diff --git a/src/battle-scene.ts b/src/battle-scene.ts
index b802466ee19..7a2e964e9e4 100644
--- a/src/battle-scene.ts
+++ b/src/battle-scene.ts
@@ -303,7 +303,7 @@ export default class BattleScene extends SceneBase {
   /** Session save data that pertains to Mystery Encounters */
   public mysteryEncounterSaveData: MysteryEncounterSaveData = new MysteryEncounterSaveData();
   /** If the previous wave was a MysteryEncounter, tracks the object with this variable. Mostly used for visual object cleanup */
-  public lastMysteryEncounter?: MysteryEncounter;
+  public lastMysteryEncounter: MysteryEncounter | undefined = undefined;
   /** Combined Biome and Wave count text */
   private biomeWaveText: Phaser.GameObjects.Text;
   private moneyText: Phaser.GameObjects.Text;
diff --git a/src/battle.ts b/src/battle.ts
index 245705f4801..4ec0bfedd9e 100644
--- a/src/battle.ts
+++ b/src/battle.ts
@@ -90,9 +90,9 @@ export default class Battle {
   public playerFaintsHistory: FaintLogEntry[] = [];
   public enemyFaintsHistory: FaintLogEntry[] = [];
 
-  public mysteryEncounterType?: MysteryEncounterType;
+  public mysteryEncounterType?: MysteryEncounterType | undefined;
   /** If the current battle is a Mystery Encounter, this will always be defined */
-  public mysteryEncounter?: MysteryEncounter;
+  public mysteryEncounter?: MysteryEncounter | undefined;
 
   private rngCounter = 0;
 
diff --git a/src/data/abilities/ability.ts b/src/data/abilities/ability.ts
index 120d1d413c4..b5a9bf09b98 100644
--- a/src/data/abilities/ability.ts
+++ b/src/data/abilities/ability.ts
@@ -1763,14 +1763,11 @@ export class PostDefendPerishSongAbAttr extends PostDefendAbAttr {
 }
 
 export class PostDefendWeatherChangeAbAttr extends PostDefendAbAttr {
-  private weatherType: WeatherType;
-  protected condition?: PokemonDefendCondition;
-
-  constructor(weatherType: WeatherType, condition?: PokemonDefendCondition) {
+  constructor(
+    private weatherType: WeatherType,
+    protected condition?: PokemonDefendCondition,
+  ) {
     super();
-
-    this.weatherType = weatherType;
-    this.condition = condition;
   }
 
   override canApplyPostDefend(
@@ -2758,13 +2755,10 @@ export class GorillaTacticsAbAttr extends ExecutedMoveAbAttr {
 }
 
 export class PostAttackStealHeldItemAbAttr extends PostAttackAbAttr {
-  private stealCondition: PokemonAttackCondition | null;
   private stolenItem?: PokemonHeldItemModifier;
 
-  constructor(stealCondition?: PokemonAttackCondition) {
+  constructor(private stealCondition?: PokemonAttackCondition) {
     super();
-
-    this.stealCondition = stealCondition ?? null;
   }
 
   override canApplyPostAttack(
@@ -2791,7 +2785,6 @@ export class PostAttackStealHeldItemAbAttr extends PostAttackAbAttr {
         }
       }
     }
-    this.stolenItem = undefined;
     return false;
   }
 
@@ -2817,7 +2810,6 @@ export class PostAttackStealHeldItemAbAttr extends PostAttackAbAttr {
         }),
       );
     }
-    this.stolenItem = undefined;
   }
 
   getTargetHeldItems(target: Pokemon): PokemonHeldItemModifier[] {
@@ -2945,13 +2937,10 @@ export class PostAttackApplyBattlerTagAbAttr extends PostAttackAbAttr {
 }
 
 export class PostDefendStealHeldItemAbAttr extends PostDefendAbAttr {
-  private condition?: PokemonDefendCondition;
   private stolenItem?: PokemonHeldItemModifier;
 
-  constructor(condition?: PokemonDefendCondition) {
+  constructor(private condition?: PokemonDefendCondition) {
     super();
-
-    this.condition = condition;
   }
 
   override canApplyPostDefend(
@@ -2997,7 +2986,6 @@ export class PostDefendStealHeldItemAbAttr extends PostDefendAbAttr {
         }),
       );
     }
-    this.stolenItem = undefined;
   }
 
   getTargetHeldItems(target: Pokemon): PokemonHeldItemModifier[] {
@@ -3375,18 +3363,16 @@ export class PostSummonRemoveArenaTagAbAttr extends PostSummonAbAttr {
  * Generic class to add an arena tag upon switching in
  */
 export class PostSummonAddArenaTagAbAttr extends PostSummonAbAttr {
-  private readonly tagType: ArenaTagType;
-  private readonly turnCount: number;
-  private readonly side?: ArenaTagSide;
-  private readonly quiet?: boolean;
   private sourceId: number;
 
-  constructor(showAbility: boolean, tagType: ArenaTagType, turnCount: number, side?: ArenaTagSide, quiet?: boolean) {
+  constructor(
+    showAbility: boolean,
+    private readonly tagType: ArenaTagType,
+    private readonly turnCount: number,
+    private readonly side?: ArenaTagSide,
+    private readonly quiet?: boolean,
+  ) {
     super(showAbility);
-    this.tagType = tagType;
-    this.turnCount = turnCount;
-    this.side = side;
-    this.quiet = quiet;
   }
 
   public override applyPostSummon(pokemon: Pokemon, _passive: boolean, simulated: boolean, _args: any[]): void {
@@ -4340,7 +4326,7 @@ export class ReflectStatStageChangeAbAttr extends PreStatStageChangeAbAttr {
  */
 export class ProtectStatAbAttr extends PreStatStageChangeAbAttr {
   /** {@linkcode BattleStat} to protect or `undefined` if **all** {@linkcode BattleStat} are protected */
-  private protectedStat?: BattleStat;
+  private protectedStat: BattleStat | undefined = undefined;
 
   constructor(protectedStat?: BattleStat) {
     super();
@@ -6446,7 +6432,6 @@ export class PostBattleLootAbAttr extends PostBattleAbAttr {
         }),
       );
     }
-    this.randItem = undefined;
   }
 }
 
