSpecies()!)
       ) {
         modifiers.splice(m--, 1);
@@ -3573,7 +3572,7 @@ export class BattleScene extends SceneBase {
     // Loading override or session encounter
     let encounter: MysteryEncounter | null;
     if (
-      !isNullOrUndefined(Overrides.MYSTERY_ENCOUNTER_OVERRIDE)
+      Overrides.MYSTERY_ENCOUNTER_OVERRIDE != null
       && allMysteryEncounters.hasOwnProperty(Overrides.MYSTERY_ENCOUNTER_OVERRIDE)
     ) {
       encounter = allMysteryEncounters[Overrides.MYSTERY_ENCOUNTER_OVERRIDE];
@@ -3584,7 +3583,7 @@ export class BattleScene extends SceneBase {
       encounter = allMysteryEncounters[encounterType ?? -1];
       return encounter;
     } else {
-      encounter = !isNullOrUndefined(encounterType) ? allMysteryEncounters[encounterType] : null;
+      encounter = encounterType != null ? allMysteryEncounters[encounterType] : null;
     }
 
     // Check for queued encounters first
@@ -3643,7 +3642,7 @@ export class BattleScene extends SceneBase {
             ? MysteryEncounterTier.ULTRA
             : MysteryEncounterTier.ROGUE;
 
-    if (!isNullOrUndefined(Overrides.MYSTERY_ENCOUNTER_TIER_OVERRIDE)) {
+    if (Overrides.MYSTERY_ENCOUNTER_TIER_OVERRIDE != null) {
       tier = Overrides.MYSTERY_ENCOUNTER_TIER_OVERRIDE;
     }
 
diff --git a/src/data/abilities/ability.ts b/src/data/abilities/ability.ts
index 8bb2f30b243..9e5bd8030d6 100644
--- a/src/data/abilities/ability.ts
+++ b/src/data/abilities/ability.ts
@@ -67,7 +67,6 @@ import type { Constructor } from "#utils/common";
 import {
   BooleanHolder,
   coerceArray,
-  isNullOrUndefined,
   NumberHolder,
   randSeedFloat,
   randSeedInt,
@@ -1040,7 +1039,7 @@ export class PostDefendStatStageChangeAbAttr extends PostDefendAbAttr {
 
     if (this.allOthers) {
       const ally = pokemon.getAlly();
-      const otherPokemon = !isNullOrUndefined(ally) ? pokemon.getOpponents().concat([ally]) : pokemon.getOpponents();
+      const otherPokemon = ally != null ? pokemon.getOpponents().concat([ally]) : pokemon.getOpponents();
       for (const other of otherPokemon) {
         globalScene.phaseManager.unshiftNew(
           "StatStageChangePhase",
@@ -1473,7 +1472,7 @@ export class PostDefendMoveDisableAbAttr extends PostDefendAbAttr {
 
   override canApply({ move, opponent: attacker, pokemon }: PostMoveInteractionAbAttrParams): boolean {
     return (
-      isNullOrUndefined(attacker.getTag(BattlerTagType.DISABLED))
+      attacker.getTag(BattlerTagType.DISABLED) == null
       && move.doesFlagEffectApply({ flag: MoveFlags.MAKES_CONTACT, user: attacker, target: pokemon })
       && (this.chance === -1 || pokemon.randBattleSeedInt(100) < this.chance)
     );
@@ -2810,7 +2809,7 @@ export class PostSummonAllyHealAbAttr extends PostSummonAbAttr {
 
   override apply({ pokemon, simulated }: AbAttrBaseParams): void {
     const target = pokemon.getAlly();
-    if (!simulated && !isNullOrUndefined(target)) {
+    if (!simulated && target != null) {
       globalScene.phaseManager.unshiftNew(
         "PokemonHealPhase",
         target.getBattlerIndex(),
@@ -2841,7 +2840,7 @@ export class PostSummonClearAllyStatStagesAbAttr extends PostSummonAbAttr {
 
   override apply({ pokemon, simulated }: AbAttrBaseParams): void {
     const target = pokemon.getAlly();
-    if (!simulated && !isNullOrUndefined(target)) {
+    if (!simulated && target != null) {
       for (const s of BATTLE_STATS) {
         target.setStatStage(s, 0);
       }
@@ -2960,13 +2959,13 @@ export class PostSummonHealStatusAbAttr extends PostSummonRemoveEffectAbAttr {
 
   public override canApply({ pokemon }: AbAttrBaseParams): boolean {
     const status = pokemon.status?.effect;
-    return !isNullOrUndefined(status) && (this.immuneEffects.length === 0 || this.immuneEffects.includes(status));
+    return status != null && (this.immuneEffects.length === 0 || this.immuneEffects.includes(status));
   }
 
   public override apply({ pokemon }: AbAttrBaseParams): void {
     // TODO: should probably check against simulated...
     const status = pokemon.status?.effect;
-    if (!isNullOrUndefined(status)) {
+    if (status != null) {
       this.statusHealed = status;
       pokemon.resetStatus(false);
       pokemon.updateInfo();
@@ -3102,7 +3101,7 @@ export class PostSummonCopyAllyStatsAbAttr extends PostSummonAbAttr {
     }
 
     const ally = pokemon.getAlly();
-    return !(isNullOrUndefined(ally) || ally.getStatStages().every(s => s === 0));
+    return !(ally == null || ally.getStatStages().every(s => s === 0));
   }
 
   override apply({ pokemon, simulated }: AbAttrBaseParams): void {
@@ -3110,7 +3109,7 @@ export class PostSummonCopyAllyStatsAbAttr extends PostSummonAbAttr {
       return;
     }
     const ally = pokemon.getAlly();
-    if (!isNullOrUndefined(ally)) {
+    if (ally != null) {
       for (const s of BATTLE_STATS) {
         pokemon.setStatStage(s, ally.getStatStage(s));
       }
@@ -3240,7 +3239,7 @@ export class CommanderAbAttr extends AbAttr {
     const ally = pokemon.getAlly();
     return (
       globalScene.currentBattle?.double
-      && !isNullOrUndefined(ally)
+      && ally != null
       && ally.species.speciesId === SpeciesId.DONDOZO
       && !(ally.isFainted() || ally.getTag(BattlerTagType.COMMANDED))
     );
@@ -3284,7 +3283,7 @@ export class PreSwitchOutResetStatusAbAttr extends PreSwitchOutAbAttr {
   }
 
   override canApply({ pokemon }: AbAttrBaseParams): boolean {
-    return !isNullOrUndefined(pokemon.status);
+    return pokemon.status != null;
   }
 
   override apply({ pokemon, simulated }: AbAttrBaseParams): void {
@@ -3564,7 +3563,7 @@ export class ProtectStatAbAttr extends PreStatStageChangeAbAttr {
   }
 
   override canApply({ stat, cancelled }: PreStatStageChangeAbAttrParams): boolean {
-    return !cancelled.value && (isNullOrUndefined(this.protectedStat) || stat === this.protectedStat);
+    return !cancelled.value && (this.protectedStat == null || stat === this.protectedStat);
   }
 
   /**
@@ -3800,11 +3799,7 @@ export class ConditionalUserFieldProtectStatAbAttr extends PreStatStageChangeAbA
     if (!target) {
       return false;
     }
-    return (
-      !cancelled.value
-      && (isNullOrUndefined(this.protectedStat) || stat === this.protectedStat)
-      && this.condition(target)
-    );
+    return !cancelled.value && (this.protectedStat == null || stat === this.protectedStat) && this.condition(target);
   }
 
   /**
@@ -4561,7 +4556,7 @@ export class PostTurnStatusHealAbAttr extends PostTurnAbAttr {
   }
 
   override canApply({ pokemon }: AbAttrBaseParams): boolean {
-    return !isNullOrUndefined(pokemon.status) && this.effects.includes(pokemon.status.effect) && !pokemon.isFullHp();
+    return pokemon.status != null && this.effects.includes(pokemon.status.effect) && !pokemon.isFullHp();
   }
 
   override apply({ simulated, passive, pokemon }: AbAttrBaseParams): void {
@@ -4896,7 +4891,7 @@ export class PostTurnHurtIfSleepingAbAttr extends PostTurnAbAttr {
  */
 export class FetchBallAbAttr extends PostTurnAbAttr {
   override canApply({ simulated, pokemon }: AbAttrBaseParams): boolean {
-    return !simulated && !isNullOrUndefined(globalScene.currentBattle.lastUsedPokeball) && !!pokemon.isPlayer;
+    return !simulated && globalScene.currentBattle.lastUsedPokeball != null && !!pokemon.isPlayer;
   }
 
   /**
@@ -6261,7 +6256,7 @@ class ForceSwitchOutHelper {
           true,
           500,
         );
-        if (globalScene.currentBattle.double && !isNullOrUndefined(allyPokemon)) {
+        if (globalScene.currentBattle.double && allyPokemon != null) {
           globalScene.redirectPokemonMoves(switchOutTarget, allyPokemon);
         }
       }
@@ -7113,7 +7108,7 @@ export function initAbilities() {
       .attr(PostDefendMoveDisableAbAttr, 30)
       .bypassFaint(),
     new Ability(AbilityId.HEALER, 5)
-      .conditionalAttr(pokemon => !isNullOrUndefined(pokemon.getAlly()) && randSeedInt(10) < 3, PostTurnResetStatusAbAttr, true),
+      .conditionalAttr(pokemon => pokemon.getAlly() != null && randSeedInt(10) < 3, PostTurnResetStatusAbAttr, true),
     new Ability(AbilityId.FRIEND_GUARD, 5)
       .attr(AlliedFieldDamageReductionAbAttr, 0.75)
       .ignorable(),
diff --git a/src/data/balance/pokemon-evolutions.ts b/src/data/balance/pokemon-evolutions.ts
index d364dc036b1..fd88fc5da37 100644
--- a/src/data/balance/pokemon-evolutions.ts
+++ b/src/data/balance/pokemon-evolutions.ts
@@ -14,7 +14,7 @@ import { TimeOfDay } from "#enums/time-of-day";
 import { WeatherType } from "#enums/weather-type";
 import type { Pokemon } from "#field/pokemon";
 import type { SpeciesStatBoosterItem, SpeciesStatBoosterModifierType } from "#modifiers/modifier-type";
-import { coerceArray, isNullOrUndefined, randSeedInt } from "#utils/common";
+import { coerceArray, randSeedInt } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import { toCamelCase } from "#utils/strings";
 import i18next from "i18next";
@@ -128,7 +128,7 @@ export class SpeciesEvolutionCondition {
   }
 
   public get description(): string[] {
-    if (!isNullOrUndefined(this.desc)) {
+    if (this.desc != null) {
       return this.desc;
     }
     this.desc = this.data.map(cond => {
@@ -161,7 +161,7 @@ export class SpeciesEvolutionCondition {
         case EvoCondKey.HELD_ITEM:
           return i18next.t(`pokemonEvolutions:heldItem.${toCamelCase(cond.itemKey)}`);
       }
-    }).filter(s => !isNullOrUndefined(s)); // Filter out stringless conditions
+    }).filter(s => s != null); // Filter out stringless conditions
     return this.desc;
   }
 
@@ -233,7 +233,7 @@ export class SpeciesFormEvolution {
     this.evoFormKey = evoFormKey;
     this.level = level;
     this.item = item || EvolutionItem.NONE;
-    if (!isNullOrUndefined(condition)) {
+    if (condition != null) {
       this.condition = new SpeciesEvolutionCondition(...coerceArray(condition));
     }
     this.wildDelay = wildDelay ?? SpeciesWildEvolutionDelay.NONE;
@@ -291,8 +291,8 @@ export class SpeciesFormEvolution {
     return (
       pokemon.level >= this.level &&
       // Check form key, using the fusion's form key if we're checking the fusion
-      (isNullOrUndefined(this.preFormKey) || (forFusion ? pokemon.getFusionFormKey() : pokemon.getFormKey()) === this.preFormKey) &&
-      (isNullOrUndefined(this.condition) || this.condition.conditionsFulfilled(pokemon)) &&
+      (this.preFormKey == null || (forFusion ? pokemon.getFusionFormKey() : pokemon.getFormKey()) === this.preFormKey) &&
+      (this.condition == null || this.condition.conditionsFulfilled(pokemon)) &&
       ((item ?? EvolutionItem.NONE) === (this.item ?? EvolutionItem.NONE))
     );
   }
@@ -305,11 +305,11 @@ export class SpeciesFormEvolution {
    */
   public isValidItemEvolution(pokemon: Pokemon, forFusion = false): boolean {
     return (
-      !isNullOrUndefined(this.item) &&
+      this.item != null &&
       pokemon.level >= this.level &&
       // Check form key, using the fusion's form key if we're checking the fusion
-      (isNullOrUndefined(this.preFormKey) || (forFusion ? pokemon.getFusionFormKey() : pokemon.getFormKey()) === this.preFormKey) &&
-      (isNullOrUndefined(this.condition) || this.condition.conditionsFulfilled(pokemon))
+      (this.preFormKey == null || (forFusion ? pokemon.getFusionFormKey() : pokemon.getFormKey()) === this.preFormKey) &&
+      (this.condition == null || this.condition.conditionsFulfilled(pokemon))
     );
   }
 
diff --git a/src/data/battle-anims.ts b/src/data/battle-anims.ts
index 1dcb7d7eebf..573a1730796 100644
--- a/src/data/battle-anims.ts
+++ b/src/data/battle-anims.ts
@@ -7,7 +7,7 @@ import { AnimBlendType, AnimFocus, AnimFrameTarget, ChargeAnim, CommonAnim } fro
 import { MoveFlags } from "#enums/move-flags";
 import { MoveId } from "#enums/move-id";
 import type { Pokemon } from "#field/pokemon";
-import { coerceArray, getFrameMs, isNullOrUndefined, type nil } from "#utils/common";
+import { coerceArray, getFrameMs, type nil } from "#utils/common";
 import { getEnumKeys, getEnumValues } from "#utils/enums";
 import { toKebabCase } from "#utils/strings";
 import Phaser from "phaser";
@@ -388,7 +388,7 @@ class AnimTimedAddBgEvent extends AnimTimedBgEvent {
     moveAnim.bgSprite.setAlpha(this.opacity / 255);
     globalScene.field.add(moveAnim.bgSprite);
     const fieldPokemon = globalScene.getEnemyPokemon(false) ?? globalScene.getPlayerPokemon(false);
-    if (!isNullOrUndefined(priority)) {
+    if (priority != null) {
       globalScene.field.moveTo(moveAnim.bgSprite as Phaser.GameObjects.GameObject, priority);
     } else if (fieldPokemon?.isOnField()) {
       globalScene.field.moveBelow(moveAnim.bgSprite as Phaser.GameObjects.GameObject, fieldPokemon);
@@ -524,7 +524,7 @@ export async function initEncounterAnims(encounterAnim: EncounterAnim | Encounte
   const encounterAnimNames = getEnumKeys(EncounterAnim);
   const encounterAnimFetches: Promise<Map<EncounterAnim, AnimConfig>>[] = [];
   for (const anim of anims) {
-    if (encounterAnims.has(anim) && !isNullOrUndefined(encounterAnims.get(anim))) {
+    if (encounterAnims.has(anim) && encounterAnims.get(anim) != null) {
       continue;
     }
     encounterAnimFetches.push(
@@ -1240,7 +1240,7 @@ export abstract class BattleAnim {
 
           const graphicIndex = graphicFrameCount++;
           const moveSprite = sprites[graphicIndex];
-          if (!isNullOrUndefined(frame.priority)) {
+          if (frame.priority != null) {
             const setSpritePriority = (priority: number) => {
               if (existingFieldSprites.length > priority) {
                 // Move to specified priority index
diff --git a/src/data/battler-tags.ts b/src/data/battler-tags.ts
index c495cdaa604..711a2bd0b44 100644
--- a/src/data/battler-tags.ts
+++ b/src/data/battler-tags.ts
@@ -49,7 +49,7 @@ import type {
   TypeBoostTagType,
 } from "#types/battler-tags";
 import type { Mutable } from "#types/type-helpers";
-import { BooleanHolder, coerceArray, getFrameMs, isNullOrUndefined, NumberHolder, toDmgValue } from "#utils/common";
+import { BooleanHolder, coerceArray, getFrameMs, NumberHolder, toDmgValue } from "#utils/common";
 import { toCamelCase } from "#utils/strings";
 
 /**
@@ -378,7 +378,7 @@ export class DisabledTag extends MoveRestrictionBattlerTag {
     // Disable fails against struggle or an empty move history
     // TODO: Confirm if this is redundant given Disable/Cursed Body's disable conditions
     const move = pokemon.getLastNonVirtualMove();
-    if (isNullOrUndefined(move) || move.move === MoveId.STRUGGLE) {
+    if (move == null || move.move === MoveId.STRUGGLE) {
       return;
     }
 
@@ -451,7 +451,7 @@ export class GorillaTacticsTag extends MoveRestrictionBattlerTag {
   override canAdd(pokemon: Pokemon): boolean {
     // Choice items ignore struggle, so Gorilla Tactics should too
     const lastSelectedMove = pokemon.getLastNonVirtualMove();
-    return !isNullOrUndefined(lastSelectedMove) && lastSelectedMove.move !== MoveId.STRUGGLE;
+    return lastSelectedMove != null && lastSelectedMove.move !== MoveId.STRUGGLE;
   }
 
   /**
@@ -1305,7 +1305,7 @@ export class EncoreTag extends MoveRestrictionBattlerTag {
   override lapse(pokemon: Pokemon, lapseType: BattlerTagLapseType): boolean {
     if (lapseType === BattlerTagLapseType.CUSTOM) {
       const encoredMove = pokemon.getMoveset().find(m => m.moveId === this.moveId);
-      return !isNullOrUndefined(encoredMove) && encoredMove.getPpRatio() > 0;
+      return encoredMove != null && encoredMove.getPpRatio() > 0;
     }
     return super.lapse(pokemon, lapseType);
   }
diff --git a/src/data/daily-run.ts b/src/data/daily-run.ts
index addaebdd238..5f49b9adbb0 100644
--- a/src/data/daily-run.ts
+++ b/src/data/daily-run.ts
@@ -7,7 +7,7 @@ import { BiomeId } from "#enums/biome-id";
 import { PartyMemberStrength } from "#enums/party-member-strength";
 import { SpeciesId } from "#enums/species-id";
 import type { Starter } from "#ui/starter-select-ui-handler";
-import { isNullOrUndefined, randSeedGauss, randSeedInt, randSeedItem } from "#utils/common";
+import { randSeedGauss, randSeedInt, randSeedItem } from "#utils/common";
 import { getEnumValues } from "#utils/enums";
 import { getPokemonSpecies, getPokemonSpeciesForm } from "#utils/pokemon-utils";
 
@@ -32,7 +32,7 @@ export function getDailyRunStarters(seed: string): Starter[] {
       const startingLevel = globalScene.gameMode.getStartingLevel();
 
       const eventStarters = getDailyEventSeedStarters(seed);
-      if (!isNullOrUndefined(eventStarters)) {
+      if (eventStarters != null) {
         starters.push(...eventStarters);
         return;
       }
@@ -127,7 +127,7 @@ const dailyBiomeWeights: BiomeWeights = {
 
 export function getDailyStartingBiome(): BiomeId {
   const eventBiome = getDailyEventSeedBiome(globalScene.seed);
-  if (!isNullOrUndefined(eventBiome)) {
+  if (eventBiome != null) {
     return eventBiome;
   }
 
diff --git a/src/data/moves/move-utils.ts b/src/data/moves/move-utils.ts
index eedeea53087..1fe0880317b 100644
--- a/src/data/moves/move-utils.ts
+++ b/src/data/moves/move-utils.ts
@@ -7,7 +7,7 @@ import { PokemonType } from "#enums/pokemon-type";
 import type { Pokemon } from "#field/pokemon";
 import { applyMoveAttrs } from "#moves/apply-attrs";
 import type { Move, MoveTargetSet, UserMoveConditionFunc } from "#moves/move";
-import { isNullOrUndefined, NumberHolder } from "#utils/common";
+import { NumberHolder } from "#utils/common";
 