@@ -7003,7 +6988,6 @@ export class PostSummonStatStageChangeOnArenaAbAttr extends PostSummonStatStageC
 export class FormBlockDamageAbAttr extends ReceivedMoveDamageMultiplierAbAttr {
   private multiplier: number;
   private tagType: BattlerTagType;
-  private recoilDamageFunc?: (pokemon: Pokemon) => number;
   private triggerMessageFunc: (pokemon: Pokemon, abilityName: string) => string;
 
   constructor(
@@ -7011,13 +6995,12 @@ export class FormBlockDamageAbAttr extends ReceivedMoveDamageMultiplierAbAttr {
     multiplier: number,
     tagType: BattlerTagType,
     triggerMessageFunc: (pokemon: Pokemon, abilityName: string) => string,
-    recoilDamageFunc?: (pokemon: Pokemon) => number,
+    private recoilDamageFunc?: (pokemon: Pokemon) => number,
   ) {
     super(condition, multiplier);
 
     this.multiplier = multiplier;
     this.tagType = tagType;
-    this.recoilDamageFunc = recoilDamageFunc;
     this.triggerMessageFunc = triggerMessageFunc;
   }
 
diff --git a/src/data/battler-tags.ts b/src/data/battler-tags.ts
index 8405fd1dd4d..062f9b1f14a 100644
--- a/src/data/battler-tags.ts
+++ b/src/data/battler-tags.ts
@@ -45,7 +45,6 @@ export class BattlerTag {
   public lapseTypes: BattlerTagLapseType[];
   public turnCount: number;
   public sourceMove: MoveId;
-  public sourceId?: number;
   public isBatonPassable: boolean;
 
   constructor(
@@ -53,14 +52,13 @@ export class BattlerTag {
     lapseType: BattlerTagLapseType | BattlerTagLapseType[],
     turnCount: number,
     sourceMove?: MoveId,
-    sourceId?: number,
+    public sourceId?: number,
     isBatonPassable = false,
   ) {
     this.tagType = tagType;
     this.lapseTypes = coerceArray(lapseType);
     this.turnCount = turnCount;
     this.sourceMove = sourceMove!; // TODO: is this bang correct?
-    this.sourceId = sourceId;
     this.isBatonPassable = isBatonPassable;
   }
 
diff --git a/src/data/egg.ts b/src/data/egg.ts
index a6e2e04a5fe..cbe5be3bef8 100644
--- a/src/data/egg.ts
+++ b/src/data/egg.ts
@@ -101,7 +101,7 @@ export class Egg {
 
   private _overrideHiddenAbility: boolean;
 
-  private _eggDescriptor?: string;
+  private _eggDescriptor: string | undefined;
 
   ////
   // #endregion
diff --git a/src/data/moves/move.ts b/src/data/moves/move.ts
index f94c59bb463..df170a670df 100644
--- a/src/data/moves/move.ts
+++ b/src/data/moves/move.ts
@@ -632,7 +632,7 @@ export default abstract class Move implements Localizable {
   doesFlagEffectApply({ flag, user, target, isFollowUp = false }: {
     flag: MoveFlags;
     user: Pokemon;
-    target?: Pokemon;
+    target?: Pokemon | undefined;
     isFollowUp?: boolean;
   }): boolean {
     // special cases below, eg: if the move flag is MAKES_CONTACT, and the user pokemon has an ability that ignores contact (like "Long Reach"), then overrides and move does not make contact
@@ -1223,15 +1223,14 @@ interface MoveEffectAttrOptions {
  * @see {@linkcode apply}
  */
 export class MoveEffectAttr extends MoveAttr {
-  /**
-   * A container for this attribute's optional parameters
-   * @see {@linkcode MoveEffectAttrOptions} for supported params.
-   */
-  protected options?: MoveEffectAttrOptions;
-
-  constructor(selfTarget?: boolean, options?: MoveEffectAttrOptions) {
+  constructor(
+    selfTarget?: boolean,
+    /**
+     * A container for this attribute's optional parameters
+     * @see {@linkcode MoveEffectAttrOptions} for supported params.
+     */
+    protected options?: MoveEffectAttrOptions) {
     super(selfTarget);
-    this.options = options;
   }
 
   /**
@@ -1434,7 +1433,7 @@ export class PreMoveMessageAttr extends MoveAttr {
  * @extends MoveAttr
  */
 export class PreUseInterruptAttr extends MoveAttr {
-  protected message?: string | ((user: Pokemon, target: Pokemon, move: Move) => string);
+  protected message: string | ((user: Pokemon, target: Pokemon, move: Move) => string);
   protected overridesFailedMessage: boolean;
   protected conditionFunc: MoveConditionFunc;
 
@@ -1442,10 +1441,10 @@ export class PreUseInterruptAttr extends MoveAttr {
    * Create a new MoveInterruptedMessageAttr.
    * @param message The message to display when the move is interrupted, or a function that formats the message based on the user, target, and move.
    */
-  constructor(message?: string | ((user: Pokemon, target: Pokemon, move: Move) => string), conditionFunc?: MoveConditionFunc) {
+  constructor(message: string | ((user: Pokemon, target: Pokemon, move: Move) => string), conditionFunc: MoveConditionFunc) {
     super();
     this.message = message;
-    this.conditionFunc = conditionFunc ?? (() => true);
+    this.conditionFunc = conditionFunc;
   }
 
   /**
@@ -2191,15 +2190,16 @@ export class SandHealAttr extends WeatherHealAttr {
  * @extends HealAttr
  * @see {@linkcode apply}
  */
+// TODO: Convert other healing attributes (WeatherHealAttr, etc) to base off of this class
 export class BoostHealAttr extends HealAttr {
   /** Healing received when {@linkcode condition} is false */
   private normalHealRatio: number;
   /** Healing received when {@linkcode condition} is true */
   private boostedHealRatio: number;
   /** The lambda expression to check against when boosting the healing value */
-  private condition?: MoveConditionFunc;
+  private condition: MoveConditionFunc;
 