 /**
  * Return whether the move targets the field
@@ -78,7 +78,7 @@ export function getMoveTargets(user: Pokemon, move: MoveId, replaceTarget?: Move
     case MoveTarget.OTHER:
     case MoveTarget.ALL_NEAR_OTHERS:
     case MoveTarget.ALL_OTHERS:
-      set = !isNullOrUndefined(ally) ? opponents.concat([ally]) : opponents;
+      set = ally != null ? opponents.concat([ally]) : opponents;
       multiple = moveTarget === MoveTarget.ALL_NEAR_OTHERS || moveTarget === MoveTarget.ALL_OTHERS;
       break;
     case MoveTarget.NEAR_ENEMY:
@@ -95,22 +95,22 @@ export function getMoveTargets(user: Pokemon, move: MoveId, replaceTarget?: Move
       return { targets: [-1 as BattlerIndex], multiple: false };
     case MoveTarget.NEAR_ALLY:
     case MoveTarget.ALLY:
-      set = !isNullOrUndefined(ally) ? [ally] : [];
+      set = ally != null ? [ally] : [];
       break;
     case MoveTarget.USER_OR_NEAR_ALLY:
     case MoveTarget.USER_AND_ALLIES:
     case MoveTarget.USER_SIDE:
-      set = !isNullOrUndefined(ally) ? [user, ally] : [user];
+      set = ally != null ? [user, ally] : [user];
       multiple = moveTarget !== MoveTarget.USER_OR_NEAR_ALLY;
       break;
     case MoveTarget.ALL:
     case MoveTarget.BOTH_SIDES:
-      set = (!isNullOrUndefined(ally) ? [user, ally] : [user]).concat(opponents);
+      set = (ally != null ? [user, ally] : [user]).concat(opponents);
       multiple = true;
       break;
     case MoveTarget.CURSE:
       {
-        const extraTargets = !isNullOrUndefined(ally) ? [ally] : [];
+        const extraTargets = ally != null ? [ally] : [];
         set = user.getTypes(true).includes(PokemonType.GHOST) ? opponents.concat(extraTargets) : [user];
       }
       break;
diff --git a/src/data/moves/move.ts b/src/data/moves/move.ts
index 19d145a36d5..91d61c15b8b 100644
--- a/src/data/moves/move.ts
+++ b/src/data/moves/move.ts
@@ -88,7 +88,7 @@ import type { AttackMoveResult } from "#types/attack-move-result";
 import type { Localizable } from "#types/locales";
 import type { ChargingMove, MoveAttrMap, MoveAttrString, MoveClassMap, MoveKindString, MoveMessageFunc } from "#types/move-types";
 import type { TurnMove } from "#types/turn-move";
-import { BooleanHolder, coerceArray, type Constructor, isNullOrUndefined, NumberHolder, randSeedFloat, randSeedInt, randSeedItem, toDmgValue } from "#utils/common";
+import { BooleanHolder, coerceArray, type Constructor, NumberHolder, randSeedFloat, randSeedInt, randSeedItem, toDmgValue } from "#utils/common";
 import { getEnumValues } from "#utils/enums";
 import { toCamelCase, toTitleCase } from "#utils/strings";
 import i18next from "i18next";
@@ -835,7 +835,7 @@ export abstract class Move implements Localizable {
 
     applyAbAttrs("VariableMovePowerAbAttr", abAttrParams);
     const ally = source.getAlly();
-    if (!isNullOrUndefined(ally)) {
+    if (ally != null) {
       applyAbAttrs("AllyMoveCategoryPowerBoostAbAttr", {...abAttrParams, pokemon: ally});
     }
 
@@ -965,7 +965,7 @@ export abstract class Move implements Localizable {
 
     // ...and cannot enhance Pollen Puff when targeting an ally.
     const ally = user.getAlly();
-    const exceptPollenPuffAlly: boolean = this.id === MoveId.POLLEN_PUFF && !isNullOrUndefined(ally) && targets.includes(ally.getBattlerIndex())
+    const exceptPollenPuffAlly: boolean = this.id === MoveId.POLLEN_PUFF && ally != null && targets.includes(ally.getBattlerIndex())
 
     return (!restrictSpread || !isMultiTarget)
       && !this.isChargingMove()
@@ -2114,7 +2114,7 @@ export class FlameBurstAttr extends MoveEffectAttr {
     const targetAlly = target.getAlly();
     const cancelled = new BooleanHolder(false);
 
-    if (!isNullOrUndefined(targetAlly)) {
+    if (targetAlly != null) {
       applyAbAttrs("BlockNonDirectDamageAbAttr", {pokemon: targetAlly, cancelled});
     }
 
@@ -2127,7 +2127,7 @@ export class FlameBurstAttr extends MoveEffectAttr {
   }
 
   getTargetBenefitScore(user: Pokemon, target: Pokemon, move: Move): number {
-    return !isNullOrUndefined(target.getAlly()) ? -5 : 0;
+    return target.getAlly() != null ? -5 : 0;
   }
 }
 
@@ -3156,7 +3156,7 @@ export class WeatherInstantChargeAttr extends InstantChargeAttr {
     super((user, move) => {
       const currentWeather = globalScene.arena.weather;
 
-      if (isNullOrUndefined(currentWeather?.weatherType)) {
+      if (currentWeather?.weatherType == null) {
         return false;
       } else {
         return !currentWeather?.isEffectSuppressed()
@@ -6293,7 +6293,7 @@ export class RevivalBlessingAttr extends MoveEffectAttr {
       pokemon.heal(Math.min(toDmgValue(0.5 * pokemon.getMaxHp()), pokemon.getMaxHp()));
       globalScene.phaseManager.queueMessage(i18next.t("moveTriggers:revivalBlessing", { pokemonName: getPokemonNameWithAffix(pokemon) }), 0, true);
       const allyPokemon = user.getAlly();
-      if (globalScene.currentBattle.double && globalScene.getEnemyParty().length > 1 && !isNullOrUndefined(allyPokemon)) {
+      if (globalScene.currentBattle.double && globalScene.getEnemyParty().length > 1 && allyPokemon != null) {
         // Handle cases where revived pokemon needs to get switched in on same turn
         if (allyPokemon.isFainted() || allyPokemon === pokemon) {
           // Enemy switch phase should be removed and replaced with the revived pkmn switching in
@@ -6462,7 +6462,7 @@ export class ForceSwitchOutAttr extends MoveEffectAttr {
         globalScene.phaseManager.queueMessage(i18next.t("moveTriggers:fled", { pokemonName: getPokemonNameWithAffix(switchOutTarget) }), null, true, 500);
 
         // in double battles redirect potential moves off fled pokemon
-        if (globalScene.currentBattle.double && !isNullOrUndefined(allyPokemon)) {
+        if (globalScene.currentBattle.double && allyPokemon != null) {
           globalScene.redirectPokemonMoves(switchOutTarget, allyPokemon);
         }
       }
@@ -7122,7 +7122,7 @@ export class CopyMoveAttr extends CallMoveAttr {
   getCondition(): MoveConditionFunc {
     return (_user, target, _move) => {
       const lastMove = this.mirrorMove ? target.getLastNonVirtualMove(false, false)?.move : globalScene.currentBattle.lastMove;
-      return !isNullOrUndefined(lastMove) && !this.invalidMoves.has(lastMove);
+      return lastMove != null && !this.invalidMoves.has(lastMove);
     };
   }
 }
@@ -7169,7 +7169,7 @@ export class RepeatMoveAttr extends MoveEffectAttr {
       && firstTarget !== target.getAlly()
     ) {
       const ally = firstTarget.getAlly();
-      if (!isNullOrUndefined(ally) && ally.isActive()) {
+      if (ally != null && ally.isActive()) {
         moveTargets = [ ally.getBattlerIndex() ];
       }
     }
@@ -7476,7 +7476,7 @@ export class SketchAttr extends MoveEffectAttr {
       }
 
       const targetMove = target.getLastNonVirtualMove();
-      return !isNullOrUndefined(targetMove)
+      return targetMove != null
         && !invalidSketchMoves.has(targetMove.move)
         && user.getMoveset().every(m => m.moveId !== targetMove.move)
     };
@@ -7533,7 +7533,7 @@ export class AbilityCopyAttr extends MoveEffectAttr {
     user.setTempAbility(target.getAbility());
     const ally = user.getAlly();
 
-    if (this.copyToPartner && globalScene.currentBattle?.double && !isNullOrUndefined(ally) && ally.hp) { // TODO is this the best way to check that the ally is active?
+    if (this.copyToPartner && globalScene.currentBattle?.double && ally != null && ally.hp) { // TODO is this the best way to check that the ally is active?
       globalScene.phaseManager.queueMessage(i18next.t("moveTriggers:copiedTargetAbility", { pokemonName: getPokemonNameWithAffix(ally), targetName: getPokemonNameWithAffix(target), abilityName: allAbilities[target.getAbility().id].name }));
       ally.setTempAbility(target.getAbility());
     }
@@ -8054,7 +8054,7 @@ const failIfGhostTypeCondition: MoveConditionFunc = (user: Pokemon, target: Poke
 const failIfNoTargetHeldItemsCondition: MoveConditionFunc = (user: Pokemon, target: Pokemon, move: Move) => target.getHeldItems().filter(i => i.isTransferable)?.length > 0;
 
 const attackedByItemMessageFunc = (user: Pokemon, target: Pokemon, move: Move) => {
-  if (isNullOrUndefined(target)) { // Fix bug when used against targets that have both fainted
+  if (target == null) { // Fix bug when used against targets that have both fainted
     return "";
   }
   const heldItems = target.getHeldItems().filter(i => i.isTransferable);
@@ -8611,7 +8611,7 @@ export function initMoves() {
       .attr(AddBattlerTagAttr, BattlerTagType.DISABLED, false, true)
       .condition((_user, target, _move) => {
         const lastNonVirtualMove = target.getLastNonVirtualMove();
-        return !isNullOrUndefined(lastNonVirtualMove) && lastNonVirtualMove.move !== MoveId.STRUGGLE;
+        return lastNonVirtualMove != null && lastNonVirtualMove.move !== MoveId.STRUGGLE;
       })
       .ignoresSubstitute()
       .reflectable(),
@@ -9955,7 +9955,7 @@ export function initMoves() {
       .condition(failOnGravityCondition)
       .condition((_user, target, _move) => ![ SpeciesId.DIGLETT, SpeciesId.DUGTRIO, SpeciesId.ALOLA_DIGLETT, SpeciesId.ALOLA_DUGTRIO, SpeciesId.SANDYGAST, SpeciesId.PALOSSAND, SpeciesId.WIGLETT, SpeciesId.WUGTRIO ].includes(target.species.speciesId))
       .condition((_user, target, _move) => !(target.species.speciesId === SpeciesId.GENGAR && target.getFormKey() === "mega"))
-      .condition((_user, target, _move) => isNullOrUndefined(target.getTag(BattlerTagType.INGRAIN)) && isNullOrUndefined(target.getTag(BattlerTagType.IGNORE_FLYING)))
+      .condition((_user, target, _move) => target.getTag(BattlerTagType.INGRAIN) == null && target.getTag(BattlerTagType.IGNORE_FLYING) == null)
       .attr(AddBattlerTagAttr, BattlerTagType.TELEKINESIS, false, true, 3)
       .attr(AddBattlerTagAttr, BattlerTagType.FLOATING, false, true, 3)
       .reflectable(),
diff --git a/src/data/mystery-encounters/encounters/bug-type-superfan-encounter.ts b/src/data/mystery-encounters/encounters/bug-type-superfan-encounter.ts
index cb34190a584..00e98048ada 100644
--- a/src/data/mystery-encounters/encounters/bug-type-superfan-encounter.ts
+++ b/src/data/mystery-encounters/encounters/bug-type-superfan-encounter.ts
@@ -48,7 +48,7 @@ import { getRandomPartyMemberFunc, trainerConfigs } from "#trainers/trainer-conf
 import { TrainerPartyCompoundTemplate, TrainerPartyTemplate } from "#trainers/trainer-party-template";
 import type { OptionSelectItem } from "#ui/abstract-option-select-ui-handler";
 import { MoveInfoOverlay } from "#ui/move-info-overlay";
-import { isNullOrUndefined, randSeedInt, randSeedShuffle } from "#utils/common";
+import { randSeedInt, randSeedShuffle } from "#utils/common";
 import i18next from "i18next";
 
 /** the i18n namespace for the encounter */
@@ -571,7 +571,7 @@ function getTrainerConfigForWave(waveIndex: number) {
       .setPartyMemberFunc(
         4,
         getRandomPartyMemberFunc([pool3Mon.species], TrainerSlot.TRAINER, true, p => {
-          if (!isNullOrUndefined(pool3Mon.formIndex)) {
+          if (pool3Mon.formIndex != null) {
             p.formIndex = pool3Mon.formIndex;
             p.generateAndPopulateMoveset();
             p.generateName();
@@ -603,7 +603,7 @@ function getTrainerConfigForWave(waveIndex: number) {
       .setPartyMemberFunc(
         3,
         getRandomPartyMemberFunc([pool3Mon.species], TrainerSlot.TRAINER, true, p => {
-          if (!isNullOrUndefined(pool3Mon.formIndex)) {
+          if (pool3Mon.formIndex != null) {
             p.formIndex = pool3Mon.formIndex;
             p.generateAndPopulateMoveset();
             p.generateName();
@@ -613,7 +613,7 @@ function getTrainerConfigForWave(waveIndex: number) {
       .setPartyMemberFunc(
         4,
         getRandomPartyMemberFunc([pool3Mon2.species], TrainerSlot.TRAINER, true, p => {
-          if (!isNullOrUndefined(pool3Mon2.formIndex)) {
+          if (pool3Mon2.formIndex != null) {
             p.formIndex = pool3Mon2.formIndex;
             p.generateAndPopulateMoveset();
             p.generateName();
@@ -648,7 +648,7 @@ function getTrainerConfigForWave(waveIndex: number) {
       .setPartyMemberFunc(
         3,
         getRandomPartyMemberFunc([pool3Mon.species], TrainerSlot.TRAINER, true, p => {
-          if (!isNullOrUndefined(pool3Mon.formIndex)) {
+          if (pool3Mon.formIndex != null) {
             p.formIndex = pool3Mon.formIndex;
             p.generateAndPopulateMoveset();
             p.generateName();
@@ -687,7 +687,7 @@ function getTrainerConfigForWave(waveIndex: number) {
       .setPartyMemberFunc(
         2,
         getRandomPartyMemberFunc([pool3Mon.species], TrainerSlot.TRAINER, true, p => {
-          if (!isNullOrUndefined(pool3Mon.formIndex)) {
+          if (pool3Mon.formIndex != null) {
             p.formIndex = pool3Mon.formIndex;
             p.generateAndPopulateMoveset();
             p.generateName();
@@ -697,7 +697,7 @@ function getTrainerConfigForWave(waveIndex: number) {
       .setPartyMemberFunc(
         3,
         getRandomPartyMemberFunc([pool3Mon2.species], TrainerSlot.TRAINER, true, p => {
-          if (!isNullOrUndefined(pool3Mon2.formIndex)) {
+          if (pool3Mon2.formIndex != null) {
             p.formIndex = pool3Mon2.formIndex;
             p.generateAndPopulateMoveset();
             p.generateName();
diff --git a/src/data/mystery-encounters/encounters/dark-deal-encounter.ts b/src/data/mystery-encounters/encounters/dark-deal-encounter.ts
index 65d22bfc6de..426eafb5e67 100644
--- a/src/data/mystery-encounters/encounters/dark-deal-encounter.ts
+++ b/src/data/mystery-encounters/encounters/dark-deal-encounter.ts
@@ -15,7 +15,7 @@ import { getRandomPlayerPokemon, getRandomSpeciesByStarterCost } from "#mystery-
 import type { MysteryEncounter } from "#mystery-encounters/mystery-encounter";
 import { MysteryEncounterBuilder } from "#mystery-encounters/mystery-encounter";
 import { MysteryEncounterOptionBuilder } from "#mystery-encounters/mystery-encounter-option";
-import { isNullOrUndefined, randSeedInt } from "#utils/common";
+import { randSeedInt } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 
 /** i18n namespace for encounter */
@@ -192,7 +192,7 @@ export const DarkDealEncounter: MysteryEncounter = MysteryEncounterBuilder.withE
             };
           }),
         };
-        if (!isNullOrUndefined(bossSpecies.forms) && bossSpecies.forms.length > 0) {
+        if (bossSpecies.forms != null && bossSpecies.forms.length > 0) {
           pokemonConfig.formIndex = 0;
         }
         const config: EnemyPartyConfig = {
diff --git a/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts b/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts
index 1cc31eaa21f..0f37a1fae94 100644
--- a/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts
+++ b/src/data/mystery-encounters/encounters/fiery-fallout-encounter.ts
@@ -45,7 +45,7 @@ import {
   TypeRequirement,
 } from "#mystery-encounters/mystery-encounter-requirements";
 import { FIRE_RESISTANT_ABILITIES } from "#mystery-encounters/requirement-groups";
-import { isNullOrUndefined, randSeedInt } from "#utils/common";
+import { randSeedInt } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 
 /** the i18n namespace for the encounter */
@@ -238,7 +238,7 @@ export const FieryFalloutEncounter: MysteryEncounter = MysteryEncounterBuilder.w
 
       // Burn random member
       const burnable = nonFireTypes.filter(
-        p => isNullOrUndefined(p.status) || isNullOrUndefined(p.status.effect) || p.status.effect === StatusEffect.NONE,
+        p => p.status == null || p.status.effect == null || p.status.effect === StatusEffect.NONE,
       );
       if (burnable?.length > 0) {
         const roll = randSeedInt(burnable.length);
diff --git a/src/data/mystery-encounters/encounters/global-trade-system-encounter.ts b/src/data/mystery-encounters/encounters/global-trade-system-encounter.ts
index 823f016029e..e2166e99f6a 100644
--- a/src/data/mystery-encounters/encounters/global-trade-system-encounter.ts
+++ b/src/data/mystery-encounters/encounters/global-trade-system-encounter.ts
@@ -43,7 +43,7 @@ import { PartySizeRequirement } from "#mystery-encounters/mystery-encounter-requ
 import { PokemonData } from "#system/pokemon-data";
 import { MusicPreference } from "#system/settings";
 import type { OptionSelectItem } from "#ui/abstract-option-select-ui-handler";
-import { isNullOrUndefined, NumberHolder, randInt, randSeedInt, randSeedItem, randSeedShuffle } from "#utils/common";
+import { NumberHolder, randInt, randSeedInt, randSeedItem, randSeedShuffle } from "#utils/common";
 import { getEnumKeys } from "#utils/enums";
 import { getRandomLocaleEntry } from "#utils/i18n";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
@@ -537,7 +537,7 @@ function generateTradeOption(alreadyUsedSpecies: PokemonSpecies[], originalBst?:
     bstCap = originalBst + 100;
     bstMin = originalBst - 100;
   }
-  while (isNullOrUndefined(newSpecies)) {
+  while (newSpecies == null) {
     // Get all non-legendary species that fall within the Bst range requirements
     let validSpecies = allSpecies.filter(s => {
       const isLegendaryOrMythical = s.legendary || s.subLegendary || s.mythical;
@@ -550,7 +550,7 @@ function generateTradeOption(alreadyUsedSpecies: PokemonSpecies[], originalBst?:
     if (validSpecies?.length > 20) {
       validSpecies = randSeedShuffle(validSpecies);
       newSpecies = validSpecies.pop();
-      while (isNullOrUndefined(newSpecies) || alreadyUsedSpecies.includes(newSpecies)) {
+      while (newSpecies == null || alreadyUsedSpecies.includes(newSpecies)) {
         newSpecies = validSpecies.pop();
       }
     } else {
diff --git a/src/data/mystery-encounters/encounters/the-pokemon-salesman-encounter.ts b/src/data/mystery-encounters/encounters/the-pokemon-salesman-encounter.ts
index 292c866c0ee..51efa0c7586 100644
--- a/src/data/mystery-encounters/encounters/the-pokemon-salesman-encounter.ts
+++ b/src/data/mystery-encounters/encounters/the-pokemon-salesman-encounter.ts
@@ -28,7 +28,7 @@ import { MysteryEncounterBuilder } from "#mystery-encounters/mystery-encounter";
 import { MysteryEncounterOptionBuilder } from "#mystery-encounters/mystery-encounter-option";
 import { MoneyRequirement } from "#mystery-encounters/mystery-encounter-requirements";
 import { PokemonData } from "#system/pokemon-data";
-import { isNullOrUndefined, randSeedInt, randSeedItem } from "#utils/common";
+import { randSeedInt, randSeedItem } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 
 /** the i18n namespace for this encounter */
@@ -81,7 +81,7 @@ export const ThePokemonSalesmanEncounter: MysteryEncounter = MysteryEncounterBui
     let tries = 0;
 