-  constructor(normalHealRatio: number = 0.5, boostedHealRatio: number = 2 / 3, showAnim?: boolean, selfTarget?: boolean, condition?: MoveConditionFunc) {
+  constructor(normalHealRatio: number, boostedHealRatio: number, condition: MoveConditionFunc, showAnim?: boolean, selfTarget?: boolean, ) {
     super(normalHealRatio, showAnim, selfTarget);
     this.normalHealRatio = normalHealRatio;
     this.boostedHealRatio = boostedHealRatio;
@@ -2213,8 +2213,8 @@ export class BoostHealAttr extends HealAttr {
    * @param args N/A
    * @returns true if the move was successful
    */
-  apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
-    const healRatio: number = (this.condition ? this.condition(user, target, move) : false) ? this.boostedHealRatio : this.normalHealRatio;
+  override apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): boolean {
+    const healRatio = this.condition(user, target, move) ? this.boostedHealRatio : this.normalHealRatio;
     this.addHealPhase(target, healRatio);
     return true;
   }
@@ -2500,7 +2500,7 @@ export class WaterShurikenMultiHitTypeAttr extends ChangeMultiHitTypeAttr {
 
 export class StatusEffectAttr extends MoveEffectAttr {
   public effect: StatusEffect;
-  public turnsRemaining?: number;
+  public turnsRemaining?: number | undefined;
   public overrideStatus: boolean = false;
 
   constructor(effect: StatusEffect, selfTarget?: boolean, turnsRemaining?: number, overrideStatus: boolean = false) {
@@ -3224,19 +3224,18 @@ interface StatStageChangeAttrOptions extends MoveEffectAttrOptions {
  * @see {@linkcode apply}
  */
 export class StatStageChangeAttr extends MoveEffectAttr {
-  public stats: BattleStat[];
-  public stages: number;
-  /**
-   * Container for optional parameters to this attribute.
-   * @see {@linkcode StatStageChangeAttrOptions} for available optional params
-   */
-  protected override options?: StatStageChangeAttrOptions;
-
-  constructor(stats: BattleStat[], stages: number, selfTarget?: boolean, options?: StatStageChangeAttrOptions) {
+  constructor(
+    public stats: BattleStat[],
+    public stages: number,
+    selfTarget?: boolean,
+    /**
+     * Container for optional parameters to this attribute.
+     * @see {@linkcode StatStageChangeAttrOptions} for available optional params
+     */
+    protected override options?: StatStageChangeAttrOptions) {
     super(selfTarget, options);
     this.stats = stats;
     this.stages = stages;
-    this.options = options;
   }
 
   /**
@@ -3468,16 +3467,16 @@ export class SecretPowerAttr extends MoveEffectAttr {
 export class PostVictoryStatStageChangeAttr extends MoveAttr {
   private stats: BattleStat[];
   private stages: number;
-  private condition?: MoveConditionFunc;
   private showMessage: boolean;
 
-  constructor(stats: BattleStat[], stages: number, selfTarget?: boolean, condition?: MoveConditionFunc, showMessage: boolean = true, firstHitOnly: boolean = false) {
+  constructor(stats: BattleStat[], stages: number, selfTarget?: boolean, private condition?: MoveConditionFunc, showMessage: boolean = true, firstHitOnly: boolean = false) {
     super();
     this.stats = stats;
     this.stages = stages;
     this.condition = condition;
     this.showMessage = showMessage;
   }
+
   applyPostVictory(user: Pokemon, target: Pokemon, move: Move): void {
     if (this.condition && !this.condition(user, target, move)) {
       return;
@@ -10501,7 +10500,7 @@ export function initMoves() {
       .attr(StatStageChangeAttr, [ Stat.SPD ], -1, true)
       .punchingMove(),
     new StatusMove(MoveId.FLORAL_HEALING, PokemonType.FAIRY, -1, 10, -1, 0, 7)
-      .attr(BoostHealAttr, 0.5, 2 / 3, true, false, (user, target, move) => globalScene.arena.terrain?.terrainType === TerrainType.GRASSY)
+      .attr(BoostHealAttr, 0.5, 2 / 3, (user, target, move) => globalScene.arena.terrain?.terrainType === TerrainType.GRASSY, true, false)
       .triageMove()
       .reflectable(),
     new AttackMove(MoveId.HIGH_HORSEPOWER, PokemonType.GROUND, MoveCategory.PHYSICAL, 95, 95, 10, -1, 0, 7),
diff --git a/src/data/moves/pokemon-move.ts b/src/data/moves/pokemon-move.ts
index daad199fbbd..01ef944c55c 100644
--- a/src/data/moves/pokemon-move.ts
+++ b/src/data/moves/pokemon-move.ts
@@ -18,21 +18,19 @@ import type Move from "./move";
  * @see {@linkcode getName} - returns name of {@linkcode Move}.
  **/
 export class PokemonMove {
-  public moveId: MoveId;
-  public ppUsed: number;
-  public ppUp: number;
-
-  /**
-   * If defined and nonzero, overrides the maximum PP of the move (e.g., due to move being copied by Transform).
-   * This also nullifies all effects of `ppUp`.
-   */
-  public maxPpOverride?: number;
-
-  constructor(moveId: MoveId, ppUsed = 0, ppUp = 0, maxPpOverride?: number) {
+  constructor(
+    public moveId: MoveId,
+    public ppUsed = 0,
+    public ppUp = 0,
+    /**
+     * If defined and nonzero, overrides the maximum PP of the move (e.g., due to move being copied by Transform).
+     * This also nullifies all effects of `ppUp`.
+     */
+    public maxPpOverride = 0,
+  ) {
     this.moveId = moveId;
     this.ppUsed = ppUsed;
     this.ppUp = ppUp;
-    this.maxPpOverride = maxPpOverride;
   }
 
   /**
diff --git a/src/data/mystery-encounters/mystery-encounter-option.ts b/src/data/mystery-encounters/mystery-encounter-option.ts
index 53b53392bb8..80399e39687 100644
--- a/src/data/mystery-encounters/mystery-encounter-option.ts
+++ b/src/data/mystery-encounters/mystery-encounter-option.ts
@@ -45,7 +45,7 @@ export default class MysteryEncounterOption implements IMysteryEncounterOption {
   requirements: EncounterSceneRequirement[];
   primaryPokemonRequirements: EncounterPokemonRequirement[];
   secondaryPokemonRequirements: EncounterPokemonRequirement[];
-  primaryPokemon?: PlayerPokemon;
+  primaryPokemon?: PlayerPokemon | undefined;
   secondaryPokemon?: PlayerPokemon[];
   excludePrimaryFromSecondaryRequirements: boolean;
 
diff --git a/src/data/mystery-encounters/mystery-encounter.ts b/src/data/mystery-encounters/mystery-encounter.ts
index fa97a7f4d40..3f9ee5cc0ee 100644
--- a/src/data/mystery-encounters/mystery-encounter.ts
+++ b/src/data/mystery-encounters/mystery-encounter.ts
@@ -53,9 +53,9 @@ export interface IMysteryEncounter {
   options: [MysteryEncounterOption, MysteryEncounterOption, ...MysteryEncounterOption[]];
   spriteConfigs: MysteryEncounterSpriteConfig[];
   encounterTier: MysteryEncounterTier;
-  encounterAnimations?: EncounterAnim[];
-  disallowedGameModes?: GameModes[];
-  disallowedChallenges?: Challenges[];
+  encounterAnimations?: EncounterAnim[] | undefined;
+  disallowedGameModes?: GameModes[] | undefined;
+  disallowedChallenges?: Challenges[] | undefined;
   hideBattleIntroMessage: boolean;
   autoHideIntroVisuals: boolean;
   enterIntroVisualsFromRight: boolean;
@@ -68,11 +68,11 @@ export interface IMysteryEncounter {
   skipToFightInput: boolean;
   preventGameStatsUpdates: boolean;
 
-  onInit?: () => boolean;
-  onVisualsStart?: () => boolean;
-  doEncounterExp?: () => boolean;
-  doEncounterRewards?: () => boolean;
-  doContinueEncounter?: () => Promise<void>;
+  onInit?: (() => boolean) | undefined;
+  onVisualsStart?: (() => boolean) | undefined;
+  doEncounterExp?: (() => boolean) | undefined;
+  doEncounterRewards?: (() => boolean) | undefined;
+  doContinueEncounter?: (() => Promise<void>) | undefined;
 
   requirements: EncounterSceneRequirement[];
   primaryPokemonRequirements: EncounterPokemonRequirement[];
@@ -105,15 +105,15 @@ export default class MysteryEncounter implements IMysteryEncounter {
    * Custom battle animations that are configured for encounter effects and visuals
    * Specify here so that assets are loaded on initialization of encounter
    */
-  encounterAnimations?: EncounterAnim[];
+  encounterAnimations?: EncounterAnim[] | undefined;
   /**
    * If specified, defines any game modes where the {@linkcode MysteryEncounter} should *NOT* spawn
    */
-  disallowedGameModes?: GameModes[];
+  disallowedGameModes?: GameModes[] | undefined;
   /**
    * If specified, defines any challenges (from Challenge game mode) where the {@linkcode MysteryEncounter} should *NOT* spawn
    */
-  disallowedChallenges?: Challenges[];
+  disallowedChallenges?: Challenges[] | undefined;
   /**
    * If true, hides "A Wild X Appeared" etc. messages
    * Default true
@@ -172,24 +172,24 @@ export default class MysteryEncounter implements IMysteryEncounter {
   // #region Event callback functions
 
   /** Event when Encounter is first loaded, use it for data conditioning */
-  onInit?: () => boolean;
+  onInit?: (() => boolean) | undefined;
   /** Event when battlefield visuals have finished sliding in and the encounter dialogue begins */
-  onVisualsStart?: () => boolean;
+  onVisualsStart?: (() => boolean) | undefined;
   /** Event triggered prior to {@linkcode CommandPhase}, during {@linkcode TurnInitPhase} */
-  onTurnStart?: () => boolean;
+  onTurnStart?: (() => boolean) | undefined;
   /** Event prior to any rewards logic in {@linkcode MysteryEncounterRewardsPhase} */
-  onRewards?: () => Promise<void>;
+  onRewards?: (() => Promise<void>) | undefined;
   /** Will provide the player party EXP before rewards are displayed for that wave */
-  doEncounterExp?: () => boolean;
+  doEncounterExp?: (() => boolean) | undefined;
   /** Will provide the player a rewards shop for that wave */
-  doEncounterRewards?: () => boolean;
+  doEncounterRewards?: (() => boolean) | undefined;
   /** Will execute callback during VictoryPhase of a continuousEncounter */
-  doContinueEncounter?: () => Promise<void>;
+  doContinueEncounter?: (() => Promise<void>) | undefined;
   /**
    * Can perform special logic when a ME battle is lost, before GameOver/battle retry prompt.
    * Should return `true` if it is treated as "real" Game Over, `false` if not.
    */
-  onGameOver?: () => boolean;
+  onGameOver?: (() => boolean) | undefined;
 
   /**
    * Requirements
@@ -204,8 +204,8 @@ export default class MysteryEncounter implements IMysteryEncounter {
    */
   secondaryPokemonRequirements: EncounterPokemonRequirement[];
   excludePrimaryFromSupportRequirements: boolean;
-  primaryPokemon?: PlayerPokemon;
-  secondaryPokemon?: PlayerPokemon[];
+  primaryPokemon?: PlayerPokemon | undefined;
+  secondaryPokemon?: PlayerPokemon[] | undefined;
 
   // #region Post-construct / Auto-populated params
   localizationKey: string;
@@ -224,7 +224,7 @@ export default class MysteryEncounter implements IMysteryEncounter {
    * Otherwise, will be undefined
    * You probably shouldn't do anything directly with this unless you have a very specific need
    */
-  introVisuals?: MysteryEncounterIntroVisuals;
+  introVisuals?: MysteryEncounterIntroVisuals | undefined;
 
   // #region Flags
 
@@ -252,7 +252,7 @@ export default class MysteryEncounter implements IMysteryEncounter {
   /**
    * Will be set by option select handlers automatically, and can be used to refer to which option was chosen by later phases
    */
-  selectedOption?: MysteryEncounterOption;
+  selectedOption?: MysteryEncounterOption | undefined;
   /**
    * Array containing data pertaining to free moves used at the start of a battle mystery envounter.
    */
diff --git a/src/data/status-effect.ts b/src/data/status-effect.ts
index a90304c9f7d..f3bc6a3eb84 100644
--- a/src/data/status-effect.ts
+++ b/src/data/status-effect.ts
@@ -7,9 +7,9 @@ export class Status {
   public effect: StatusEffect;
   /** Toxic damage is `1/16 max HP * toxicTurnCount` */
   public toxicTurnCount = 0;
-  public sleepTurnsRemaining?: number;
+  public sleepTurnsRemaining = 0;
 