     // Reroll any species that don't have HAs
-    while ((isNullOrUndefined(species.abilityHidden) || species.abilityHidden === AbilityId.NONE) && tries < 5) {
+    while ((species.abilityHidden == null || species.abilityHidden === AbilityId.NONE) && tries < 5) {
       species = getSalesmanSpeciesOffer();
       tries++;
     }
@@ -110,7 +110,7 @@ export const ThePokemonSalesmanEncounter: MysteryEncounter = MysteryEncounterBui
      */
     if (
       r === 0
-      || ((isNullOrUndefined(species.abilityHidden) || species.abilityHidden === AbilityId.NONE)
+      || ((species.abilityHidden == null || species.abilityHidden === AbilityId.NONE)
         && validEventEncounters.length === 0)
     ) {
       // If you roll 1%, give shiny Magikarp with random variant
@@ -118,7 +118,7 @@ export const ThePokemonSalesmanEncounter: MysteryEncounter = MysteryEncounterBui
       pokemon = new PlayerPokemon(species, 5, 2, undefined, undefined, true);
     } else if (
       validEventEncounters.length > 0
-      && (r <= EVENT_THRESHOLD || isNullOrUndefined(species.abilityHidden) || species.abilityHidden === AbilityId.NONE)
+      && (r <= EVENT_THRESHOLD || species.abilityHidden == null || species.abilityHidden === AbilityId.NONE)
     ) {
       tries = 0;
       do {
diff --git a/src/data/mystery-encounters/encounters/training-session-encounter.ts b/src/data/mystery-encounters/encounters/training-session-encounter.ts
index 033a54cc5f5..1f3778a5d2c 100644
--- a/src/data/mystery-encounters/encounters/training-session-encounter.ts
+++ b/src/data/mystery-encounters/encounters/training-session-encounter.ts
@@ -28,7 +28,7 @@ import { MysteryEncounterOptionBuilder } from "#mystery-encounters/mystery-encou
 import { PokemonData } from "#system/pokemon-data";
 import type { HeldModifierConfig } from "#types/held-modifier-config";
 import type { OptionSelectItem } from "#ui/abstract-option-select-ui-handler";
-import { isNullOrUndefined, randSeedShuffle } from "#utils/common";
+import { randSeedShuffle } from "#utils/common";
 import { getEnumValues } from "#utils/enums";
 import i18next from "i18next";
 
@@ -324,7 +324,7 @@ export const TrainingSessionEncounter: MysteryEncounter = MysteryEncounterBuilde
             // Only update the fusion's dex data if the Pokemon is already caught in dex (ignore rentals)
             const rootFusionSpecies = playerPokemon.fusionSpecies?.getRootSpeciesId();
             if (
-              !isNullOrUndefined(rootFusionSpecies)
+              rootFusionSpecies != null
               && speciesStarterCosts.hasOwnProperty(rootFusionSpecies)
               && !!globalScene.gameData.dexData[rootFusionSpecies].caughtAttr
             ) {
diff --git a/src/data/mystery-encounters/encounters/uncommon-breed-encounter.ts b/src/data/mystery-encounters/encounters/uncommon-breed-encounter.ts
index 7bbc4a57757..cd61a6852f7 100644
--- a/src/data/mystery-encounters/encounters/uncommon-breed-encounter.ts
+++ b/src/data/mystery-encounters/encounters/uncommon-breed-encounter.ts
@@ -32,7 +32,7 @@ import { MysteryEncounterOptionBuilder } from "#mystery-encounters/mystery-encou
 import { MoveRequirement, PersistentModifierRequirement } from "#mystery-encounters/mystery-encounter-requirements";
 import { CHARMING_MOVES } from "#mystery-encounters/requirement-groups";
 import { PokemonData } from "#system/pokemon-data";
-import { isNullOrUndefined, randSeedInt } from "#utils/common";
+import { randSeedInt } from "#utils/common";
 
 /** the i18n namespace for the encounter */
 const namespace = "mysteryEncounters/uncommonBreed";
@@ -167,7 +167,7 @@ export const UncommonBreedEncounter: MysteryEncounter = MysteryEncounterBuilder.
       const encounter = globalScene.currentBattle.mysteryEncounter!;
 
       const eggMove = encounter.misc.eggMove;
-      if (!isNullOrUndefined(eggMove)) {
+      if (eggMove != null) {
         // Check what type of move the egg move is to determine target
         const pokemonMove = new PokemonMove(eggMove);
         const move = pokemonMove.getMove();
diff --git a/src/data/mystery-encounters/encounters/weird-dream-encounter.ts b/src/data/mystery-encounters/encounters/weird-dream-encounter.ts
index 1fcbd2961d1..abd81fb92ea 100644
--- a/src/data/mystery-encounters/encounters/weird-dream-encounter.ts
+++ b/src/data/mystery-encounters/encounters/weird-dream-encounter.ts
@@ -41,7 +41,7 @@ import { PokemonData } from "#system/pokemon-data";
 import { trainerConfigs } from "#trainers/trainer-config";
 import { TrainerPartyTemplate } from "#trainers/trainer-party-template";
 import type { HeldModifierConfig } from "#types/held-modifier-config";
-import { isNullOrUndefined, NumberHolder, randSeedInt, randSeedShuffle } from "#utils/common";
+import { NumberHolder, randSeedInt, randSeedShuffle } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 
 /** i18n namespace for encounter */
@@ -634,7 +634,7 @@ function getTransformedSpecies(
   alreadyUsedSpecies: PokemonSpecies[],
 ): PokemonSpecies {
   let newSpecies: PokemonSpecies | undefined;
-  while (isNullOrUndefined(newSpecies)) {
+  while (newSpecies == null) {
     const bstCap = originalBst + bstSearchRange[1];
     const bstMin = Math.max(originalBst + bstSearchRange[0], 0);
 
@@ -655,7 +655,7 @@ function getTransformedSpecies(
     if (validSpecies?.length > 20) {
       validSpecies = randSeedShuffle(validSpecies);
       newSpecies = validSpecies.pop();
-      while (isNullOrUndefined(newSpecies) || alreadyUsedSpecies.includes(newSpecies)) {
+      while (newSpecies == null || alreadyUsedSpecies.includes(newSpecies)) {
         newSpecies = validSpecies.pop();
       }
     } else {
@@ -771,12 +771,12 @@ async function addEggMoveToNewPokemonMoveset(
   if (eggMoves) {
     const eggMoveIndices = randSeedShuffle([0, 1, 2, 3]);
     let randomEggMoveIndex = eggMoveIndices.pop();
-    let randomEggMove = !isNullOrUndefined(randomEggMoveIndex) ? eggMoves[randomEggMoveIndex] : null;
+    let randomEggMove = randomEggMoveIndex != null ? eggMoves[randomEggMoveIndex] : null;
     let retries = 0;
     while (retries < 3 && (!randomEggMove || newPokemon.moveset.some(m => m.moveId === randomEggMove))) {
       // If Pokemon already knows this move, roll for another egg move
       randomEggMoveIndex = eggMoveIndices.pop();
-      randomEggMove = !isNullOrUndefined(randomEggMoveIndex) ? eggMoves[randomEggMoveIndex] : null;
+      randomEggMove = randomEggMoveIndex != null ? eggMoves[randomEggMoveIndex] : null;
       retries++;
     }
 
@@ -791,11 +791,7 @@ async function addEggMoveToNewPokemonMoveset(
       }
 
       // For pokemon that the player owns (including ones just caught), unlock the egg move
-      if (
-        !forBattle
-        && !isNullOrUndefined(randomEggMoveIndex)
-        && !!globalScene.gameData.dexData[speciesRootForm].caughtAttr
-      ) {
+      if (!forBattle && randomEggMoveIndex != null && !!globalScene.gameData.dexData[speciesRootForm].caughtAttr) {
         await globalScene.gameData.setEggMoveUnlocked(getPokemonSpecies(speciesRootForm), randomEggMoveIndex, true);
       }
     }
diff --git a/src/data/mystery-encounters/mystery-encounter-option.ts b/src/data/mystery-encounters/mystery-encounter-option.ts
index fc7bb15d343..1b3b260414d 100644
--- a/src/data/mystery-encounters/mystery-encounter-option.ts
+++ b/src/data/mystery-encounters/mystery-encounter-option.ts
@@ -12,7 +12,7 @@ import {
   MoneyRequirement,
   TypeRequirement,
 } from "#mystery-encounters/mystery-encounter-requirements";
-import { isNullOrUndefined, randSeedInt } from "#utils/common";
+import { randSeedInt } from "#utils/common";
 
 // biome-ignore lint/suspicious/noConfusingVoidType: void unions in callbacks are OK
 export type OptionPhaseCallback = () => Promise<void | boolean>;
@@ -62,7 +62,7 @@ export class MysteryEncounterOption implements IMysteryEncounterOption {
   onPostOptionPhase?: OptionPhaseCallback;
 
   constructor(option: IMysteryEncounterOption | null) {
-    if (!isNullOrUndefined(option)) {
+    if (option != null) {
       Object.assign(this, option);
     }
     this.hasDexProgress = this.hasDexProgress ?? false;
diff --git a/src/data/mystery-encounters/mystery-encounter-requirements.ts b/src/data/mystery-encounters/mystery-encounter-requirements.ts
index f20d513419e..85906044b77 100644
--- a/src/data/mystery-encounters/mystery-encounter-requirements.ts
+++ b/src/data/mystery-encounters/mystery-encounter-requirements.ts
@@ -15,7 +15,7 @@ import { WeatherType } from "#enums/weather-type";
 import type { PlayerPokemon } from "#field/pokemon";
 import { AttackTypeBoosterModifier } from "#modifiers/modifier";
 import type { AttackTypeBoosterModifierType } from "#modifiers/modifier-type";
-import { coerceArray, isNullOrUndefined } from "#utils/common";
+import { coerceArray } from "#utils/common";
 
 export interface EncounterRequirement {
   meetsRequirement(): boolean; // Boolean to see if a requirement is met
@@ -219,7 +219,7 @@ export class WaveRangeRequirement extends EncounterSceneRequirement {
   }
 
   override meetsRequirement(): boolean {
-    if (!isNullOrUndefined(this.waveRange) && this.waveRange[0] <= this.waveRange[1]) {
+    if (this.waveRange != null && this.waveRange[0] <= this.waveRange[1]) {
       const waveIndex = globalScene.currentBattle.waveIndex;
       if (
         (waveIndex >= 0 && this.waveRange[0] >= 0 && this.waveRange[0] > waveIndex)
@@ -275,11 +275,7 @@ export class TimeOfDayRequirement extends EncounterSceneRequirement {
 
   override meetsRequirement(): boolean {
     const timeOfDay = globalScene.arena?.getTimeOfDay();
-    return !(
-      !isNullOrUndefined(timeOfDay)
-      && this.requiredTimeOfDay?.length > 0
-      && !this.requiredTimeOfDay.includes(timeOfDay)
-    );
+    return !(timeOfDay != null && this.requiredTimeOfDay?.length > 0 && !this.requiredTimeOfDay.includes(timeOfDay));
   }
 
   override getDialogueToken(_pokemon?: PlayerPokemon): [string, string] {
@@ -298,7 +294,7 @@ export class WeatherRequirement extends EncounterSceneRequirement {
   override meetsRequirement(): boolean {
     const currentWeather = globalScene.arena.weather?.weatherType;
     return !(
-      !isNullOrUndefined(currentWeather)
+      currentWeather != null
       && this.requiredWeather?.length > 0
       && !this.requiredWeather.includes(currentWeather!)
     );
@@ -307,7 +303,7 @@ export class WeatherRequirement extends EncounterSceneRequirement {
   override getDialogueToken(_pokemon?: PlayerPokemon): [string, string] {
     const currentWeather = globalScene.arena.weather?.weatherType;
     let token = "";
-    if (!isNullOrUndefined(currentWeather)) {
+    if (currentWeather != null) {
       token = WeatherType[currentWeather].replace("_", " ").toLocaleLowerCase();
     }
     return ["weather", token];
@@ -331,7 +327,7 @@ export class PartySizeRequirement extends EncounterSceneRequirement {
   }
 
   override meetsRequirement(): boolean {
-    if (!isNullOrUndefined(this.partySizeRange) && this.partySizeRange[0] <= this.partySizeRange[1]) {
+    if (this.partySizeRange != null && this.partySizeRange[0] <= this.partySizeRange[1]) {
       const partySize = this.excludeDisallowedPokemon
         ? globalScene.getPokemonAllowedInBattle().length
         : globalScene.getPlayerParty().length;
@@ -363,7 +359,7 @@ export class PersistentModifierRequirement extends EncounterSceneRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredHeldItemModifiers?.length < 0) {
+    if (partyPokemon == null || this.requiredHeldItemModifiers?.length < 0) {
       return false;
     }
     let modifierCount = 0;
@@ -396,7 +392,7 @@ export class MoneyRequirement extends EncounterSceneRequirement {
 
   override meetsRequirement(): boolean {
     const money = globalScene.money;
-    if (isNullOrUndefined(money)) {
+    if (money == null) {
       return false;
     }
 
@@ -429,7 +425,7 @@ export class SpeciesRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredSpecies?.length < 0) {
+    if (partyPokemon == null || this.requiredSpecies?.length < 0) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -469,7 +465,7 @@ export class NatureRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredNature?.length < 0) {
+    if (partyPokemon == null || this.requiredNature?.length < 0) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -484,7 +480,7 @@ export class NatureRequirement extends EncounterPokemonRequirement {
   }
 
   override getDialogueToken(pokemon?: PlayerPokemon): [string, string] {
-    if (!isNullOrUndefined(pokemon?.nature) && this.requiredNature.includes(pokemon.nature)) {
+    if (pokemon?.nature != null && this.requiredNature.includes(pokemon.nature)) {
       return ["nature", Nature[pokemon.nature]];
     }
     return ["nature", ""];
@@ -508,7 +504,7 @@ export class TypeRequirement extends EncounterPokemonRequirement {
   override meetsRequirement(): boolean {
     let partyPokemon = globalScene.getPlayerParty();
 
-    if (isNullOrUndefined(partyPokemon)) {
+    if (partyPokemon == null) {
       return false;
     }
 
@@ -561,7 +557,7 @@ export class MoveRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredMoves?.length < 0) {
+    if (partyPokemon == null || this.requiredMoves?.length < 0) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -612,7 +608,7 @@ export class CompatibleMoveRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredMoves?.length < 0) {
+    if (partyPokemon == null || this.requiredMoves?.length < 0) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -668,7 +664,7 @@ export class AbilityRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredAbilities?.length < 0) {
+    if (partyPokemon == null || this.requiredAbilities?.length < 0) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -692,7 +688,7 @@ export class AbilityRequirement extends EncounterPokemonRequirement {
 
   override getDialogueToken(pokemon?: PlayerPokemon): [string, string] {
     const matchingAbility = this.requiredAbilities.find(a => pokemon?.hasAbility(a, false));
-    if (!isNullOrUndefined(matchingAbility)) {
+    if (matchingAbility != null) {
       return ["ability", allAbilities[matchingAbility].name];
     }
     return ["ability", ""];
@@ -713,7 +709,7 @@ export class StatusEffectRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredStatusEffect?.length < 0) {
+    if (partyPokemon == null || this.requiredStatusEffect?.length < 0) {
       return false;
     }
     const x = this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -727,11 +723,7 @@ export class StatusEffectRequirement extends EncounterPokemonRequirement {
         return this.requiredStatusEffect.some(statusEffect => {
           if (statusEffect === StatusEffect.NONE) {
             // StatusEffect.NONE also checks for null or undefined status
-            return (
-              isNullOrUndefined(pokemon.status)
-              || isNullOrUndefined(pokemon.status.effect)
-              || pokemon.status.effect === statusEffect
-            );
+            return pokemon.status == null || pokemon.status.effect == null || pokemon.status.effect === statusEffect;
           }
           return pokemon.status?.effect === statusEffect;
         });
@@ -742,11 +734,7 @@ export class StatusEffectRequirement extends EncounterPokemonRequirement {
       return !this.requiredStatusEffect.some(statusEffect => {
         if (statusEffect === StatusEffect.NONE) {
           // StatusEffect.NONE also checks for null or undefined status
-          return (
-            isNullOrUndefined(pokemon.status)
-            || isNullOrUndefined(pokemon.status.effect)
-            || pokemon.status.effect === statusEffect
-          );
+          return pokemon.status == null || pokemon.status.effect == null || pokemon.status.effect === statusEffect;
         }
         return pokemon.status?.effect === statusEffect;
       });
@@ -756,9 +744,7 @@ export class StatusEffectRequirement extends EncounterPokemonRequirement {
   override getDialogueToken(pokemon?: PlayerPokemon): [string, string] {
     const reqStatus = this.requiredStatusEffect.filter(a => {
       if (a === StatusEffect.NONE) {
-        return (
-          isNullOrUndefined(pokemon?.status) || isNullOrUndefined(pokemon.status.effect) || pokemon.status.effect === a
-        );
+        return pokemon?.status == null || pokemon.status.effect == null || pokemon.status.effect === a;
       }
       return pokemon!.status?.effect === a;
     });
@@ -788,7 +774,7 @@ export class CanFormChangeWithItemRequirement extends EncounterPokemonRequiremen
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon) || this.requiredFormChangeItem?.length < 0) {
+    if (partyPokemon == null || this.requiredFormChangeItem?.length < 0) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -847,7 +833,7 @@ export class HeldItemRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon)) {
+    if (partyPokemon == null) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -911,7 +897,7 @@ export class AttackTypeBoosterHeldItemTypeRequirement extends EncounterPokemonRe
 
   override meetsRequirement(): boolean {
     const partyPokemon = globalScene.getPlayerParty();
-    if (isNullOrUndefined(partyPokemon)) {
+    if (partyPokemon == null) {
       return false;
     }
     return this.queryParty(partyPokemon).length >= this.minNumberOfPokemon;
@@ -978,7 +964,7 @@ export class LevelRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     // Party Pokemon inside required level range
-    if (!isNullOrUndefined(this.requiredLevelRange) && this.requiredLevelRange[0] <= this.requiredLevelRange[1]) {
+    if (this.requiredLevelRange != null && this.requiredLevelRange[0] <= this.requiredLevelRange[1]) {
       const partyPokemon = globalScene.getPlayerParty();
       const pokemonInRange = this.queryParty(partyPokemon);
       if (pokemonInRange.length < this.minNumberOfPokemon) {
@@ -1019,10 +1005,7 @@ export class FriendshipRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     // Party Pokemon inside required friendship range
-    if (
-      !isNullOrUndefined(this.requiredFriendshipRange)
-      && this.requiredFriendshipRange[0] <= this.requiredFriendshipRange[1]
-    ) {
+    if (this.requiredFriendshipRange != null && this.requiredFriendshipRange[0] <= this.requiredFriendshipRange[1]) {
       const partyPokemon = globalScene.getPlayerParty();
       const pokemonInRange = this.queryParty(partyPokemon);
       if (pokemonInRange.length < this.minNumberOfPokemon) {
@@ -1071,7 +1054,7 @@ export class HealthRatioRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     // Party Pokemon's health inside required health range
-    if (!isNullOrUndefined(this.requiredHealthRange) && this.requiredHealthRange[0] <= this.requiredHealthRange[1]) {
+    if (this.requiredHealthRange != null && this.requiredHealthRange[0] <= this.requiredHealthRange[1]) {
       const partyPokemon = globalScene.getPlayerParty();
       const pokemonInRange = this.queryParty(partyPokemon);
       if (pokemonInRange.length < this.minNumberOfPokemon) {
@@ -1098,7 +1081,7 @@ export class HealthRatioRequirement extends EncounterPokemonRequirement {
 