-  constructor(effect: StatusEffect, toxicTurnCount = 0, sleepTurnsRemaining?: number) {
+  constructor(effect: StatusEffect, toxicTurnCount = 0, sleepTurnsRemaining = 0) {
     this.effect = effect;
     this.toxicTurnCount = toxicTurnCount;
     this.sleepTurnsRemaining = sleepTurnsRemaining;
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index e9cc4f70d70..8eb530aa47c 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -263,14 +263,14 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
   public isTerastallized: boolean;
   public stellarTypesBoosted: PokemonType[];
 
-  public fusionSpecies: PokemonSpecies | null;
+  public fusionSpecies?: PokemonSpecies | undefined = undefined;
   public fusionFormIndex: number;
   public fusionAbilityIndex: number;
   public fusionShiny: boolean;
   public fusionVariant: Variant;
   public fusionGender: Gender;
   public fusionLuck: number;
-  public fusionCustomPokemonData: CustomPokemonData | null;
+  public fusionCustomPokemonData?: CustomPokemonData | undefined = undefined;
   public fusionTeraType: PokemonType;
 
   public customPokemonData: CustomPokemonData = new CustomPokemonData();
@@ -372,7 +372,7 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
           ? dataSource.fusionSpecies
           : dataSource.fusionSpecies
             ? getPokemonSpecies(dataSource.fusionSpecies)
-            : null;
+            : undefined;
       this.fusionFormIndex = dataSource.fusionFormIndex;
       this.fusionAbilityIndex = dataSource.fusionAbilityIndex;
       this.fusionShiny = dataSource.fusionShiny;
@@ -642,7 +642,7 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
         gender: pokemon.gender,
         pokeball: pokemon.pokeball,
         fusionFormIndex: pokemon.fusionFormIndex,
-        fusionSpecies: pokemon.fusionSpecies || undefined,
+        fusionSpecies: pokemon.fusionSpecies,
         fusionGender: pokemon.fusionGender,
       };
 
@@ -2923,14 +2923,13 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
   }
 
   public clearFusionSpecies(): void {
-    this.fusionSpecies = null;
     this.fusionFormIndex = 0;
     this.fusionAbilityIndex = 0;
     this.fusionShiny = false;
     this.fusionVariant = 0;
     this.fusionGender = 0;
     this.fusionLuck = 0;
-    this.fusionCustomPokemonData = null;
+    this.fusionCustomPokemonData = this.fusionSpecies = undefined;
 
     this.generateName();
     this.calculateStats();
@@ -4749,7 +4748,7 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
       globalScene.phaseManager.unshiftNew(
         "ObtainStatusEffectPhase",
         this.getBattlerIndex(),
-        effect,
+        effect ?? StatusEffect.NONE, // This is addressed in a separate PR; please don't sue
         turnsRemaining,
         sourceText,
         sourcePokemon,
@@ -6734,7 +6733,7 @@ interface IllusionData {
   /** The pokeball of the illusion */
   pokeball: PokeballType;
   /** The fusion species of the illusion if it's a fusion */
-  fusionSpecies?: PokemonSpecies;
+  fusionSpecies?: PokemonSpecies | undefined;
   /** The fusionFormIndex of the illusion */
   fusionFormIndex?: number;
   /** The fusionGender of the illusion if it's a fusion */
@@ -6743,6 +6742,7 @@ interface IllusionData {
   level?: number;
 }
 
+// TODO: Remove `turn` and make `result` mandatory
 export interface TurnMove {
   move: MoveId;
   targets: BattlerIndex[];
diff --git a/src/modifier/modifier-type.ts b/src/modifier/modifier-type.ts
index a04a5e2be47..b5d0e597cf4 100644
--- a/src/modifier/modifier-type.ts
+++ b/src/modifier/modifier-type.ts
@@ -2499,7 +2499,7 @@ export interface CustomModifierSettings {
   guaranteedModifierTypeFuncs?: ModifierTypeFunc[];
   fillRemaining?: boolean;
   /** Set to negative value to disable rerolls completely in shop */
-  rerollMultiplier?: number;
+  rerollMultiplier?: number | undefined;
   allowLuckUpgrades?: boolean;
 }
 
diff --git a/src/phases/common-anim-phase.ts b/src/phases/common-anim-phase.ts
index abfe8ed99f0..1d2b83f5fef 100644
--- a/src/phases/common-anim-phase.ts
+++ b/src/phases/common-anim-phase.ts
@@ -9,19 +9,17 @@ export class CommonAnimPhase extends PokemonPhase {
   // we need to allow phaseName to be a union of the two
   public readonly phaseName: "CommonAnimPhase" | "PokemonHealPhase" | "WeatherEffectPhase" = "CommonAnimPhase";
   private anim: CommonAnim | null;
-  private targetIndex?: BattlerIndex;
   private playOnEmptyField: boolean;
 
   constructor(
     battlerIndex?: BattlerIndex,
-    targetIndex?: BattlerIndex,
+    private targetIndex?: BattlerIndex,
     anim: CommonAnim | null = null,
     playOnEmptyField = false,
   ) {
     super(battlerIndex);
 
     this.anim = anim;
-    this.targetIndex = targetIndex;
     this.playOnEmptyField = playOnEmptyField;
   }
 
diff --git a/src/phases/faint-phase.ts b/src/phases/faint-phase.ts
index 675a198d096..e79aef97cdf 100644
--- a/src/phases/faint-phase.ts
+++ b/src/phases/faint-phase.ts
@@ -33,16 +33,17 @@ export class FaintPhase extends PokemonPhase {
    */
   private preventInstantRevive: boolean;
 
-  /**
-   * The source Pokemon that dealt fatal damage
-   */
-  private source?: Pokemon;
-
-  constructor(battlerIndex: BattlerIndex, preventInstantRevive = false, source?: Pokemon) {
+  constructor(
+    battlerIndex: BattlerIndex,
+    preventInstantRevive = false,
+    /**
+     * The source Pokemon that dealt fatal damage
+     */
+    private source?: Pokemon,
+  ) {
     super(battlerIndex);
 
     this.preventInstantRevive = preventInstantRevive;
-    this.source = source;
   }
 
   start() {
diff --git a/src/phases/message-phase.ts b/src/phases/message-phase.ts
index 61f9b74a037..c5c2f7ca521 100644
--- a/src/phases/message-phase.ts
+++ b/src/phases/message-phase.ts
@@ -4,17 +4,17 @@ import { Phase } from "#app/phase";
 export class MessagePhase extends Phase {
   public readonly phaseName = "MessagePhase";
   private text: string;
-  private callbackDelay?: number | null;
-  private prompt?: boolean | null;
-  private promptDelay?: number | null;
-  private speaker?: string;
+  private callbackDelay: number | null = null;
+  private prompt: boolean | null;
+  private promptDelay: number | null;
+  private speaker = "";
 
   constructor(
     text: string,
-    callbackDelay?: number | null,
-    prompt?: boolean | null,
-    promptDelay?: number | null,
-    speaker?: string,
+    callbackDelay: number | null = null,
+    prompt: boolean | null = null,
+    promptDelay: number | null = null,
+    speaker = "",
   ) {
     super();
 
diff --git a/src/phases/mystery-encounter-phases.ts b/src/phases/mystery-encounter-phases.ts
index 9aae796211f..af59c74b00d 100644
--- a/src/phases/mystery-encounter-phases.ts
+++ b/src/phases/mystery-encounter-phases.ts
@@ -30,15 +30,13 @@ import { isNullOrUndefined, randSeedItem } from "#app/utils/common";
 export class MysteryEncounterPhase extends Phase {
   public readonly phaseName = "MysteryEncounterPhase";
   private readonly FIRST_DIALOGUE_PROMPT_DELAY = 300;
-  optionSelectSettings?: OptionSelectSettings;
 
   /**
    * Mostly useful for having repeated queries during a single encounter, where the queries and options may differ each time
    * @param optionSelectSettings allows overriding the typical options of an encounter with new ones
    */
-  constructor(optionSelectSettings?: OptionSelectSettings) {
+  constructor(public optionSelectSettings?: OptionSelectSettings) {
     super();
-    this.optionSelectSettings = optionSelectSettings;
   }
 
   /**
@@ -574,7 +572,7 @@ export class MysteryEncounterRewardsPhase extends Phase {
 export class PostMysteryEncounterPhase extends Phase {
   public readonly phaseName = "PostMysteryEncounterPhase";
   private readonly FIRST_DIALOGUE_PROMPT_DELAY = 750;
-  onPostOptionSelect?: OptionPhaseCallback;
+  onPostOptionSelect?: OptionPhaseCallback | undefined;
 
   constructor() {
     super();
diff --git a/src/phases/obtain-status-effect-phase.ts b/src/phases/obtain-status-effect-phase.ts
index dc26d070029..8926a77a4e7 100644
--- a/src/phases/obtain-status-effect-phase.ts
+++ b/src/phases/obtain-status-effect-phase.ts
@@ -13,24 +13,15 @@ import { isNullOrUndefined } from "#app/utils/common";
 
 export class ObtainStatusEffectPhase extends PokemonPhase {
   public readonly phaseName = "ObtainStatusEffectPhase";
-  private statusEffect?: StatusEffect;
-  private turnsRemaining?: number;
-  private sourceText?: string | null;
-  private sourcePokemon?: Pokemon | null;
 
   constructor(
     battlerIndex: BattlerIndex,
-    statusEffect?: StatusEffect,
-    turnsRemaining?: number,
-    sourceText?: string | null,
-    sourcePokemon?: Pokemon | null,
+    private statusEffect: StatusEffect,
+    private turnsRemaining = 0,
+    private sourceText: string | null = null,
+    private sourcePokemon: Pokemon | null = null,
   ) {
     super(battlerIndex);
-
-    this.statusEffect = statusEffect;
-    this.turnsRemaining = turnsRemaining;
-    this.sourceText = sourceText;
-    this.sourcePokemon = sourcePokemon;
   }
 
   start() {
diff --git a/src/phases/party-exp-phase.ts b/src/phases/party-exp-phase.ts
index 30fc97d9105..b9d75e3e42a 100644
--- a/src/phases/party-exp-phase.ts
+++ b/src/phases/party-exp-phase.ts
@@ -7,16 +7,13 @@ import { Phase } from "#app/phase";
  */
 export class PartyExpPhase extends Phase {
   public readonly phaseName = "PartyExpPhase";
-  expValue: number;
-  useWaveIndexMultiplier?: boolean;
-  pokemonParticipantIds?: Set<number>;
 
-  constructor(expValue: number, useWaveIndexMultiplier?: boolean, pokemonParticipantIds?: Set<number>) {
+  constructor(
+    public expValue: number,
+    public useWaveIndexMultiplier?: boolean,
+    public pokemonParticipantIds?: Set<number>,
+  ) {
     super();
-
-    this.expValue = expValue;
-    this.useWaveIndexMultiplier = useWaveIndexMultiplier;
-    this.pokemonParticipantIds = pokemonParticipantIds;
   }
 
   /**
diff --git a/src/phases/post-game-over-phase.ts b/src/phases/post-game-over-phase.ts
index 8e19dcd5498..2fd47760a5b 100644
--- a/src/phases/post-game-over-phase.ts
+++ b/src/phases/post-game-over-phase.ts
@@ -4,12 +4,9 @@ import type { EndCardPhase } from "./end-card-phase";
 
 export class PostGameOverPhase extends Phase {
   public readonly phaseName = "PostGameOverPhase";
-  private endCardPhase?: EndCardPhase;
 
-  constructor(endCardPhase?: EndCardPhase) {
+  constructor(private endCardPhase?: EndCardPhase) {
     super();
-
-    this.endCardPhase = endCardPhase;
   }
 
   start() {
diff --git a/src/phases/reload-session-phase.ts b/src/phases/reload-session-phase.ts
index ac9337753c4..0ed92b4049e 100644
--- a/src/phases/reload-session-phase.ts
+++ b/src/phases/reload-session-phase.ts
@@ -5,12 +5,9 @@ import { fixedInt } from "#app/utils/common";
 
 export class ReloadSessionPhase extends Phase {
   public readonly phaseName = "ReloadSessionPhase";
-  private systemDataStr?: string;
 