   override getDialogueToken(pokemon?: PlayerPokemon): [string, string] {
     const hpRatio = pokemon?.getHpRatio();
-    if (!isNullOrUndefined(hpRatio)) {
+    if (hpRatio != null) {
       return ["healthRatio", Math.floor(hpRatio * 100).toString() + "%"];
     }
     return ["healthRatio", ""];
@@ -1119,7 +1102,7 @@ export class WeightRequirement extends EncounterPokemonRequirement {
 
   override meetsRequirement(): boolean {
     // Party Pokemon's weight inside required weight range
-    if (!isNullOrUndefined(this.requiredWeightRange) && this.requiredWeightRange[0] <= this.requiredWeightRange[1]) {
+    if (this.requiredWeightRange != null && this.requiredWeightRange[0] <= this.requiredWeightRange[1]) {
       const partyPokemon = globalScene.getPlayerParty();
       const pokemonInRange = this.queryParty(partyPokemon);
       if (pokemonInRange.length < this.minNumberOfPokemon) {
diff --git a/src/data/mystery-encounters/mystery-encounter-save-data.ts b/src/data/mystery-encounters/mystery-encounter-save-data.ts
index f04abccba5f..71cd2517a95 100644
--- a/src/data/mystery-encounters/mystery-encounter-save-data.ts
+++ b/src/data/mystery-encounters/mystery-encounter-save-data.ts
@@ -1,7 +1,6 @@
 import { BASE_MYSTERY_ENCOUNTER_SPAWN_WEIGHT } from "#app/constants";
 import type { MysteryEncounterTier } from "#enums/mystery-encounter-tier";
 import type { MysteryEncounterType } from "#enums/mystery-encounter-type";
-import { isNullOrUndefined } from "#utils/common";
 
 export class SeenEncounterData {
   type: MysteryEncounterType;
@@ -28,7 +27,7 @@ export class MysteryEncounterSaveData {
   queuedEncounters: QueuedEncounter[] = [];
 
   constructor(data?: MysteryEncounterSaveData) {
-    if (!isNullOrUndefined(data)) {
+    if (data != null) {
       Object.assign(this, data);
     }
 
diff --git a/src/data/mystery-encounters/mystery-encounter.ts b/src/data/mystery-encounters/mystery-encounter.ts
index 273e14248e6..f18660b5d71 100644
--- a/src/data/mystery-encounters/mystery-encounter.ts
+++ b/src/data/mystery-encounters/mystery-encounter.ts
@@ -25,7 +25,7 @@ import {
   StatusEffectRequirement,
   WaveRangeRequirement,
 } from "#mystery-encounters/mystery-encounter-requirements";
-import { coerceArray, isNullOrUndefined, randSeedInt } from "#utils/common";
+import { coerceArray, randSeedInt } from "#utils/common";
 import { capitalizeFirstLetter } from "#utils/strings";
 
 export interface EncounterStartOfBattleEffect {
@@ -275,7 +275,7 @@ export class MysteryEncounter implements IMysteryEncounter {
   private seedOffset?: any;
 
   constructor(encounter: IMysteryEncounter | null) {
-    if (!isNullOrUndefined(encounter)) {
+    if (encounter != null) {
       Object.assign(this, encounter);
     }
     this.encounterTier = this.encounterTier ?? MysteryEncounterTier.COMMON;
diff --git a/src/data/mystery-encounters/requirements/can-learn-move-requirement.ts b/src/data/mystery-encounters/requirements/can-learn-move-requirement.ts
index 26602b8ae31..a5810406ef9 100644
--- a/src/data/mystery-encounters/requirements/can-learn-move-requirement.ts
+++ b/src/data/mystery-encounters/requirements/can-learn-move-requirement.ts
@@ -3,7 +3,7 @@ import type { MoveId } from "#enums/move-id";
 import type { PlayerPokemon } from "#field/pokemon";
 import { PokemonMove } from "#moves/pokemon-move";
 import { EncounterPokemonRequirement } from "#mystery-encounters/mystery-encounter-requirements";
-import { coerceArray, isNullOrUndefined } from "#utils/common";
+import { coerceArray } from "#utils/common";
 
 /**
  * {@linkcode CanLearnMoveRequirement} options
@@ -44,7 +44,7 @@ export class CanLearnMoveRequirement extends EncounterPokemonRequirement {
       .getPlayerParty()
       .filter(pkm => (this.includeFainted ? pkm.isAllowedInChallenge() : pkm.isAllowedInBattle()));
 
-    if (isNullOrUndefined(partyPokemon) || this.requiredMoves?.length < 0) {
+    if (partyPokemon == null || this.requiredMoves?.length < 0) {
       return false;
     }
 
diff --git a/src/data/mystery-encounters/utils/encounter-dialogue-utils.ts b/src/data/mystery-encounters/utils/encounter-dialogue-utils.ts
index 1a9b008f9e9..be681f731e8 100644
--- a/src/data/mystery-encounters/utils/encounter-dialogue-utils.ts
+++ b/src/data/mystery-encounters/utils/encounter-dialogue-utils.ts
@@ -1,7 +1,6 @@
 import { globalScene } from "#app/global-scene";
 import type { TextStyle } from "#enums/text-style";
 import { getTextWithColors } from "#ui/text";
-import { isNullOrUndefined } from "#utils/common";
 import i18next from "i18next";
 
 /**
@@ -11,7 +10,7 @@ import i18next from "i18next";
  * @param primaryStyle Can define a text style to be applied to the entire string. Must be defined for BBCodeText styles to be applied correctly
  */
 export function getEncounterText(keyOrString?: string, primaryStyle?: TextStyle): string | null {
-  if (isNullOrUndefined(keyOrString)) {
+  if (keyOrString == null) {
     return null;
   }
 
diff --git a/src/data/mystery-encounters/utils/encounter-phase-utils.ts b/src/data/mystery-encounters/utils/encounter-phase-utils.ts
index aa569f11aca..86cd3fa3a32 100644
--- a/src/data/mystery-encounters/utils/encounter-phase-utils.ts
+++ b/src/data/mystery-encounters/utils/encounter-phase-utils.ts
@@ -49,7 +49,7 @@ import type { HeldModifierConfig } from "#types/held-modifier-config";
 import type { OptionSelectConfig, OptionSelectItem } from "#ui/abstract-option-select-ui-handler";
 import type { PartyOption, PokemonSelectFilter } from "#ui/party-ui-handler";
 import { PartyUiMode } from "#ui/party-ui-handler";
-import { coerceArray, isNullOrUndefined, randomString, randSeedInt, randSeedItem } from "#utils/common";
+import { coerceArray, randomString, randSeedInt, randSeedItem } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import i18next from "i18next";
 
@@ -143,7 +143,7 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
   const trainerType = partyConfig?.trainerType;
   const partyTrainerConfig = partyConfig?.trainerConfig;
   let trainerConfig: TrainerConfig;
-  if (!isNullOrUndefined(trainerType) || partyTrainerConfig) {
+  if (trainerType != null || partyTrainerConfig) {
     globalScene.currentBattle.mysteryEncounter!.encounterMode = MysteryEncounterMode.TRAINER_BATTLE;
     if (globalScene.currentBattle.trainer) {
       globalScene.currentBattle.trainer.setVisible(false);
@@ -154,7 +154,7 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
 
     const doubleTrainer = trainerConfig.doubleOnly || (trainerConfig.hasDouble && !!partyConfig.doubleBattle);
     doubleBattle = doubleTrainer;
-    const trainerFemale = isNullOrUndefined(partyConfig.female) ? !!randSeedInt(2) : partyConfig.female;
+    const trainerFemale = partyConfig.female == null ? !!randSeedInt(2) : partyConfig.female;
     const newTrainer = new Trainer(
       trainerConfig.trainerType,
       doubleTrainer ? TrainerVariant.DOUBLE : trainerFemale ? TrainerVariant.FEMALE : TrainerVariant.DEFAULT,
@@ -202,7 +202,7 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
     let dataSource: PokemonData | undefined;
     let isBoss = false;
     if (!loaded) {
-      if ((!isNullOrUndefined(trainerType) || trainerConfig) && battle.trainer) {
+      if ((trainerType != null || trainerConfig) && battle.trainer) {
         // Allows overriding a trainer's pokemon to use specific species/data
         if (partyConfig?.pokemonConfigs && e < partyConfig.pokemonConfigs.length) {
           const config = partyConfig.pokemonConfigs[e];
@@ -258,7 +258,7 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
       enemyPokemon.resetSummonData();
     }
 
-    if ((!loaded && isNullOrUndefined(partyConfig.countAsSeen)) || partyConfig.countAsSeen) {
+    if ((!loaded && partyConfig.countAsSeen == null) || partyConfig.countAsSeen) {
       globalScene.gameData.setPokemonSeen(enemyPokemon, true, !!(trainerType || trainerConfig));
     }
 
@@ -266,7 +266,7 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
       const config = partyConfig.pokemonConfigs[e];
 
       // Set form
-      if (!isNullOrUndefined(config.nickname)) {
+      if (config.nickname != null) {
         enemyPokemon.nickname = btoa(unescape(encodeURIComponent(config.nickname)));
       }
 
@@ -276,22 +276,22 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
       }
 
       // Set form
-      if (!isNullOrUndefined(config.formIndex)) {
+      if (config.formIndex != null) {
         enemyPokemon.formIndex = config.formIndex;
       }
 
       // Set shiny
-      if (!isNullOrUndefined(config.shiny)) {
+      if (config.shiny != null) {
         enemyPokemon.shiny = config.shiny;
       }
 
       // Set Variant
-      if (enemyPokemon.shiny && !isNullOrUndefined(config.variant)) {
+      if (enemyPokemon.shiny && config.variant != null) {
         enemyPokemon.variant = config.variant;
       }
 
       // Set custom mystery encounter data fields (such as sprite scale, custom abilities, types, etc.)
-      if (!isNullOrUndefined(config.customPokemonData)) {
+      if (config.customPokemonData != null) {
         enemyPokemon.customPokemonData = config.customPokemonData;
       }
 
@@ -300,7 +300,7 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
         let segments =
           config.bossSegments
           ?? globalScene.getEncounterBossSegments(globalScene.currentBattle.waveIndex, level, enemySpecies, true);
-        if (!isNullOrUndefined(config.bossSegmentModifier)) {
+        if (config.bossSegmentModifier != null) {
           segments += config.bossSegmentModifier;
         }
         enemyPokemon.setBoss(true, segments);
@@ -335,18 +335,18 @@ export async function initBattleWithEnemyConfig(partyConfig: EnemyPartyConfig):
       }
 
       // Set ability
-      if (!isNullOrUndefined(config.abilityIndex)) {
+      if (config.abilityIndex != null) {
         enemyPokemon.abilityIndex = config.abilityIndex;
       }
 
       // Set gender
-      if (!isNullOrUndefined(config.gender)) {
+      if (config.gender != null) {
         enemyPokemon.gender = config.gender!;
         enemyPokemon.summonData.gender = config.gender;
       }
 
       // Set AI type
-      if (!isNullOrUndefined(config.aiType)) {
+      if (config.aiType != null) {
         enemyPokemon.aiType = config.aiType;
       }
 
diff --git a/src/data/mystery-encounters/utils/encounter-pokemon-utils.ts b/src/data/mystery-encounters/utils/encounter-pokemon-utils.ts
index 8f6c78fab9c..01d4659d379 100644
--- a/src/data/mystery-encounters/utils/encounter-pokemon-utils.ts
+++ b/src/data/mystery-encounters/utils/encounter-pokemon-utils.ts
@@ -35,7 +35,7 @@ import type { PartyOption } from "#ui/party-ui-handler";
 import { PartyUiMode } from "#ui/party-ui-handler";
 import { SummaryUiMode } from "#ui/summary-ui-handler";
 import { applyChallenges } from "#utils/challenge-utils";
-import { BooleanHolder, isNullOrUndefined, randSeedInt } from "#utils/common";
+import { BooleanHolder, randSeedInt } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import i18next from "i18next";
 
@@ -276,7 +276,7 @@ export function getRandomSpeciesByStarterCost(
 
   if (types && types.length > 0) {
     filteredSpecies = filteredSpecies.filter(
-      s => types.includes(s[0].type1) || (!isNullOrUndefined(s[0].type2) && types.includes(s[0].type2)),
+      s => types.includes(s[0].type1) || (s[0].type2 != null && types.includes(s[0].type2)),
     );
   }
 
diff --git a/src/data/pokemon-species.ts b/src/data/pokemon-species.ts
index 2d76c2c0400..7c00bf5dff7 100644
--- a/src/data/pokemon-species.ts
+++ b/src/data/pokemon-species.ts
@@ -28,7 +28,7 @@ import type { Variant, VariantSet } from "#sprites/variant";
 import { populateVariantColorCache, variantColorCache, variantData } from "#sprites/variant";
 import type { Localizable } from "#types/locales";
 import type { StarterMoveset } from "#types/save-data";
-import { isNullOrUndefined, randSeedFloat, randSeedGauss, randSeedInt } from "#utils/common";
+import { randSeedFloat, randSeedGauss, randSeedInt } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import { toCamelCase, toPascalCase } from "#utils/strings";
 import { argbFromRgba, QuantizerCelebi, rgbaFromArgb } from "@material/material-color-utilities";
@@ -197,7 +197,7 @@ export abstract class PokemonSpeciesForm {
    * @returns The id of the ability
    */
   getPassiveAbility(formIndex?: number): AbilityId {
-    if (isNullOrUndefined(formIndex)) {
+    if (formIndex == null) {
       formIndex = this.formIndex;
     }
     let starterSpeciesId = this.speciesId;
@@ -551,7 +551,7 @@ export abstract class PokemonSpeciesForm {
     const spriteKey = this.getSpriteKey(female, formIndex, shiny, variant, back);
     globalScene.loadPokemonAtlas(spriteKey, this.getSpriteAtlasPath(female, formIndex, shiny, variant, back));
     globalScene.load.audio(this.getCryKey(formIndex), `audio/${this.getCryKey(formIndex)}.m4a`);
-    if (!isNullOrUndefined(variant)) {
+    if (variant != null) {
       await this.loadVariantColors(spriteKey, female, variant, back, formIndex);
     }
     return new Promise<void>(resolve => {
@@ -579,7 +579,7 @@ export abstract class PokemonSpeciesForm {
         const spritePath = this.getSpriteAtlasPath(female, formIndex, shiny, variant, back)
           .replace("variant/", "")
           .replace(/_[1-3]$/, "");
-        if (!isNullOrUndefined(variant)) {
+        if (variant != null) {
           loadPokemonVariantAssets(spriteKey, spritePath, variant).then(() => resolve());
         }
       });
@@ -791,7 +791,7 @@ export class PokemonSpecies extends PokemonSpeciesForm implements Localizable {
    * @returns A randomly rolled gender based on this Species' {@linkcode malePercent}.
    */
   generateGender(): Gender {
-    if (isNullOrUndefined(this.malePercent)) {
+    if (this.malePercent == null) {
       return Gender.GENDERLESS;
     }
 
diff --git a/src/data/pokemon/pokemon-data.ts b/src/data/pokemon/pokemon-data.ts
index 87ffbbab4cd..4fbb70bccb2 100644
--- a/src/data/pokemon/pokemon-data.ts
+++ b/src/data/pokemon/pokemon-data.ts
@@ -16,7 +16,6 @@ import type { AttackMoveResult } from "#types/attack-move-result";
 import type { IllusionData } from "#types/illusion-data";
 import type { TurnMove } from "#types/turn-move";
 import type { CoerceNullPropertiesToUndefined } from "#types/type-helpers";
-import { isNullOrUndefined } from "#utils/common";
 import { getPokemonSpeciesForm } from "#utils/pokemon-utils";
 
 /**
@@ -64,14 +63,14 @@ function deserializePokemonSpeciesForm(value: SerializedSpeciesForm | PokemonSpe
   // @ts-expect-error: We may be deserializing a PokemonSpeciesForm, but we catch later on
   let { id, formIdx } = value;
 
-  if (isNullOrUndefined(id) || isNullOrUndefined(formIdx)) {
+  if (id == null || formIdx == null) {
     // @ts-expect-error: Typescript doesn't know that in block, `value` must be a PokemonSpeciesForm
     id = value.speciesId;
     // @ts-expect-error: Same as above (plus we are accessing a protected property)
     formIdx = value._formIndex;
   }
   // If for some reason either of these fields are null/undefined, we cannot reconstruct the species form
-  if (isNullOrUndefined(id) || isNullOrUndefined(formIdx)) {
+  if (id == null || formIdx == null) {
     return null;
   }
   return getPokemonSpeciesForm(id, formIdx);
@@ -151,13 +150,13 @@ export class PokemonSummonData {
   public moveHistory: TurnMove[] = [];
 
   constructor(source?: PokemonSummonData | SerializedPokemonSummonData) {
-    if (isNullOrUndefined(source)) {
+    if (source == null) {
       return;
     }
 
     // TODO: Rework this into an actual generic function for use elsewhere
     for (const [key, value] of Object.entries(source)) {
-      if (isNullOrUndefined(value) && this.hasOwnProperty(key)) {
+      if (value == null && this.hasOwnProperty(key)) {
         continue;
       }
 
@@ -171,7 +170,7 @@ export class PokemonSummonData {
         const illusionData = {
           ...value,
         };
-        if (!isNullOrUndefined(illusionData.fusionSpecies)) {
+        if (illusionData.fusionSpecies != null) {
           switch (typeof illusionData.fusionSpecies) {
             case "object":
               illusionData.fusionSpecies = allSpecies[illusionData.fusionSpecies.speciesId];
@@ -224,18 +223,18 @@ export class PokemonSummonData {
         CoerceNullPropertiesToUndefined<PokemonSummonData>,
         "speciesForm" | "fusionSpeciesForm" | "illusion"
       >),
-      speciesForm: isNullOrUndefined(speciesForm)
-        ? undefined
-        : { id: speciesForm.speciesId, formIdx: speciesForm.formIndex },
-      fusionSpeciesForm: isNullOrUndefined(fusionSpeciesForm)
-        ? undefined
-        : { id: fusionSpeciesForm.speciesId, formIdx: fusionSpeciesForm.formIndex },
-      illusion: isNullOrUndefined(illusion)
-        ? undefined
-        : {
-            ...(this.illusion as Omit<typeof illusion, "fusionSpecies">),
-            fusionSpecies: illusionSpeciesForm?.speciesId,
-          },
+      speciesForm: speciesForm == null ? undefined : { id: speciesForm.speciesId, formIdx: speciesForm.formIndex },
+      fusionSpeciesForm:
+        fusionSpeciesForm == null
+          ? undefined
+          : { id: fusionSpeciesForm.speciesId, formIdx: fusionSpeciesForm.formIndex },
+      illusion:
+        illusion == null
+          ? undefined
+          : {
+              ...(this.illusion as Omit<typeof illusion, "fusionSpecies">),
+              fusionSpecies: illusionSpeciesForm?.speciesId,
+            },
     };
     // Replace `null` with `undefined`, as `undefined` never gets serialized
     for (const [key, value] of Object.entries(t)) {
@@ -278,7 +277,7 @@ export class PokemonBattleData {
   public berriesEaten: BerryType[] = [];
 
   constructor(source?: PokemonBattleData | Partial<PokemonBattleData>) {
-    if (!isNullOrUndefined(source)) {
+    if (source != null) {
       this.hitCount = source.hitCount ?? 0;
       this.hasEatenBerry = source.hasEatenBerry ?? false;
       this.berriesEaten = source.berriesEaten ?? [];
diff --git a/src/data/trainers/trainer-config.ts b/src/data/trainers/trainer-config.ts
index 9d891444829..55362017cee 100644
--- a/src/data/trainers/trainer-config.ts
+++ b/src/data/trainers/trainer-config.ts
@@ -41,7 +41,7 @@ import type {
   TrainerConfigs,
   TrainerTierPools,
 } from "#types/trainer-funcs";
-import { coerceArray, isNullOrUndefined, randSeedInt, randSeedIntRange, randSeedItem } from "#utils/common";
+import { coerceArray, randSeedInt, randSeedIntRange, randSeedItem } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import { toCamelCase, toTitleCase } from "#utils/strings";
 import i18next from "i18next";
@@ -474,7 +474,7 @@ export class TrainerConfig {
         .fill(null)
         .map((_, i) => i)
         .filter(i => shedinjaCanTera || party[i].species.speciesId !== SpeciesId.SHEDINJA); // Shedinja can only Tera on Bug specialty type (or no specialty type)
-      const setPartySlot = !isNullOrUndefined(slot) ? Phaser.Math.Wrap(slot, 0, party.length) : -1; // If we have a tera slot defined, wrap it to party size.
+      const setPartySlot = slot != null ? Phaser.Math.Wrap(slot, 0, party.length) : -1; // If we have a tera slot defined, wrap it to party size.
       for (let t = 0; t < Math.min(count(), party.length); t++) {
         const randomIndex =
           partyMemberIndexes.indexOf(setPartySlot) > -1 ? setPartySlot : randSeedItem(partyMemberIndexes);
@@ -537,7 +537,7 @@ export class TrainerConfig {
       initI18n();
     }
 
-    if (!isNullOrUndefined(specialtyType)) {
+    if (specialtyType != null) {
       this.setSpecialtyType(specialtyType);
     }
 
@@ -612,7 +612,7 @@ export class TrainerConfig {
     signatureSpecies.forEach((speciesPool, s) => {
       this.setPartyMemberFunc(-(s + 1), getRandomPartyMemberFunc(coerceArray(speciesPool)));
     });
-    if (!isNullOrUndefined(specialtyType)) {
+    if (specialtyType != null) {
       this.setSpeciesFilter(p => p.isOfType(specialtyType));
       this.setSpecialtyType(specialtyType);
     }
@@ -717,7 +717,7 @@ export class TrainerConfig {
     });
 
     // Set species filter and specialty type if provided, otherwise filter by base total.
-    if (!isNullOrUndefined(specialtyType)) {
+    if (specialtyType != null) {
       this.setSpeciesFilter(p => p.isOfType(specialtyType) && p.baseTotal >= ELITE_FOUR_MINIMUM_BST);
       this.setSpecialtyType(specialtyType);
     } else {
@@ -895,7 +895,7 @@ export class TrainerConfig {
    * @returns `true` if `specialtyType` is defined and not {@link PokemonType.UNKNOWN}
    */
   hasSpecialtyType(): boolean {
-    return !isNullOrUndefined(this.specialtyType) && this.specialtyType !== PokemonType.UNKNOWN;
+    return this.specialtyType != null && this.specialtyType !== PokemonType.UNKNOWN;
   }
 
   /**
@@ -2942,7 +2942,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.SLOWBRO, SpeciesId.GALAR_SLOWBRO], TrainerSlot.TRAINER, true, p => {
         // Tera Ice Slowbro/G-Slowbro
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.ICE_BEAM)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.ICE_BEAM)) {
           // Check if Ice Beam is in the moveset, if not, replace the third move with Ice Beam.
           p.moveset[2] = new PokemonMove(MoveId.ICE_BEAM);
         }
@@ -2967,7 +2967,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.STEELIX], TrainerSlot.TRAINER, true, p => {
         // Tera Fighting Steelix
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.BODY_PRESS)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.BODY_PRESS)) {
           // Check if Body Press is in the moveset, if not, replace the third move with Body Press.
           p.moveset[2] = new PokemonMove(MoveId.BODY_PRESS);
         }
@@ -2992,7 +2992,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.ARBOK, SpeciesId.WEEZING], TrainerSlot.TRAINER, true, p => {
         // Tera Ghost Arbok/Weezing
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3018,7 +3018,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.GYARADOS, SpeciesId.AERODACTYL], TrainerSlot.TRAINER, true, p => {
         // Tera Dragon Gyarados/Aerodactyl
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3079,7 +3079,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.GENGAR], TrainerSlot.TRAINER, true, p => {
         // Tera Dark Gengar
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.DARK_PULSE)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.DARK_PULSE)) {
           // Check if Dark Pulse is in the moveset, if not, replace the third move with Dark Pulse.
           p.moveset[2] = new PokemonMove(MoveId.DARK_PULSE);
         }
@@ -3163,7 +3163,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.DHELMISE], TrainerSlot.TRAINER, true, p => {
         // Tera Dragon Dhelmise
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3193,7 +3193,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.setBoss(true, 2);
         p.abilityIndex = 1; // Sniper
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.X_SCISSOR)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.X_SCISSOR)) {
           // Check if X-Scissor is in the moveset, if not, replace the third move with X-Scissor.
           p.moveset[2] = new PokemonMove(MoveId.X_SCISSOR);
         }
@@ -3232,7 +3232,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.STEELIX, SpeciesId.LOPUNNY], TrainerSlot.TRAINER, true, p => {
         // Tera Fire Steelix/Lopunny
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3375,7 +3375,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.CERULEDGE], TrainerSlot.TRAINER, true, p => {
         // Tera Steel Ceruledge
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.IRON_HEAD)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.IRON_HEAD)) {
           // Check if Iron Head is in the moveset, if not, replace the third move with Iron Head.
           p.moveset[2] = new PokemonMove(MoveId.IRON_HEAD);
         }
@@ -3413,7 +3413,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.INCINEROAR], TrainerSlot.TRAINER, true, p => {
         // Tera Fighting Incineroar
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.CROSS_CHOP)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.CROSS_CHOP)) {
           // Check if Cross Chop is in the moveset, if not, replace the third move with Cross Chop.
           p.moveset[2] = new PokemonMove(MoveId.CROSS_CHOP);
         }
@@ -3486,7 +3486,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.DECIDUEYE], TrainerSlot.TRAINER, true, p => {
         // Tera Flying Decidueye
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.BRAVE_BIRD)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.BRAVE_BIRD)) {
           // Check if Brave Bird is in the moveset, if not, replace the third move with Brave Bird.
           p.moveset[2] = new PokemonMove(MoveId.BRAVE_BIRD);
         }
@@ -3511,7 +3511,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.TOXICROAK], TrainerSlot.TRAINER, true, p => {
         // Tera Dark Toxicroak
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.SUCKER_PUNCH)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.SUCKER_PUNCH)) {
           // Check if Sucker Punch is in the moveset, if not, replace the third move with Sucker Punch.
           p.moveset[2] = new PokemonMove(MoveId.SUCKER_PUNCH);
         }
@@ -3536,7 +3536,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.EISCUE], TrainerSlot.TRAINER, true, p => {
         // Tera Water Eiscue
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.LIQUIDATION)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.LIQUIDATION)) {
           // Check if Liquidation is in the moveset, if not, replace the third move with Liquidation.
           p.moveset[2] = new PokemonMove(MoveId.LIQUIDATION);
         }
@@ -3598,7 +3598,7 @@ export const trainerConfigs: TrainerConfigs = {
         // Tera Dragon Torkoal
         p.abilityIndex = 1; // Drought
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3695,7 +3695,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.EXEGGUTOR], TrainerSlot.TRAINER, true, p => {
         // Tera Fire Exeggutor
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3705,7 +3705,7 @@ export const trainerConfigs: TrainerConfigs = {
       3,
       getRandomPartyMemberFunc([SpeciesId.TALONFLAME], TrainerSlot.TRAINER, true, p => {
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.SUNNY_DAY)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.SUNNY_DAY)) {
           // Check if Sunny Day is in the moveset, if not, replace the third move with Sunny Day.
           p.moveset[2] = new PokemonMove(MoveId.SUNNY_DAY);
         }
@@ -3728,7 +3728,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.REUNICLUS], TrainerSlot.TRAINER, true, p => {
         // Tera Steel Reuniclus
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.FLASH_CANNON)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.FLASH_CANNON)) {
           // Check if Flash Cannon is in the moveset, if not, replace the third move with Flash Cannon.
           p.moveset[2] = new PokemonMove(MoveId.FLASH_CANNON);
         }
@@ -3756,7 +3756,7 @@ export const trainerConfigs: TrainerConfigs = {
         // Tera Fairy Excadrill
         p.setBoss(true, 2);
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -3771,7 +3771,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.SCEPTILE], TrainerSlot.TRAINER, true, p => {
         // Tera Dragon Sceptile
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.DUAL_CHOP)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.DUAL_CHOP)) {
           // Check if Dual Chop is in the moveset, if not, replace the third move with Dual Chop.
           p.moveset[2] = new PokemonMove(MoveId.DUAL_CHOP);
         }
@@ -3841,7 +3841,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.formIndex = 1; // Partner Pikachu
         p.gender = Gender.MALE;
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.VOLT_TACKLE)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.VOLT_TACKLE)) {
           // Check if Volt Tackle is in the moveset, if not, replace the first move with Volt Tackle.
           p.moveset[0] = new PokemonMove(MoveId.VOLT_TACKLE);
         }
@@ -4072,7 +4072,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.KELDEO], TrainerSlot.TRAINER, true, p => {
         p.pokeball = PokeballType.ROGUE_BALL;
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.SECRET_SWORD)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.SECRET_SWORD)) {
           // Check if Secret Sword is in the moveset, if not, replace the third move with Secret Sword.
           p.moveset[2] = new PokemonMove(MoveId.SECRET_SWORD);
         }
@@ -4401,7 +4401,7 @@ export const trainerConfigs: TrainerConfigs = {
       5,
       getRandomPartyMemberFunc([SpeciesId.KINGAMBIT], TrainerSlot.TRAINER, true, p => {
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -4480,7 +4480,7 @@ export const trainerConfigs: TrainerConfigs = {
       4,
       getRandomPartyMemberFunc([SpeciesId.TERAPAGOS], TrainerSlot.TRAINER, true, p => {
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_STARSTORM)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_STARSTORM)) {
           // Check if Tera Starstorm is in the moveset, if not, replace the first move with Tera Starstorm.
           p.moveset[0] = new PokemonMove(MoveId.TERA_STARSTORM);
         }
@@ -4494,7 +4494,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.setBoss(true, 2);
         p.teraType = PokemonType.FIGHTING;
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TERA_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TERA_BLAST)) {
           // Check if Tera Blast is in the moveset, if not, replace the third move with Tera Blast.
           p.moveset[2] = new PokemonMove(MoveId.TERA_BLAST);
         }
@@ -5054,7 +5054,7 @@ export const trainerConfigs: TrainerConfigs = {
       2,
       getRandomPartyMemberFunc([SpeciesId.HONCHKROW], TrainerSlot.TRAINER, true, p => {
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.SUCKER_PUNCH)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.SUCKER_PUNCH)) {
           // Check if Sucker Punch is in the moveset, if not, replace the third move with Sucker Punch.
           p.moveset[2] = new PokemonMove(MoveId.SUCKER_PUNCH);
         }
@@ -5517,7 +5517,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.formIndex = randSeedInt(18); // Random Silvally Form
         p.generateAndPopulateMoveset();
         p.pokeball = PokeballType.ROGUE_BALL;
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.MULTI_ATTACK)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.MULTI_ATTACK)) {
           // Check if Multi Attack is in the moveset, if not, replace the first move with Multi Attack.
           p.moveset[0] = new PokemonMove(MoveId.MULTI_ATTACK);
         }
@@ -5590,7 +5590,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.GOLISOPOD], TrainerSlot.TRAINER, true, p => {
         p.setBoss(true, 2);
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.FIRST_IMPRESSION)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.FIRST_IMPRESSION)) {
           // Check if First Impression is in the moveset, if not, replace the third move with First Impression.
           p.moveset[2] = new PokemonMove(MoveId.FIRST_IMPRESSION);
           p.gender = Gender.MALE;
@@ -5607,7 +5607,7 @@ export const trainerConfigs: TrainerConfigs = {
       getRandomPartyMemberFunc([SpeciesId.GOLISOPOD], TrainerSlot.TRAINER, true, p => {
         p.setBoss(true, 2);
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.FIRST_IMPRESSION)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.FIRST_IMPRESSION)) {
           // Check if First Impression is in the moveset, if not, replace the third move with First Impression.
           p.moveset[2] = new PokemonMove(MoveId.FIRST_IMPRESSION);
           p.abilityIndex = 2; // Anticipation
@@ -5643,7 +5643,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.generateAndPopulateMoveset();
         p.pokeball = PokeballType.ROGUE_BALL;
         p.formIndex = randSeedInt(4, 1); // Shock, Burn, Chill, or Douse Drive
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.TECHNO_BLAST)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.TECHNO_BLAST)) {
           // Check if Techno Blast is in the moveset, if not, replace the third move with Techno Blast.
           p.moveset[2] = new PokemonMove(MoveId.TECHNO_BLAST);
         }
@@ -5778,7 +5778,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.setBoss(true, 2);
         p.abilityIndex = 2; // Pixilate
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.HYPER_VOICE)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.HYPER_VOICE)) {
           // Check if Hyper Voice is in the moveset, if not, replace the second move with Hyper Voice.
           p.moveset[1] = new PokemonMove(MoveId.HYPER_VOICE);
           p.gender = Gender.FEMALE;
@@ -5807,7 +5807,7 @@ export const trainerConfigs: TrainerConfigs = {
         p.setBoss(true, 2);
         p.abilityIndex = 2; // Pixilate
         p.generateAndPopulateMoveset();
-        if (!p.moveset.some(move => !isNullOrUndefined(move) && move.moveId === MoveId.HYPER_VOICE)) {
+        if (!p.moveset.some(move => move != null && move.moveId === MoveId.HYPER_VOICE)) {
           // Check if Hyper Voice is in the moveset, if not, replace the second move with Hyper Voice.
           p.moveset[1] = new PokemonMove(MoveId.HYPER_VOICE);
           p.gender = Gender.FEMALE;
diff --git a/src/field/arena.ts b/src/field/arena.ts
index ff7379b2a4a..c4708be1336 100644
--- a/src/field/arena.ts
+++ b/src/field/arena.ts
@@ -36,7 +36,7 @@ import type { Pokemon } from "#field/pokemon";
 import { FieldEffectModifier } from "#modifiers/modifier";
 import type { Move } from "#moves/move";
 import type { AbstractConstructor } from "#types/type-helpers";
-import { type Constructor, isNullOrUndefined, NumberHolder, randSeedInt } from "#utils/common";
+import { type Constructor, NumberHolder, randSeedInt } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 
 export class Arena {
@@ -339,7 +339,7 @@ export class Arena {
 
     const weatherDuration = new NumberHolder(0);
 
-    if (!isNullOrUndefined(user)) {
+    if (user != null) {
       weatherDuration.value = 5;
       globalScene.applyModifier(FieldEffectModifier, user.isPlayer(), user, weatherDuration);
     }
@@ -420,7 +420,7 @@ export class Arena {
 
     const terrainDuration = new NumberHolder(0);
 
-    if (!isNullOrUndefined(user)) {
+    if (user != null) {
       terrainDuration.value = 5;
       globalScene.applyModifier(FieldEffectModifier, user.isPlayer(), user, terrainDuration);
     }
diff --git a/src/field/mystery-encounter-intro.ts b/src/field/mystery-encounter-intro.ts
index cf0a0f30529..4b4a234251a 100644
--- a/src/field/mystery-encounter-intro.ts
+++ b/src/field/mystery-encounter-intro.ts
@@ -5,7 +5,6 @@ import { getSpriteKeysFromSpecies } from "#mystery-encounters/encounter-pokemon-
 import type { MysteryEncounter } from "#mystery-encounters/mystery-encounter";
 import { loadPokemonVariantAssets } from "#sprites/pokemon-sprite";
 import type { Variant } from "#sprites/variant";
-import { isNullOrUndefined } from "#utils/common";
 import type { GameObjects } from "phaser";
 
 type PlayAnimationConfig = Phaser.Types.Animations.PlayAnimationConfig;
@@ -98,7 +97,7 @@ export class MysteryEncounterIntroVisuals extends Phaser.GameObjects.Container {
         ...config,
       };
 
-      if (!isNullOrUndefined(result.species)) {
+      if (result.species != null) {
         const keys = getSpriteKeysFromSpecies(result.species, undefined, undefined, result.isShiny, result.variant);
         result.spriteKey = keys.spriteKey;
         result.fileRoot = keys.fileRoot;
@@ -205,12 +204,12 @@ export class MysteryEncounterIntroVisuals extends Phaser.GameObjects.Container {
         n++;
       }
 
-      if (!isNullOrUndefined(pokemonShinySparkle)) {
+      if (pokemonShinySparkle != null) {
         // Offset the sparkle to match the Pokemon's position
         pokemonShinySparkle.setPosition(sprite.x, sprite.y);
       }
 
-      if (!isNullOrUndefined(alpha)) {
+      if (alpha != null) {
         sprite.setAlpha(alpha);
         tintSprite.setAlpha(alpha);
       }
@@ -234,7 +233,7 @@ export class MysteryEncounterIntroVisuals extends Phaser.GameObjects.Container {
       this.spriteConfigs.forEach(config => {
         if (config.isPokemon) {
           globalScene.loadPokemonAtlas(config.spriteKey, config.fileRoot);
-          if (config.isShiny && !isNullOrUndefined(config.variant)) {
+          if (config.isShiny && config.variant != null) {
             shinyPromises.push(loadPokemonVariantAssets(config.spriteKey, config.fileRoot, config.variant));
           }
         } else if (config.isItem) {
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index d3ff7312977..e6a46ac926f 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -159,7 +159,6 @@ import {
   fixedInt,
   getIvsFromId,
   isBetween,
-  isNullOrUndefined,
   NumberHolder,
   randSeedFloat,
   randSeedInt,
@@ -886,7 +885,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
         return this.fallbackVariantColor(cacheKey, spritePath, useExpSprite, battleSpritePath, error);
       })
       .then(c => {
-        if (!isNullOrUndefined(c)) {
+        if (c != null) {
           variantColorCache[cacheKey] = c;
         }
       });
@@ -1475,7 +1474,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
     }
 
     const ally = this.getAlly();
-    if (!isNullOrUndefined(ally)) {
+    if (ally != null) {
       applyAbAttrs("AllyStatMultiplierAbAttr", {
         pokemon: ally,
         stat,
@@ -1658,7 +1657,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
     if (useIllusion && this.summonData.illusion) {
       return this.summonData.illusion.gender;
     }
-    if (!ignoreOverride && !isNullOrUndefined(this.summonData.gender)) {
+    if (!ignoreOverride && this.summonData.gender != null) {
       return this.summonData.gender;
     }
     return this.gender;
@@ -1674,7 +1673,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
     if (useIllusion && this.summonData.illusion?.fusionGender) {
       return this.summonData.illusion.fusionGender;
     }
-    if (!ignoreOverride && !isNullOrUndefined(this.summonData.fusionGender)) {
+    if (!ignoreOverride && this.summonData.fusionGender != null) {
       return this.summonData.fusionGender;
     }
     return this.fusionGender;
@@ -1793,7 +1792,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
    * @returns Whether this Pokemon has this species as either its base or fusion counterpart.
    */
   hasSpecies(species: SpeciesId, formKey?: string): boolean {
-    if (isNullOrUndefined(formKey)) {
+    if (formKey == null) {
       return this.species.speciesId === species || this.fusionSpecies?.speciesId === species;
     }
 