-  constructor(systemDataStr?: string) {
+  constructor(private systemDataStr?: string) {
     super();
-
-    this.systemDataStr = systemDataStr;
   }
 
   start(): void {
diff --git a/src/phases/select-modifier-phase.ts b/src/phases/select-modifier-phase.ts
index 53e1f5bc282..fc4d18c8d6d 100644
--- a/src/phases/select-modifier-phase.ts
+++ b/src/phases/select-modifier-phase.ts
@@ -35,25 +35,15 @@ export type ModifierSelectCallback = (rowCursor: number, cursor: number) => bool
 
 export class SelectModifierPhase extends BattlePhase {
   public readonly phaseName = "SelectModifierPhase";
-  private rerollCount: number;
-  private modifierTiers?: ModifierTier[];
-  private customModifierSettings?: CustomModifierSettings;
-  private isCopy: boolean;
-
   private typeOptions: ModifierTypeOption[];
 
   constructor(
-    rerollCount = 0,
-    modifierTiers?: ModifierTier[],
-    customModifierSettings?: CustomModifierSettings,
-    isCopy = false,
+    private rerollCount = 0,
+    private modifierTiers?: ModifierTier[],
+    private customModifierSettings?: CustomModifierSettings,
+    private isCopy = false,
   ) {
     super();
-
-    this.rerollCount = rerollCount;
-    this.modifierTiers = modifierTiers;
-    this.customModifierSettings = customModifierSettings;
-    this.isCopy = isCopy;
   }
 
   start() {
diff --git a/src/plugins/api/api-base.ts b/src/plugins/api/api-base.ts
index f55ffe2d3fd..3dcb9f6282f 100644
--- a/src/plugins/api/api-base.ts
+++ b/src/plugins/api/api-base.ts
@@ -32,8 +32,8 @@ export abstract class ApiBase {
    * @param bodyData The body-data to send.
    * @param dataType The data-type of the {@linkcode bodyData}.
    */
-  protected async doPost<D = undefined>(path: string, bodyData?: D, dataType: DataType = "json") {
-    let body: string | undefined = undefined;
+  protected async doPost<D = null>(path: string, bodyData?: D, dataType: DataType = "json") {
+    let body: string | null = null;
     const headers: HeadersInit = {};
 
     if (bodyData) {
diff --git a/src/plugins/api/pokerogue-admin-api.ts b/src/plugins/api/pokerogue-admin-api.ts
index 5923f286430..cc24c1c1e97 100644
--- a/src/plugins/api/pokerogue-admin-api.ts
+++ b/src/plugins/api/pokerogue-admin-api.ts
@@ -112,7 +112,9 @@ export class PokerogueAdminApi extends ApiBase {
    * @param params The {@linkcode SearchAccountRequest} to send
    * @returns an array of {@linkcode SearchAccountResponse} and error. Both can be `undefined`
    */
-  public async searchAccount(params: SearchAccountRequest): Promise<[data?: SearchAccountResponse, error?: string]> {
+  public async searchAccount(
+    params: SearchAccountRequest,
+  ): Promise<[data?: SearchAccountResponse | undefined, error?: string | undefined]> {
     try {
       const urlSearchParams = this.toUrlSearchParams(params);
       const response = await this.doGet(`/admin/account/adminSearch?${urlSearchParams}`);
diff --git a/src/system/game-data.ts b/src/system/game-data.ts
index e933c5704f9..ec61831553f 100644
--- a/src/system/game-data.ts
+++ b/src/system/game-data.ts
@@ -164,15 +164,15 @@ export interface StarterMoveData {
 }
 
 export interface StarterAttributes {
-  nature?: number;
-  ability?: number;
-  variant?: number;
-  form?: number;
-  female?: boolean;
-  shiny?: boolean;
-  favorite?: boolean;
-  nickname?: string;
-  tera?: PokemonType;
+  nature?: number | undefined;
+  ability?: number | undefined;
+  variant?: number | undefined;
+  form?: number | undefined;
+  female?: boolean | undefined;
+  shiny?: boolean | undefined;
+  favorite?: boolean | undefined;
+  nickname?: string | undefined;
+  tera?: PokemonType | undefined;
 }
 
 export interface DexAttrProps {
diff --git a/src/system/version_migration/versions/v1_10_0.ts b/src/system/version_migration/versions/v1_10_0.ts
index 4d1dedf701e..43e0fe83876 100644
--- a/src/system/version_migration/versions/v1_10_0.ts
+++ b/src/system/version_migration/versions/v1_10_0.ts
@@ -25,6 +25,7 @@ interface OldTurnMove {
 const fixMoveHistory: SessionSaveMigrator = {
   version: "1.10.0",
   migrate: (data: SessionSaveData): void => {
+    // @ts-expect-error - optional property jank
     const mapTurnMove = (tm: OldTurnMove): TurnMove => ({
       move: tm.move,
       targets: tm.targets,
diff --git a/src/ui/battle-info/enemy-battle-info.ts b/src/ui/battle-info/enemy-battle-info.ts
index 7c16f01ac38..98dab4cfe70 100644
--- a/src/ui/battle-info/enemy-battle-info.ts
+++ b/src/ui/battle-info/enemy-battle-info.ts
@@ -21,7 +21,7 @@ export class EnemyBattleInfo extends BattleInfo {
   protected effectivenessContainer: Phaser.GameObjects.Container;
   protected effectivenessWindow: Phaser.GameObjects.NineSlice;
   protected effectivenessText: Phaser.GameObjects.Text;
-  protected currentEffectiveness?: string;
+  protected currentEffectiveness?: string | undefined;
   // #endregion
 
   override get statOrder(): Stat[] {
diff --git a/src/ui/dropdown.ts b/src/ui/dropdown.ts
index 2cbd1f0dfa9..8192dc5d63e 100644
--- a/src/ui/dropdown.ts
+++ b/src/ui/dropdown.ts
@@ -36,15 +36,11 @@ export enum SortCriteria {
 }
 