@@ -1941,7 +1940,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
             secondType = fusionType1;
           }
 
-          if (secondType === PokemonType.UNKNOWN && isNullOrUndefined(fusionType2)) {
+          if (secondType === PokemonType.UNKNOWN && fusionType2 == null) {
             // If second pokemon was monotype and shared its primary type
             secondType =
               customTypes
@@ -2024,12 +2023,12 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
       return allAbilities[Overrides.ENEMY_ABILITY_OVERRIDE];
     }
     if (this.isFusion()) {
-      if (!isNullOrUndefined(this.fusionCustomPokemonData?.ability) && this.fusionCustomPokemonData.ability !== -1) {
+      if (this.fusionCustomPokemonData?.ability != null && this.fusionCustomPokemonData.ability !== -1) {
         return allAbilities[this.fusionCustomPokemonData.ability];
       }
       return allAbilities[this.getFusionSpeciesForm(ignoreOverride).getAbility(this.fusionAbilityIndex)];
     }
-    if (!isNullOrUndefined(this.customPokemonData.ability) && this.customPokemonData.ability !== -1) {
+    if (this.customPokemonData.ability != null && this.customPokemonData.ability !== -1) {
       return allAbilities[this.customPokemonData.ability];
     }
     let abilityId = this.getSpeciesForm(ignoreOverride).getAbility(this.abilityIndex);
@@ -2053,7 +2052,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
     if (Overrides.ENEMY_PASSIVE_ABILITY_OVERRIDE && this.isEnemy()) {
       return allAbilities[Overrides.ENEMY_PASSIVE_ABILITY_OVERRIDE];
     }
-    if (!isNullOrUndefined(this.customPokemonData.passive) && this.customPokemonData.passive !== -1) {
+    if (this.customPokemonData.passive != null && this.customPokemonData.passive !== -1) {
       return allAbilities[this.customPokemonData.passive];
     }
 
@@ -2237,7 +2236,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
   public getWeight(): number {
     const autotomizedTag = this.getTag(AutotomizedTag);
     let weightRemoved = 0;
-    if (!isNullOrUndefined(autotomizedTag)) {
+    if (autotomizedTag != null) {
       weightRemoved = 100 * autotomizedTag.autotomizeCount;
     }
     const minWeight = 0.1;
@@ -2384,7 +2383,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
     cancelled?: BooleanHolder,
     useIllusion = false,
   ): TypeDamageMultiplier {
-    if (!isNullOrUndefined(this.turnData?.moveEffectiveness)) {
+    if (this.turnData?.moveEffectiveness != null) {
       return this.turnData?.moveEffectiveness;
     }
 
@@ -3606,7 +3605,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
     });
 
     const ally = this.getAlly();
-    if (!isNullOrUndefined(ally)) {
+    if (ally != null) {
       const ignore =
         this.hasAbilityWithAttr("MoveAbilityBypassAbAttr") || sourceMove.hasFlag(MoveFlags.IGNORE_ABILITIES);
       applyAbAttrs("AllyStatMultiplierAbAttr", {
@@ -4022,7 +4021,7 @@ export abstract class Pokemon extends Phaser.GameObjects.Container {
 
       const ally = this.getAlly();
       /** Additionally apply friend guard damage reduction if ally has it. */
-      if (globalScene.currentBattle.double && !isNullOrUndefined(ally) && ally.isActive(true)) {
+      if (globalScene.currentBattle.double && ally != null && ally.isActive(true)) {
         applyAbAttrs("AlliedFieldDamageReductionAbAttr", {
           ...abAttrParams,
           // Same parameters as before, except we are applying the ally's ability
@@ -6397,13 +6396,13 @@ export class EnemyPokemon extends Pokemon {
 
     if (
       speciesId in Overrides.ENEMY_FORM_OVERRIDES
-      && !isNullOrUndefined(Overrides.ENEMY_FORM_OVERRIDES[speciesId])
+      && Overrides.ENEMY_FORM_OVERRIDES[speciesId] != null
       && this.species.forms[Overrides.ENEMY_FORM_OVERRIDES[speciesId]]
     ) {
       this.formIndex = Overrides.ENEMY_FORM_OVERRIDES[speciesId];
     } else if (globalScene.gameMode.isDaily && globalScene.gameMode.isWaveFinal(globalScene.currentBattle.waveIndex)) {
       const eventBoss = getDailyEventSeedBoss(globalScene.seed);
-      if (!isNullOrUndefined(eventBoss)) {
+      if (eventBoss != null) {
         this.formIndex = eventBoss.formIndex;
       }
     }
diff --git a/src/game-mode.ts b/src/game-mode.ts
index 9ea3adf59d3..e543e3c42ca 100644
--- a/src/game-mode.ts
+++ b/src/game-mode.ts
@@ -14,7 +14,7 @@ import { SpeciesId } from "#enums/species-id";
 import type { Arena } from "#field/arena";
 import { classicFixedBattles, type FixedBattleConfigs } from "#trainers/fixed-battle-configs";
 import { applyChallenges } from "#utils/challenge-utils";
-import { BooleanHolder, isNullOrUndefined, randSeedInt, randSeedItem } from "#utils/common";
+import { BooleanHolder, randSeedInt, randSeedItem } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import i18next from "i18next";
 
@@ -146,7 +146,7 @@ export class GameMode implements GameModeConfig {
    * - Town
    */
   getStartingBiome(): BiomeId {
-    if (!isNullOrUndefined(Overrides.STARTING_BIOME_OVERRIDE)) {
+    if (Overrides.STARTING_BIOME_OVERRIDE != null) {
       return Overrides.STARTING_BIOME_OVERRIDE;
     }
 
@@ -234,7 +234,7 @@ export class GameMode implements GameModeConfig {
   getOverrideSpecies(waveIndex: number): PokemonSpecies | null {
     if (this.isDaily && this.isWaveFinal(waveIndex)) {
       const eventBoss = getDailyEventSeedBoss(globalScene.seed);
-      if (!isNullOrUndefined(eventBoss)) {
+      if (eventBoss != null) {
         // Cannot set form index here, it will be overriden when adding it as enemy pokemon.
         return getPokemonSpecies(eventBoss.speciesId);
       }
diff --git a/src/modifier/init-modifier-pools.ts b/src/modifier/init-modifier-pools.ts
index ba12920407d..e6ec69eac7f 100644
--- a/src/modifier/init-modifier-pools.ts
+++ b/src/modifier/init-modifier-pools.ts
@@ -31,7 +31,6 @@ import {
 } from "#modifiers/modifier-pools";
 import { WeightedModifierType } from "#modifiers/modifier-type";
 import type { WeightedModifierTypeWeightFunc } from "#types/modifier-types";
-import { isNullOrUndefined } from "#utils/common";
 
 /**
  * Initialize the wild modifier pool
@@ -409,7 +408,7 @@ function initUltraModifierPool() {
           if (!isHoldingOrb) {
             const moveset = p
               .getMoveset(true)
-              .filter(m => !isNullOrUndefined(m))
+              .filter(m => m != null)
               .map(m => m.moveId);
             const canSetStatus = p.canSetStatus(StatusEffect.TOXIC, true, true, null, true);
 
@@ -455,7 +454,7 @@ function initUltraModifierPool() {
           if (!isHoldingOrb) {
             const moveset = p
               .getMoveset(true)
-              .filter(m => !isNullOrUndefined(m))
+              .filter(m => m != null)
               .map(m => m.moveId);
             const canSetStatus = p.canSetStatus(StatusEffect.BURN, true, true, null, true);
 
diff --git a/src/modifier/modifier-type.ts b/src/modifier/modifier-type.ts
index 579fb75ce3b..d67011bc145 100644
--- a/src/modifier/modifier-type.ts
+++ b/src/modifier/modifier-type.ts
@@ -119,15 +119,7 @@ import type { PokemonMoveSelectFilter, PokemonSelectFilter } from "#ui/party-ui-
 import { PartyUiHandler } from "#ui/party-ui-handler";
 import { getModifierTierTextTint } from "#ui/text";
 import { applyChallenges } from "#utils/challenge-utils";
-import {
-  BooleanHolder,
-  formatMoney,
-  isNullOrUndefined,
-  NumberHolder,
-  padInt,
-  randSeedInt,
-  randSeedItem,
-} from "#utils/common";
+import { BooleanHolder, formatMoney, NumberHolder, padInt, randSeedInt, randSeedItem } from "#utils/common";
 import { getEnumKeys, getEnumValues } from "#utils/enums";
 import { getModifierPoolForType, getModifierType } from "#utils/modifier-utils";
 import { toCamelCase } from "#utils/strings";
@@ -263,7 +255,7 @@ export class ModifierType {
             this.tier = modifier.modifierType.tier;
             return this;
           }
-          if (isNullOrUndefined(defaultTier)) {
+          if (defaultTier == null) {
             // If weight is 0, keep track of the first tier where the item was found
             defaultTier = modifier.modifierType.tier;
           }
@@ -2920,7 +2912,7 @@ export function getPartyLuckValue(party: Pokemon[]): number {
     globalScene.executeWithSeedOffset(
       () => {
         const eventLuck = getDailyEventSeedLuck(globalScene.seed);
-        if (!isNullOrUndefined(eventLuck)) {
+        if (eventLuck != null) {
           DailyLuck.value = eventLuck;
           return;
         }
diff --git a/src/modifier/modifier.ts b/src/modifier/modifier.ts
index 1f470e592c2..82e64316edd 100644
--- a/src/modifier/modifier.ts
+++ b/src/modifier/modifier.ts
@@ -42,7 +42,7 @@ import type {
 import type { VoucherType } from "#system/voucher";
 import type { ModifierInstanceMap, ModifierString } from "#types/modifier-types";
 import { addTextObject } from "#ui/text";
-import { BooleanHolder, hslToHex, isNullOrUndefined, NumberHolder, randSeedFloat, toDmgValue } from "#utils/common";
+import { BooleanHolder, hslToHex, NumberHolder, randSeedFloat, toDmgValue } from "#utils/common";
 import { getModifierType } from "#utils/modifier-utils";
 import i18next from "i18next";
 
@@ -2113,10 +2113,7 @@ export class PokemonHpRestoreModifier extends ConsumablePokemonModifier {
    * @returns `true` if the {@linkcode PokemonHpRestoreModifier} should be applied
    */
   override shouldApply(playerPokemon?: PlayerPokemon, multiplier?: number): boolean {
-    return (
-      super.shouldApply(playerPokemon)
-      && (this.fainted || (!isNullOrUndefined(multiplier) && typeof multiplier === "number"))
-    );
+    return super.shouldApply(playerPokemon) && (this.fainted || (multiplier != null && typeof multiplier === "number"));
   }
 
   /**
@@ -2753,10 +2750,10 @@ export class PokemonMultiHitModifier extends PokemonHeldItemModifier {
       return false;
     }
 
-    if (!isNullOrUndefined(count)) {
+    if (count != null) {
       return this.applyHitCountBoost(count);
     }
-    if (!isNullOrUndefined(damageMultiplier)) {
+    if (damageMultiplier != null) {
       return this.applyDamageModifier(pokemon, damageMultiplier);
     }
 
diff --git a/src/phases/faint-phase.ts b/src/phases/faint-phase.ts
index 349dfcfa8e5..821d16c6546 100644
--- a/src/phases/faint-phase.ts
+++ b/src/phases/faint-phase.ts
@@ -17,7 +17,6 @@ import type { EnemyPokemon, PlayerPokemon, Pokemon } from "#field/pokemon";
 import { PokemonInstantReviveModifier } from "#modifiers/modifier";
 import { PokemonMove } from "#moves/pokemon-move";
 import { PokemonPhase } from "#phases/pokemon-phase";
-import { isNullOrUndefined } from "#utils/common";
 import i18next from "i18next";
 
 export class FaintPhase extends PokemonPhase {
@@ -187,7 +186,7 @@ export class FaintPhase extends PokemonPhase {
 
     // in double battles redirect potential moves off fainted pokemon
     const allyPokemon = pokemon.getAlly();
-    if (globalScene.currentBattle.double && !isNullOrUndefined(allyPokemon)) {
+    if (globalScene.currentBattle.double && allyPokemon != null) {
       globalScene.redirectPokemonMoves(pokemon, allyPokemon);
     }
 
diff --git a/src/phases/move-effect-phase.ts b/src/phases/move-effect-phase.ts
index 6c143f1f8a1..717bc670558 100644
--- a/src/phases/move-effect-phase.ts
+++ b/src/phases/move-effect-phase.ts
@@ -40,7 +40,7 @@ import { DamageAchv } from "#system/achv";
 import type { DamageResult } from "#types/damage-result";
 import type { TurnMove } from "#types/turn-move";
 import type { nil } from "#utils/common";
-import { BooleanHolder, isNullOrUndefined, NumberHolder } from "#utils/common";
+import { BooleanHolder, NumberHolder } from "#utils/common";
 import i18next from "i18next";
 
 export type HitCheckEntry = [HitCheckResult, TypeDamageMultiplier];
@@ -740,7 +740,7 @@ export class MoveEffectPhase extends PokemonPhase {
       (attr: MoveAttr) =>
         attr.is("MoveEffectAttr")
         && attr.trigger === triggerType
-        && (isNullOrUndefined(selfTarget) || attr.selfTarget === selfTarget)
+        && (selfTarget == null || attr.selfTarget === selfTarget)
         && (!attr.firstHitOnly || this.firstHit)
         && (!attr.lastHitOnly || this.lastHit)
         && (!attr.firstTargetOnly || (firstTarget ?? true)),
@@ -765,7 +765,7 @@ export class MoveEffectPhase extends PokemonPhase {
    */
   protected applyMoveEffects(target: Pokemon, effectiveness: TypeDamageMultiplier, firstTarget: boolean): void {
     const user = this.getUserPokemon();
-    if (isNullOrUndefined(user)) {
+    if (user == null) {
       return;
     }
 
diff --git a/src/phases/mystery-encounter-phases.ts b/src/phases/mystery-encounter-phases.ts
index df670deaf26..4f50b40c965 100644
--- a/src/phases/mystery-encounter-phases.ts
+++ b/src/phases/mystery-encounter-phases.ts
@@ -14,7 +14,7 @@ import type { OptionSelectSettings } from "#mystery-encounters/encounter-phase-u
 import { transitionMysteryEncounterIntroVisuals } from "#mystery-encounters/encounter-phase-utils";
 import type { MysteryEncounterOption, OptionPhaseCallback } from "#mystery-encounters/mystery-encounter-option";
 import { SeenEncounterData } from "#mystery-encounters/mystery-encounter-save-data";
-import { isNullOrUndefined, randSeedItem } from "#utils/common";
+import { randSeedItem } from "#utils/common";
 import i18next from "i18next";
 
 /**
@@ -93,7 +93,7 @@ export class MysteryEncounterPhase extends Phase {
     if (option.onPreOptionPhase) {
       globalScene.executeWithSeedOffset(async () => {
         return await option.onPreOptionPhase!().then(result => {
-          if (isNullOrUndefined(result) || result) {
+          if (result == null || result) {
             this.continueEncounter();
           }
         });
@@ -578,7 +578,7 @@ export class PostMysteryEncounterPhase extends Phase {
     if (this.onPostOptionSelect) {
       globalScene.executeWithSeedOffset(async () => {
         return await this.onPostOptionSelect!().then(result => {
-          if (isNullOrUndefined(result) || result) {
+          if (result == null || result) {
             this.continueEncounter();
           }
         });
diff --git a/src/phases/pokemon-anim-phase.ts b/src/phases/pokemon-anim-phase.ts
index 39e9c609aec..c45f201641c 100644
--- a/src/phases/pokemon-anim-phase.ts
+++ b/src/phases/pokemon-anim-phase.ts
@@ -4,7 +4,6 @@ import { PokemonAnimType } from "#enums/pokemon-anim-type";
 import { SpeciesId } from "#enums/species-id";
 import type { Pokemon } from "#field/pokemon";
 import { BattlePhase } from "#phases/battle-phase";
-import { isNullOrUndefined } from "#utils/common";
 
 export class PokemonAnimPhase extends BattlePhase {
   public readonly phaseName = "PokemonAnimPhase";
@@ -52,7 +51,7 @@ export class PokemonAnimPhase extends BattlePhase {
 
   private doSubstituteAddAnim(): void {
     const substitute = this.pokemon.getTag(SubstituteTag);
-    if (isNullOrUndefined(substitute)) {
+    if (substitute == null) {
       this.end();
       return;
     }
@@ -336,7 +335,7 @@ export class PokemonAnimPhase extends BattlePhase {
     // Note: unlike the other Commander animation, this is played through the
     // Dondozo instead of the Tatsugiri.
     const tatsugiri = this.pokemon.getAlly();
-    if (isNullOrUndefined(tatsugiri)) {
+    if (tatsugiri == null) {
       console.warn("Aborting COMMANDER_REMOVE anim: Tatsugiri is undefined");
       this.end();
       return;
diff --git a/src/phases/revival-blessing-phase.ts b/src/phases/revival-blessing-phase.ts
index fdb108d62ac..5d75f2c9b47 100644
--- a/src/phases/revival-blessing-phase.ts
+++ b/src/phases/revival-blessing-phase.ts
@@ -5,7 +5,7 @@ import type { PlayerPokemon } from "#field/pokemon";
 import { BattlePhase } from "#phases/battle-phase";
 import type { PartyOption } from "#ui/party-ui-handler";
 import { PartyUiHandler, PartyUiMode } from "#ui/party-ui-handler";
-import { isNullOrUndefined, toDmgValue } from "#utils/common";
+import { toDmgValue } from "#utils/common";
 import i18next from "i18next";
 
 /**
@@ -42,11 +42,7 @@ export class RevivalBlessingPhase extends BattlePhase {
           );
 
           const allyPokemon = this.user.getAlly();
-          if (
-            globalScene.currentBattle.double
-            && globalScene.getPlayerParty().length > 1
-            && !isNullOrUndefined(allyPokemon)
-          ) {
+          if (globalScene.currentBattle.double && globalScene.getPlayerParty().length > 1 && allyPokemon != null) {
             if (slotIndex <= 1) {
               // Revived ally pokemon
               globalScene.phaseManager.unshiftNew(
diff --git a/src/phases/select-modifier-phase.ts b/src/phases/select-modifier-phase.ts
index 3c378a95b2a..2031fc5c5f1 100644
--- a/src/phases/select-modifier-phase.ts
+++ b/src/phases/select-modifier-phase.ts
@@ -27,7 +27,7 @@ import { BattlePhase } from "#phases/battle-phase";
 import type { ModifierSelectUiHandler } from "#ui/modifier-select-ui-handler";
 import { SHOP_OPTIONS_ROW_LIMIT } from "#ui/modifier-select-ui-handler";
 import { PartyOption, PartyUiHandler, PartyUiMode } from "#ui/party-ui-handler";
-import { isNullOrUndefined, NumberHolder } from "#utils/common";
+import { NumberHolder } from "#utils/common";
 import i18next from "i18next";
 
 export type ModifierSelectCallback = (rowCursor: number, cursor: number) => boolean;
@@ -429,7 +429,7 @@ export class SelectModifierPhase extends BattlePhase {
     }
 
     let multiplier = 1;
-    if (!isNullOrUndefined(this.customModifierSettings?.rerollMultiplier)) {
+    if (this.customModifierSettings?.rerollMultiplier != null) {
       if (this.customModifierSettings.rerollMultiplier < 0) {
         // Completely overrides reroll cost to -1 and early exits
         return -1;
diff --git a/src/phases/select-starter-phase.ts b/src/phases/select-starter-phase.ts
index a08394e3acb..27e2150fd06 100644
--- a/src/phases/select-starter-phase.ts
+++ b/src/phases/select-starter-phase.ts
@@ -10,7 +10,6 @@ import { overrideHeldItems, overrideModifiers } from "#modifiers/modifier";
 import { SaveSlotUiMode } from "#ui/save-slot-select-ui-handler";
 import type { Starter } from "#ui/starter-select-ui-handler";
 import { applyChallenges } from "#utils/challenge-utils";
-import { isNullOrUndefined } from "#utils/common";
 import { getPokemonSpecies } from "#utils/pokemon-utils";
 import SoundFade from "phaser3-rex-plugins/plugins/soundfade";
 
@@ -51,7 +50,7 @@ export class SelectStarterPhase extends Phase {
       let starterFormIndex = Math.min(starterProps.formIndex, Math.max(starter.species.forms.length - 1, 0));
       if (
         starter.species.speciesId in Overrides.STARTER_FORM_OVERRIDES
-        && !isNullOrUndefined(Overrides.STARTER_FORM_OVERRIDES[starter.species.speciesId])
+        && Overrides.STARTER_FORM_OVERRIDES[starter.species.speciesId] != null
         && starter.species.forms[Overrides.STARTER_FORM_OVERRIDES[starter.species.speciesId]!]
       ) {
         starterFormIndex = Overrides.STARTER_FORM_OVERRIDES[starter.species.speciesId]!;
@@ -89,7 +88,7 @@ export class SelectStarterPhase extends Phase {
         starterPokemon.nickname = starter.nickname;
       }
 
-      if (!isNullOrUndefined(starter.teraType)) {
+      if (starter.teraType != null) {
         starterPokemon.teraType = starter.teraType;
       } else {
         starterPokemon.teraType = starterPokemon.species.type1;
diff --git a/src/phases/stat-stage-change-phase.ts b/src/phases/stat-stage-change-phase.ts
index 6c15342ddeb..2731c037d5f 100644
--- a/src/phases/stat-stage-change-phase.ts
+++ b/src/phases/stat-stage-change-phase.ts
@@ -13,7 +13,7 @@ import type { Pokemon } from "#field/pokemon";
 import { ResetNegativeStatStageModifier } from "#modifiers/modifier";
 import { PokemonPhase } from "#phases/pokemon-phase";
 import type { ConditionalUserFieldProtectStatAbAttrParams, PreStatStageChangeAbAttrParams } from "#types/ability-types";
-import { BooleanHolder, isNullOrUndefined, NumberHolder } from "#utils/common";
+import { BooleanHolder, NumberHolder } from "#utils/common";
 import i18next from "i18next";
 
 export type StatStageChangeCallback = (
@@ -153,7 +153,7 @@ export class StatStageChangePhase extends PokemonPhase {
         applyAbAttrs("ConditionalUserFieldProtectStatAbAttr", abAttrParams);
         // TODO: Consider skipping this call if `cancelled` is false.
         const ally = pokemon.getAlly();
-        if (!isNullOrUndefined(ally)) {
+        if (ally != null) {
           applyAbAttrs("ConditionalUserFieldProtectStatAbAttr", { ...abAttrParams, pokemon: ally });
         }
 
diff --git a/src/phases/title-phase.ts b/src/phases/title-phase.ts
index d422766bf09..414be4c820c 100644
--- a/src/phases/title-phase.ts
+++ b/src/phases/title-phase.ts
@@ -18,7 +18,7 @@ import { vouchers } from "#system/voucher";
 import type { SessionSaveData } from "#types/save-data";
 import type { OptionSelectConfig, OptionSelectItem } from "#ui/abstract-option-select-ui-handler";
 import { SaveSlotUiMode } from "#ui/save-slot-select-ui-handler";
-import { isLocal, isLocalServerConnected, isNullOrUndefined } from "#utils/common";
+import { isLocal, isLocalServerConnected } from "#utils/common";
 import i18next from "i18next";
 
 export class TitlePhase extends Phase {
@@ -289,7 +289,7 @@ export class TitlePhase extends Phase {
       } else {
         // Grab first 10 chars of ISO date format (YYYY-MM-DD) and convert to base64
         let seed: string = btoa(new Date().toISOString().substring(0, 10));
-        if (!isNullOrUndefined(Overrides.DAILY_RUN_SEED_OVERRIDE)) {
+        if (Overrides.DAILY_RUN_SEED_OVERRIDE != null) {
           seed = Overrides.DAILY_RUN_SEED_OVERRIDE;
         }
         generateDaily(seed);
diff --git a/src/sprites/variant.ts b/src/sprites/variant.ts
index 28d7ed13839..9d7a20bc058 100644
--- a/src/sprites/variant.ts
+++ b/src/sprites/variant.ts
@@ -2,7 +2,6 @@ import { globalScene } from "#app/global-scene";
 import { VariantTier } from "#enums/variant-tier";
 import type { Pokemon } from "#field/pokemon";
 import { hasExpSprite } from "#sprites/sprite-utils";
-import { isNullOrUndefined } from "#utils/common";
 
 export type Variant = 0 | 1 | 2;
 
@@ -138,7 +137,7 @@ export async function populateVariantColorCache(
       return fallbackVariantColor(cacheKey, spritePath, useExpSprite, battleSpritePath, error);
     })
     .then(c => {
-      if (!isNullOrUndefined(c)) {
+      if (c != null) {
         variantColorCache[cacheKey] = c;
       }
     });
diff --git a/src/system/ribbons/ribbon-methods.ts b/src/system/ribbons/ribbon-methods.ts
index 138c0be7b51..f1aeb9fefc2 100644
--- a/src/system/ribbons/ribbon-methods.ts
+++ b/src/system/ribbons/ribbon-methods.ts
@@ -2,7 +2,6 @@ import { globalScene } from "#app/global-scene";
 import { pokemonPrevolutions } from "#balance/pokemon-evolutions";
 import type { SpeciesId } from "#enums/species-id";
 import type { RibbonFlag } from "#system/ribbons/ribbon-data";
-import { isNullOrUndefined } from "#utils/common";
 
 /**
  * Award one or more ribbons to a species and its pre-evolutions
@@ -14,7 +13,7 @@ export function awardRibbonsToSpeciesLine(id: SpeciesId, ribbons: RibbonFlag): v
   const dexData = globalScene.gameData.dexData;
   dexData[id].ribbons.award(ribbons);
   // Mark all pre-evolutions of the Pokémon with the same ribbon flags.
-  for (let prevoId = pokemonPrevolutions[id]; !isNullOrUndefined(prevoId); prevoId = pokemonPrevolutions[prevoId]) {
+  for (let prevoId = pokemonPrevolutions[id]; prevoId != null; prevoId = pokemonPrevolutions[prevoId]) {
     dexData[prevoId].ribbons.award(ribbons);
   }
 }
diff --git a/src/system/version-migration/versions/v1_0_4.ts b/src/system/version-migration/versions/v1_0_4.ts
index 8229b9320d5..5342396d576 100644
--- a/src/system/version-migration/versions/v1_0_4.ts
+++ b/src/system/version-migration/versions/v1_0_4.ts
@@ -8,7 +8,6 @@ import type { SessionSaveData, SystemSaveData } from "#types/save-data";
 import type { SessionSaveMigrator } from "#types/session-save-migrator";
 import type { SettingsSaveMigrator } from "#types/settings-save-migrator";
 import type { SystemSaveMigrator } from "#types/system-save-migrator";
-import { isNullOrUndefined } from "#utils/common";
 
 /**
  * Migrate ability starter data if empty for caught species.
@@ -82,7 +81,7 @@ const fixLegendaryStats: SystemSaveMigrator = {
 const fixStarterData: SystemSaveMigrator = {
   version: "1.0.4",
   migrate: (data: SystemSaveData): void => {
-    if (!isNullOrUndefined(data.starterData)) {
+    if (data.starterData != null) {
       for (const starterId of defaultStarterSpecies) {
         if (data.starterData[starterId]?.abilityAttr) {
           data.starterData[starterId].abilityAttr |= AbilityAttr.ABILITY_1;
@@ -198,7 +197,7 @@ const migrateCustomPokemonData: SessionSaveMigrator = {
         pokemon["fusionMysteryEncounterPokemonData"] = null;
       }
       pokemon.customPokemonData = pokemon.customPokemonData ?? new CustomPokemonData();
-      if (!isNullOrUndefined(pokemon["natureOverride"]) && pokemon["natureOverride"] >= 0) {
+      if (pokemon["natureOverride"] != null && pokemon["natureOverride"] >= 0) {
         pokemon.customPokemonData.nature = pokemon["natureOverride"];
         pokemon["natureOverride"] = -1;
       }
diff --git a/src/system/version-migration/versions/v1_7_0.ts b/src/system/version-migration/versions/v1_7_0.ts
index 6d365cf31ac..e526ccd2c2b 100644
--- a/src/system/version-migration/versions/v1_7_0.ts
+++ b/src/system/version-migration/versions/v1_7_0.ts
@@ -3,7 +3,6 @@ import { DexAttr } from "#enums/dex-attr";
 import type { SessionSaveData, SystemSaveData } from "#types/save-data";
 import type { SessionSaveMigrator } from "#types/session-save-migrator";
 import type { SystemSaveMigrator } from "#types/system-save-migrator";
-import { isNullOrUndefined } from "#utils/common";
 import { getPokemonSpecies, getPokemonSpeciesForm } from "#utils/pokemon-utils";
 
 /**
@@ -68,13 +67,13 @@ const migrateTera: SessionSaveMigrator = {
     }
 
     data.party.forEach(p => {
-      if (isNullOrUndefined(p.teraType)) {
+      if (p.teraType == null) {
         p.teraType = getPokemonSpeciesForm(p.species, p.formIndex).type1;
       }
     });
 
     data.enemyParty.forEach(p => {
-      if (isNullOrUndefined(p.teraType)) {
+      if (p.teraType == null) {
         p.teraType = getPokemonSpeciesForm(p.species, p.formIndex).type1;
       }
     });
diff --git a/src/timed-event-manager.ts b/src/timed-event-manager.ts
index ed92a1c9ca5..7db89b2a0ef 100644
--- a/src/timed-event-manager.ts
+++ b/src/timed-event-manager.ts
@@ -9,7 +9,6 @@ import { TextStyle } from "#enums/text-style";
 import { WeatherType } from "#enums/weather-type";
 import { addTextObject } from "#ui/text";
 import type { nil } from "#utils/common";
-import { isNullOrUndefined } from "#utils/common";
 import i18next from "i18next";
 
 export enum EventType {
@@ -428,7 +427,7 @@ export class TimedEventManager {
 
   getEventBannerLangs(): string[] {
     const ret: string[] = [];
-    ret.push(...timedEvents.find(te => this.isActive(te) && !isNullOrUndefined(te.availableLangs))?.availableLangs!);
+    ret.push(...timedEvents.find(te => this.isActive(te) && te.availableLangs != null)?.availableLangs!);
     return ret;
   }
 
@@ -437,7 +436,7 @@ export class TimedEventManager {
     timedEvents
       .filter(te => this.isActive(te))
       .map(te => {
-        if (!isNullOrUndefined(te.eventEncounters)) {
+        if (te.eventEncounters != null) {
           ret.push(...te.eventEncounters);
         }
       });
@@ -452,7 +451,7 @@ export class TimedEventManager {
     let multiplier = CLASSIC_CANDY_FRIENDSHIP_MULTIPLIER;
     const classicFriendshipEvents = timedEvents.filter(te => this.isActive(te));
     for (const fe of classicFriendshipEvents) {
-      if (!isNullOrUndefined(fe.classicFriendshipMultiplier) && fe.classicFriendshipMultiplier > multiplier) {
+      if (fe.classicFriendshipMultiplier != null && fe.classicFriendshipMultiplier > multiplier) {
         multiplier = fe.classicFriendshipMultiplier;
       }
     }
@@ -476,7 +475,7 @@ export class TimedEventManager {
     timedEvents
       .filter(te => this.isActive(te))
       .map(te => {
-        if (!isNullOrUndefined(te.delibirdyBuff)) {
+        if (te.delibirdyBuff != null) {
           ret.push(...te.delibirdyBuff);
         }
       });
@@ -492,7 +491,7 @@ export class TimedEventManager {
     timedEvents
       .filter(te => this.isActive(te))
       .map(te => {
-        if (!isNullOrUndefined(te.weather)) {
+        if (te.weather != null) {
           ret.push(...te.weather);
         }
       });
@@ -504,7 +503,7 @@ export class TimedEventManager {
     timedEvents
       .filter(te => this.isActive(te))
       .map(te => {
-        if (!isNullOrUndefined(te.mysteryEncounterTierChanges)) {
+        if (te.mysteryEncounterTierChanges != null) {
           ret.push(...te.mysteryEncounterTierChanges);
         }
       });
@@ -514,7 +513,7 @@ export class TimedEventManager {
   getEventMysteryEncountersDisabled(): MysteryEncounterType[] {
     const ret: MysteryEncounterType[] = [];
     timedEvents
-      .filter(te => this.isActive(te) && !isNullOrUndefined(te.mysteryEncounterTierChanges))
+      .filter(te => this.isActive(te) && te.mysteryEncounterTierChanges != null)
       .map(te => {
         te.mysteryEncounterTierChanges?.map(metc => {
           if (metc.disable) {
@@ -531,7 +530,7 @@ export class TimedEventManager {
   ): MysteryEncounterTier {
     let ret = normal;
     timedEvents
-      .filter(te => this.isActive(te) && !isNullOrUndefined(te.mysteryEncounterTierChanges))
+      .filter(te => this.isActive(te) && te.mysteryEncounterTierChanges != null)
       .map(te => {
         te.mysteryEncounterTierChanges?.map(metc => {
           if (metc.mysteryEncounter === encounterType) {
@@ -544,7 +543,7 @@ export class TimedEventManager {
 
   getEventLuckBoost(): number {
     let ret = 0;
-    const luckEvents = timedEvents.filter(te => this.isActive(te) && !isNullOrUndefined(te.luckBoost));
+    const luckEvents = timedEvents.filter(te => this.isActive(te) && te.luckBoost != null);
     for (const le of luckEvents) {
       ret += le.luckBoost!;
     }
@@ -556,7 +555,7 @@ export class TimedEventManager {
     timedEvents
       .filter(te => this.isActive(te))
       .map(te => {
-        if (!isNullOrUndefined(te.luckBoostedSpecies)) {
+        if (te.luckBoostedSpecies != null) {
           ret.push(...te.luckBoostedSpecies.filter(s => !ret.includes(s)));
         }
       });
@@ -576,7 +575,7 @@ export class TimedEventManager {
   getFixedBattleEventRewards(wave: number): string[] {
     const ret: string[] = [];
     timedEvents
-      .filter(te => this.isActive(te) && !isNullOrUndefined(te.classicWaveRewards))
+      .filter(te => this.isActive(te) && te.classicWaveRewards != null)
       .map(te => {
         ret.push(...te.classicWaveRewards!.filter(cwr => cwr.wave === wave).map(cwr => cwr.type));
       });
@@ -586,7 +585,7 @@ export class TimedEventManager {
   // Gets the extra shiny chance for trainers due to event (odds/65536)
   getClassicTrainerShinyChance(): number {
     let ret = 0;
-    const tsEvents = timedEvents.filter(te => this.isActive(te) && !isNullOrUndefined(te.trainerShinyChance));
+    const tsEvents = timedEvents.filter(te => this.isActive(te) && te.trainerShinyChance != null);
     tsEvents.map(t => (ret += t.trainerShinyChance!));
     return ret;
   }
@@ -594,7 +593,7 @@ export class TimedEventManager {
   getEventBgmReplacement(bgm: string): string {
     let ret = bgm;
     timedEvents.map(te => {
-      if (this.isActive(te) && !isNullOrUndefined(te.music)) {
+      if (this.isActive(te) && te.music != null) {
         te.music.map(mr => {
           if (mr[0] === bgm) {
             console.log(`it is ${te.name} so instead of ${mr[0]} we play ${mr[1]}`);
diff --git a/src/ui/containers/pokedex-mon-container.ts b/src/ui/containers/pokedex-mon-container.ts
index 15ef6c9b5c8..158f42dd42a 100644
--- a/src/ui/containers/pokedex-mon-container.ts
+++ b/src/ui/containers/pokedex-mon-container.ts
@@ -3,7 +3,6 @@ import type { PokemonSpecies } from "#data/pokemon-species";
 import { TextStyle } from "#enums/text-style";
 import type { Variant } from "#sprites/variant";
 import { addTextObject } from "#ui/text";
-import { isNullOrUndefined } from "#utils/common";
 
 interface SpeciesDetails {
   shiny?: boolean;
@@ -177,16 +176,16 @@ export class PokedexMonContainer extends Phaser.GameObjects.Container {
     const defaultDexAttr = globalScene.gameData.getSpeciesDefaultDexAttr(species, false, true);
     const defaultProps = globalScene.gameData.getSpeciesDexAttrProps(species, defaultDexAttr);
 
-    if (!isNullOrUndefined(formIndex)) {
+    if (formIndex != null) {
       defaultProps.formIndex = formIndex;
     }
-    if (!isNullOrUndefined(shiny)) {
+    if (shiny != null) {
       defaultProps.shiny = shiny;
     }
-    if (!isNullOrUndefined(variant)) {
+    if (variant != null) {
       defaultProps.variant = variant;
     }
-    if (!isNullOrUndefined(female)) {
+    if (female != null) {
       defaultProps.female = female;
     }
 
diff --git a/src/ui/handlers/mystery-encounter-ui-handler.ts b/src/ui/handlers/mystery-encounter-ui-handler.ts
index bbbd3cb4af8..e4c9dfbfee3 100644
--- a/src/ui/handlers/mystery-encounter-ui-handler.ts
+++ b/src/ui/handlers/mystery-encounter-ui-handler.ts
@@ -13,7 +13,7 @@ import { PartyUiMode } from "#ui/party-ui-handler";
 import { addBBCodeTextObject, getBBCodeFrag } from "#ui/text";
 import { UiHandler } from "#ui/ui-handler";
 import { addWindow, WindowVariant } from "#ui/ui-theme";
-import { fixedInt, isNullOrUndefined } from "#utils/common";
+import { fixedInt } from "#utils/common";
 import i18next from "i18next";
 import type BBCodeText from "phaser3-rex-plugins/plugins/bbcodetext";
 
@@ -95,12 +95,10 @@ export class MysteryEncounterUiHandler extends UiHandler {
     super.show(args);
 
     this.overrideSettings = (args[0] as OptionSelectSettings) ?? {};
-    const showDescriptionContainer = isNullOrUndefined(this.overrideSettings?.hideDescription)
-      ? true
-      : !this.overrideSettings.hideDescription;
-    const slideInDescription = isNullOrUndefined(this.overrideSettings?.slideInDescription)
-      ? true
-      : this.overrideSettings.slideInDescription;
+    const showDescriptionContainer =
+      this.overrideSettings?.hideDescription == null ? true : !this.overrideSettings.hideDescription;
+    const slideInDescription =
+      this.overrideSettings?.slideInDescription == null ? true : this.overrideSettings.slideInDescription;
     const startingCursorIndex = this.overrideSettings?.startingCursorIndex ?? 0;
 
     this.cursorContainer.setVisible(true);
@@ -567,7 +565,7 @@ export class MysteryEncounterUiHandler extends UiHandler {
     }
     this.tooltipContainer.setVisible(true);
 