 export class DropDownLabel {
-  public state: DropDownState;
-  public text: string;
-  public sprite?: Phaser.GameObjects.Sprite;
-
-  constructor(label: string, sprite?: Phaser.GameObjects.Sprite, state: DropDownState = DropDownState.OFF) {
-    this.text = label || "";
-    this.sprite = sprite;
-    this.state = state;
-  }
+  constructor(
+    public text: string,
+    public sprite?: Phaser.GameObjects.Sprite,
+    public state: DropDownState = DropDownState.OFF,
+  ) {}
 }
 
 export class DropDownOption extends Phaser.GameObjects.Container {
diff --git a/src/ui/form-modal-ui-handler.ts b/src/ui/form-modal-ui-handler.ts
index 8c30b4e0bc4..56267549724 100644
--- a/src/ui/form-modal-ui-handler.ts
+++ b/src/ui/form-modal-ui-handler.ts
@@ -95,7 +95,7 @@ export abstract class FormModalUiHandler extends ModalUiHandler {
       const inputBg = addWindow(0, 0, 80, 16, false, false, 0, 0, WindowVariant.XTHIN);
 
       const isPassword = config?.isPassword;
-      const isReadOnly = config?.isReadOnly;
+      const isReadOnly = config?.isReadOnly ?? false;
       const input = addTextInputObject(4, -2, 440, 116, TextStyle.TOOLTIP_CONTENT, {
         type: isPassword ? "password" : "text",
         maxLength: isPassword ? 64 : 20,
diff --git a/src/ui/mystery-encounter-ui-handler.ts b/src/ui/mystery-encounter-ui-handler.ts
index 83ce88714f5..addfc84425d 100644
--- a/src/ui/mystery-encounter-ui-handler.ts
+++ b/src/ui/mystery-encounter-ui-handler.ts
@@ -18,7 +18,7 @@ import { globalScene } from "#app/global-scene";
 
 export default class MysteryEncounterUiHandler extends UiHandler {
   private cursorContainer: Phaser.GameObjects.Container;
-  private cursorObj?: Phaser.GameObjects.Image;
+  private cursorObj?: Phaser.GameObjects.Image | undefined;
 
   private optionsContainer: Phaser.GameObjects.Container;
   // Length = max number of allowable options (4)
@@ -26,18 +26,18 @@ export default class MysteryEncounterUiHandler extends UiHandler {
 
   private tooltipWindow: Phaser.GameObjects.NineSlice;
   private tooltipContainer: Phaser.GameObjects.Container;
-  private tooltipScrollTween?: Phaser.Tweens.Tween;
+  private tooltipScrollTween?: Phaser.Tweens.Tween | undefined;
 
   private descriptionWindow: Phaser.GameObjects.NineSlice;
   private descriptionContainer: Phaser.GameObjects.Container;
-  private descriptionScrollTween?: Phaser.Tweens.Tween;
+  private descriptionScrollTween?: Phaser.Tweens.Tween | undefined;
   private rarityBall: Phaser.GameObjects.Sprite;
 
   private dexProgressWindow: Phaser.GameObjects.NineSlice;
   private dexProgressContainer: Phaser.GameObjects.Container;
   private showDexProgress = false;
 
-  private overrideSettings?: OptionSelectSettings;
+  private overrideSettings?: OptionSelectSettings | undefined;
   private encounterOptions: MysteryEncounterOption[] = [];
   private optionsMeetsReqs: boolean[];
 
diff --git a/src/ui/pokedex-page-ui-handler.ts b/src/ui/pokedex-page-ui-handler.ts
index 32a88ab36b2..3cd4b9d44e4 100644
--- a/src/ui/pokedex-page-ui-handler.ts
+++ b/src/ui/pokedex-page-ui-handler.ts
@@ -134,10 +134,10 @@ const valueReductionMax = 2;
 const speciesContainerX = 109;
 
 interface SpeciesDetails {
-  shiny?: boolean;
-  formIndex?: number;
-  female?: boolean;
-  variant?: number;
+  shiny?: boolean | undefined;
+  formIndex?: number | undefined;
+  female?: boolean | undefined;
+  variant?: number | undefined;
 }
 
 enum MenuOptions {
diff --git a/src/ui/starter-select-ui-handler.ts b/src/ui/starter-select-ui-handler.ts
index 20f613fb694..5e1294d3a62 100644
--- a/src/ui/starter-select-ui-handler.ts
+++ b/src/ui/starter-select-ui-handler.ts
@@ -89,8 +89,8 @@ export interface Starter {
   nature: Nature;
   moveset?: StarterMoveset;
   pokerus: boolean;
-  nickname?: string;
-  teraType?: PokemonType;
+  nickname?: string | undefined;
+  teraType?: PokemonType | undefined;
 }
 
 interface LanguageSetting {
diff --git a/test/testUtils/mocks/mockClock.ts b/test/testUtils/mocks/mockClock.ts
index 67f399ae41d..beea62daeba 100644
--- a/test/testUtils/mocks/mockClock.ts
+++ b/test/testUtils/mocks/mockClock.ts
@@ -17,6 +17,7 @@ export class MockClock extends Clock {
 
   addEvent(config: Phaser.Time.TimerEvent | Phaser.Types.Time.TimerEventConfig): Phaser.Time.TimerEvent {
     const cfg = { ...config, delay: this.overrideDelay ?? config.delay };
-    return super.addEvent(cfg);
+    // Type assertion needed to get around optional property shit
+    return super.addEvent(cfg as Phaser.Time.TimerEvent | Phaser.Types.Time.TimerEventConfig);
   }
 }
diff --git a/test/testUtils/phaseInterceptor.ts b/test/testUtils/phaseInterceptor.ts
index 9d046fc85ba..e271782abf7 100644
--- a/test/testUtils/phaseInterceptor.ts
+++ b/test/testUtils/phaseInterceptor.ts
@@ -69,7 +69,7 @@ export interface PromptHandler {
   phaseTarget?: string;
   mode?: UiMode;
   callback?: () => void;
-  expireFn?: () => void;
+  expireFn?: (() => void) | undefined;
   awaitingActionInput?: boolean;
 }
 
diff --git a/tsconfig.json b/tsconfig.json
index 6af3e9ce650..4e2bdfe2bc9 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -8,6 +8,7 @@
     "strictNullChecks": true,
     "sourceMap": false,
     "strict": false, // TODO: Enable this eventually
+    "exactOptionalPropertyTypes": true,
     "rootDir": ".",
     "baseUrl": "./src",
     "paths": {