-    if (isNullOrUndefined(cursor) || cursor > this.optionsContainer.length - 2) {
+    if (cursor == null || cursor > this.optionsContainer.length - 2) {
       // Ignore hovers on view party button
       // Hide dex progress if visible
       this.showHideDexProgress(false);
diff --git a/src/ui/handlers/pokedex-page-ui-handler.ts b/src/ui/handlers/pokedex-page-ui-handler.ts
index 253309bf94a..31e2998b850 100644
--- a/src/ui/handlers/pokedex-page-ui-handler.ts
+++ b/src/ui/handlers/pokedex-page-ui-handler.ts
@@ -54,7 +54,7 @@ import { PokedexInfoOverlay } from "#ui/pokedex-info-overlay";
 import { StatsContainer } from "#ui/stats-container";
 import { addBBCodeTextObject, addTextObject, getTextColor, getTextStyleOptions } from "#ui/text";
 import { addWindow } from "#ui/ui-theme";
-import { BooleanHolder, getLocalizedSpriteKey, isNullOrUndefined, padInt, rgbHexToRgba } from "#utils/common";
+import { BooleanHolder, getLocalizedSpriteKey, padInt, rgbHexToRgba } from "#utils/common";
 import { getEnumValues } from "#utils/enums";
 import { getPokemonSpecies, getPokemonSpeciesForm } from "#utils/pokemon-utils";
 import { toCamelCase, toTitleCase } from "#utils/strings";
@@ -2424,11 +2424,7 @@ export class PokedexPageUiHandler extends MessageUiHandler {
     // We will only update the sprite if there is a change to form, shiny/variant
     // or gender for species with gender sprite differences
     const shouldUpdateSprite =
-      (species?.genderDiffs && !isNullOrUndefined(female))
-      || !isNullOrUndefined(formIndex)
-      || !isNullOrUndefined(shiny)
-      || !isNullOrUndefined(variant)
-      || forceUpdate;
+      (species?.genderDiffs && female != null) || formIndex != null || shiny != null || variant != null || forceUpdate;
 
     if (this.activeTooltip === "CANDY") {
       if (this.species && this.pokemonCandyContainer.visible) {
diff --git a/src/ui/handlers/pokedex-scan-ui-handler.ts b/src/ui/handlers/pokedex-scan-ui-handler.ts
index 1f5195588f3..18afd0598c2 100644
--- a/src/ui/handlers/pokedex-scan-ui-handler.ts
+++ b/src/ui/handlers/pokedex-scan-ui-handler.ts
@@ -6,7 +6,6 @@ import { FilterTextRow } from "#ui/filter-text";
 import type { InputFieldConfig } from "#ui/form-modal-ui-handler";
 import { FormModalUiHandler } from "#ui/form-modal-ui-handler";
 import type { ModalConfig } from "#ui/modal-ui-handler";
-import { isNullOrUndefined } from "#utils/common";
 import i18next from "i18next";
 
 export class PokedexScanUiHandler extends FormModalUiHandler {
@@ -132,7 +131,7 @@ export class PokedexScanUiHandler extends FormModalUiHandler {
           return {
             label: value,
             handler: () => {
-              if (!isNullOrUndefined(evt.data) || evt.inputType?.toLowerCase() === "deletecontentbackward") {
+              if (evt.data != null || evt.inputType?.toLowerCase() === "deletecontentbackward") {
                 inputObject.setText(value);
               }
               ui.revertMode();
diff --git a/src/ui/handlers/save-slot-select-ui-handler.ts b/src/ui/handlers/save-slot-select-ui-handler.ts
index a71be5dd070..194971a005f 100644
--- a/src/ui/handlers/save-slot-select-ui-handler.ts
+++ b/src/ui/handlers/save-slot-select-ui-handler.ts
@@ -13,7 +13,7 @@ import { MessageUiHandler } from "#ui/message-ui-handler";
 import { RunDisplayMode } from "#ui/run-info-ui-handler";
 import { addTextObject } from "#ui/text";
 import { addWindow } from "#ui/ui-theme";
-import { fixedInt, formatLargeNumber, getPlayTimeString, isNullOrUndefined } from "#utils/common";
+import { fixedInt, formatLargeNumber, getPlayTimeString } from "#utils/common";
 import i18next from "i18next";
 
 const SESSION_SLOTS_COUNT = 5;
@@ -405,7 +405,7 @@ export class SaveSlotSelectUiHandler extends MessageUiHandler {
       }
       this.setArrowVisibility(hasData);
     }
-    if (!isNullOrUndefined(prevSlotIndex)) {
+    if (prevSlotIndex != null) {
       this.revertSessionSlot(prevSlotIndex);
     }
 
diff --git a/src/ui/handlers/starter-select-ui-handler.ts b/src/ui/handlers/starter-select-ui-handler.ts
index 18be7e130e3..53e566864b1 100644
--- a/src/ui/handlers/starter-select-ui-handler.ts
+++ b/src/ui/handlers/starter-select-ui-handler.ts
@@ -66,7 +66,6 @@ import {
   BooleanHolder,
   fixedInt,
   getLocalizedSpriteKey,
-  isNullOrUndefined,
   NumberHolder,
   padInt,
   randIntRange,
@@ -2548,7 +2547,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
           case Button.CYCLE_TERA:
             if (this.canCycleTera) {
               const speciesForm = getPokemonSpeciesForm(this.lastSpecies.speciesId, starterAttributes.form ?? 0);
-              if (speciesForm.type1 === this.teraCursor && !isNullOrUndefined(speciesForm.type2)) {
+              if (speciesForm.type1 === this.teraCursor && speciesForm.type2 != null) {
                 starterAttributes.tera = speciesForm.type2;
                 originalStarterAttributes.tera = starterAttributes.tera;
                 this.setSpeciesDetails(this.lastSpecies, {
@@ -2790,7 +2789,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
    */
   switchMoveHandler(targetIndex: number, newMove: MoveId, previousMove: MoveId) {
     const starterMoveset = this.starterMoveset;
-    if (isNullOrUndefined(starterMoveset)) {
+    if (starterMoveset == null) {
       console.warn("Trying to update a non-existing moveset");
       return;
     }
@@ -3687,7 +3686,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
           );
         }
 
-        if (!isNullOrUndefined(props.formIndex)) {
+        if (props.formIndex != null) {
           // If switching forms while the pokemon is in the team, update its moveset
           this.updateSelectedStarterMoveset(species.speciesId);
         }
@@ -3809,10 +3808,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
     // We will only update the sprite if there is a change to form, shiny/variant
     // or gender for species with gender sprite differences
     const shouldUpdateSprite =
-      (species?.genderDiffs && !isNullOrUndefined(female))
-      || !isNullOrUndefined(formIndex)
-      || !isNullOrUndefined(shiny)
-      || !isNullOrUndefined(variant);
+      (species?.genderDiffs && female != null) || formIndex != null || shiny != null || variant != null;
 
     const isFreshStartChallenge = globalScene.gameMode.hasChallenge(Challenges.FRESH_START);
 
@@ -3850,7 +3846,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
       ); // TODO: is this bang correct?
       this.abilityCursor = abilityIndex !== undefined ? abilityIndex : (abilityIndex = oldAbilityIndex);
       this.natureCursor = natureIndex !== undefined ? natureIndex : (natureIndex = oldNatureIndex);
-      this.teraCursor = !isNullOrUndefined(teraType) ? teraType : (teraType = oldTeraType);
+      this.teraCursor = teraType != null ? teraType : (teraType = oldTeraType);
       const [isInParty, partyIndex]: [boolean, number] = this.isInParty(species); // we use this to firstly check if the pokemon is in the party, and if so, to get the party index in order to update the icon image
       if (isInParty) {
         this.updatePartyIcon(species, partyIndex);
@@ -3991,7 +3987,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
         this.canCycleTera =
           !this.statsMode
           && this.allowTera
-          && !isNullOrUndefined(getPokemonSpeciesForm(species.speciesId, formIndex ?? 0).type2)
+          && getPokemonSpeciesForm(species.speciesId, formIndex ?? 0).type2 != null
           && !globalScene.gameMode.hasChallenge(Challenges.FRESH_START);
       }
 
@@ -4592,7 +4588,7 @@ export class StarterSelectUiHandler extends MessageUiHandler {
       this.canCycleTera =
         !this.statsMode
         && this.allowTera
-        && !isNullOrUndefined(getPokemonSpeciesForm(this.lastSpecies.speciesId, formIndex ?? 0).type2)
+        && getPokemonSpeciesForm(this.lastSpecies.speciesId, formIndex ?? 0).type2 != null
         && !globalScene.gameMode.hasChallenge(Challenges.FRESH_START);
       this.updateInstructions();
     }
diff --git a/src/ui/handlers/summary-ui-handler.ts b/src/ui/handlers/summary-ui-handler.ts
index 1c647573cbf..c9c8229ebfd 100644
--- a/src/ui/handlers/summary-ui-handler.ts
+++ b/src/ui/handlers/summary-ui-handler.ts
@@ -27,15 +27,7 @@ import { getVariantTint } from "#sprites/variant";
 import { achvs } from "#system/achv";
 import { addBBCodeTextObject, addTextObject, getBBCodeFrag, getTextColor } from "#ui/text";
 import { UiHandler } from "#ui/ui-handler";
-import {
-  fixedInt,
-  formatStat,
-  getLocalizedSpriteKey,
-  getShinyDescriptor,
-  isNullOrUndefined,
-  padInt,
-  rgbHexToRgba,
-} from "#utils/common";
+import { fixedInt, formatStat, getLocalizedSpriteKey, getShinyDescriptor, padInt, rgbHexToRgba } from "#utils/common";
 import { getEnumValues } from "#utils/enums";
 import { toCamelCase, toTitleCase } from "#utils/strings";
 import { argbFromRgba } from "@material/material-color-utilities";
@@ -895,10 +887,7 @@ export class SummaryUiHandler extends UiHandler {
           profileContainer.add(luckText);
         }
 
-        if (
-          globalScene.gameData.achvUnlocks.hasOwnProperty(achvs.TERASTALLIZE.id)
-          && !isNullOrUndefined(this.pokemon)
-        ) {
+        if (globalScene.gameData.achvUnlocks.hasOwnProperty(achvs.TERASTALLIZE.id) && this.pokemon != null) {
           const teraIcon = globalScene.add.sprite(123, 26, "button_tera");
           teraIcon.setName("terastallize-icon");
           teraIcon.setFrame(PokemonType[this.pokemon.getTeraType()].toLowerCase());
diff --git a/src/ui/handlers/target-select-ui-handler.ts b/src/ui/handlers/target-select-ui-handler.ts
index 4e3096b96f4..bd81278c661 100644
--- a/src/ui/handlers/target-select-ui-handler.ts
+++ b/src/ui/handlers/target-select-ui-handler.ts
@@ -8,7 +8,7 @@ import type { Pokemon } from "#field/pokemon";
 import type { ModifierBar } from "#modifiers/modifier";
 import { getMoveTargets } from "#moves/move-utils";
 import { UiHandler } from "#ui/ui-handler";
-import { fixedInt, isNullOrUndefined } from "#utils/common";
+import { fixedInt } from "#utils/common";
 
 export type TargetSelectCallback = (targets: BattlerIndex[]) => void;
 
@@ -71,7 +71,7 @@ export class TargetSelectUiHandler extends UiHandler {
    */
   resetCursor(cursorN: number, user: Pokemon): void {
     if (
-      !isNullOrUndefined(cursorN)
+      cursorN != null
       && ([BattlerIndex.PLAYER, BattlerIndex.PLAYER_2].includes(cursorN) || user.tempSummonData.waveTurnCount === 1)
     ) {
       // Reset cursor on the first turn of a fight or if an ally was targeted last turn
@@ -90,13 +90,10 @@ export class TargetSelectUiHandler extends UiHandler {
       this.targetSelectCallback(button === Button.ACTION ? targetIndexes : []);
       success = true;
       if (this.fieldIndex === BattlerIndex.PLAYER) {
-        if (isNullOrUndefined(this.cursor0) || this.cursor0 !== this.cursor) {
+        if (this.cursor0 == null || this.cursor0 !== this.cursor) {
           this.cursor0 = this.cursor;
         }
-      } else if (
-        this.fieldIndex === BattlerIndex.PLAYER_2
-        && (isNullOrUndefined(this.cursor1) || this.cursor1 !== this.cursor)
-      ) {
+      } else if (this.fieldIndex === BattlerIndex.PLAYER_2 && (this.cursor1 == null || this.cursor1 !== this.cursor)) {
         this.cursor1 = this.cursor;
       }
     } else if (this.isMultipleTargets) {
diff --git a/src/ui/handlers/test-dialogue-ui-handler.ts b/src/ui/handlers/test-dialogue-ui-handler.ts
index bd5c1a1dc37..b33e6726547 100644
--- a/src/ui/handlers/test-dialogue-ui-handler.ts
+++ b/src/ui/handlers/test-dialogue-ui-handler.ts
@@ -4,7 +4,6 @@ import type { OptionSelectItem } from "#ui/abstract-option-select-ui-handler";
 import type { InputFieldConfig } from "#ui/form-modal-ui-handler";
 import { FormModalUiHandler } from "#ui/form-modal-ui-handler";
 import type { ModalConfig } from "#ui/modal-ui-handler";
-import { isNullOrUndefined } from "#utils/common";
 import i18next from "i18next";
 
 export class TestDialogueUiHandler extends FormModalUiHandler {
@@ -18,7 +17,7 @@ export class TestDialogueUiHandler extends FormModalUiHandler {
         .map((t, i) => {
           const value = Object.values(object)[i];
 
-          if (typeof value === "object" && !isNullOrUndefined(value)) {
+          if (typeof value === "object" && value != null) {
             // we check for not null or undefined here because if the language json file has a null key, the typeof will still be an object, but that object will be null, causing issues
             // If the value is an object, execute the same process
             // si el valor es un objeto ejecuta el mismo proceso
@@ -27,7 +26,7 @@ export class TestDialogueUiHandler extends FormModalUiHandler {
               t => t.length > 0,
             );
           }
-          if (typeof value === "string" || isNullOrUndefined(value)) {
+          if (typeof value === "string" || value == null) {
             // we check for null or undefined here as per above - the typeof is still an object but the value is null so we need to exit out of this and pass the null key
 
             // Return in the format expected by i18next
@@ -109,7 +108,7 @@ export class TestDialogueUiHandler extends FormModalUiHandler {
             handler: () => {
               // this is here to make sure that if you try to backspace then enter, the last known evt.data (backspace) is picked up
               // this is because evt.data is null for backspace, so without this, the autocomplete windows just closes
-              if (!isNullOrUndefined(evt.data) || evt.inputType?.toLowerCase() === "deletecontentbackward") {
+              if (evt.data != null || evt.inputType?.toLowerCase() === "deletecontentbackward") {
                 const separatedArray = inputObject.text.split(" ");
                 separatedArray[separatedArray.length - 1] = value;
                 inputObject.setText(separatedArray.join(" "));
diff --git a/src/utils/common.ts b/src/utils/common.ts
index 2734b075a53..f0166b1e74c 100644
--- a/src/utils/common.ts
+++ b/src/utils/common.ts
@@ -458,15 +458,6 @@ export function truncateString(str: string, maxLength = 10) {
   return str;
 }
 
-/**
- * Report whether a given value is nullish (`null`/`undefined`).
- * @param val - The value whose nullishness is being checked
- * @returns `true` if `val` is either `null` or `undefined`
- */
-export function isNullOrUndefined(val: any): val is null | undefined {
-  return val === null || val === undefined;
-}
-
 /**
  * This function is used in the context of a Pokémon battle game to calculate the actual integer damage value from a float result.
  * Many damage calculation formulas involve various parameters and result in float values.
diff --git a/test/abilities/healer.test.ts b/test/abilities/healer.test.ts
index 43280ff8271..fb28cd891ac 100644
--- a/test/abilities/healer.test.ts
+++ b/test/abilities/healer.test.ts
@@ -6,7 +6,6 @@ import { SpeciesId } from "#enums/species-id";
 import { StatusEffect } from "#enums/status-effect";
 import type { Pokemon } from "#field/pokemon";
 import { GameManager } from "#test/test-utils/game-manager";
-import { isNullOrUndefined } from "#utils/common";
 import Phaser from "phaser";
 import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
 
@@ -37,7 +36,7 @@ describe("Abilities - Healer", () => {
 
     // Mock healer to have a 100% chance of healing its ally
     vi.spyOn(allAbilities[AbilityId.HEALER].getAttrs("PostTurnResetStatusAbAttr")[0], "getCondition").mockReturnValue(
-      (pokemon: Pokemon) => !isNullOrUndefined(pokemon.getAlly()),
+      (pokemon: Pokemon) => pokemon.getAlly() != null,
     );
   });
 
diff --git a/test/eggs/egg.test.ts b/test/eggs/egg.test.ts
index 8b47e68f402..001adf83b37 100644
--- a/test/eggs/egg.test.ts
+++ b/test/eggs/egg.test.ts
@@ -202,7 +202,7 @@ describe("Egg Generation Tests", () => {
     const scene = game.scene;
 
     const eggMoveIndex = new Egg({ scene }).eggMoveIndex;
-    const result = !Utils.isNullOrUndefined(eggMoveIndex) && eggMoveIndex >= 0 && eggMoveIndex <= 3;
+    const result = eggMoveIndex != null && eggMoveIndex >= 0 && eggMoveIndex <= 3;
 
     expect(result).toBe(true);
   });
diff --git a/test/mystery-encounter/encounter-test-utils.ts b/test/mystery-encounter/encounter-test-utils.ts
index 7b2dbfc9aeb..4aad0e000d9 100644
--- a/test/mystery-encounter/encounter-test-utils.ts
+++ b/test/mystery-encounter/encounter-test-utils.ts
@@ -17,7 +17,6 @@ import type { MessageUiHandler } from "#ui/message-ui-handler";
 import type { MysteryEncounterUiHandler } from "#ui/mystery-encounter-ui-handler";
 import type { OptionSelectUiHandler } from "#ui/option-select-ui-handler";
 import type { PartyUiHandler } from "#ui/party-ui-handler";
-import { isNullOrUndefined } from "#utils/common";
 import { expect, vi } from "vitest";
 
 /**
@@ -147,7 +146,7 @@ export async function runSelectMysteryEncounterOption(
       break;
   }
 
-  if (!isNullOrUndefined(secondaryOptionSelect?.pokemonNo)) {
+  if (secondaryOptionSelect?.pokemonNo != null) {
     await handleSecondaryOptionSelect(game, secondaryOptionSelect.pokemonNo, secondaryOptionSelect.optionNo);
   } else {
     uiHandler.processInput(Button.ACTION);
@@ -174,7 +173,7 @@ async function handleSecondaryOptionSelect(game: GameManager, pokemonNo: number,
   partyUiHandler.processInput(Button.ACTION);
 
   // If there is a second choice to make after selecting a Pokemon
-  if (!isNullOrUndefined(optionNo)) {
+  if (optionNo != null) {
     // Wait for Summary menu to close and second options to spawn
     const secondOptionUiHandler = game.scene.ui.handlers[UiMode.OPTION_SELECT] as OptionSelectUiHandler;
     vi.spyOn(secondOptionUiHandler, "show");
diff --git a/test/test-utils/game-manager.ts b/test/test-utils/game-manager.ts
index 1b379daae88..f9db964ad26 100644
--- a/test/test-utils/game-manager.ts
+++ b/test/test-utils/game-manager.ts
@@ -52,7 +52,6 @@ import type { ModifierSelectUiHandler } from "#ui/modifier-select-ui-handler";
 import type { PartyUiHandler } from "#ui/party-ui-handler";
 import type { StarterSelectUiHandler } from "#ui/starter-select-ui-handler";
 import type { TargetSelectUiHandler } from "#ui/target-select-ui-handler";
-import { isNullOrUndefined } from "#utils/common";
 import fs from "node:fs";
 import { AES, enc } from "crypto-js";
 import { expect, vi } from "vitest";
@@ -240,7 +239,7 @@ export class GameManager {
    * @returns A Promise that resolves when the EncounterPhase ends.
    */
   async runToMysteryEncounter(encounterType?: MysteryEncounterType, species?: SpeciesId[]) {
-    if (!isNullOrUndefined(encounterType)) {
+    if (encounterType != null) {
       this.override.disableTrainerWaves();
       this.override.mysteryEncounter(encounterType);
     }
@@ -272,7 +271,7 @@ export class GameManager {
     );
 
     await this.phaseInterceptor.to("EncounterPhase");
-    if (!isNullOrUndefined(encounterType)) {
+    if (encounterType != null) {
       expect(this.scene.currentBattle?.mysteryEncounter?.encounterType).toBe(encounterType);
     }
   }