diff --git a/sim/dex-abilities.ts b/sim/dex-abilities.ts
index 882ec3ee71d7..c038d7c4b866 100644
--- a/sim/dex-abilities.ts
+++ b/sim/dex-abilities.ts
@@ -1,5 +1,5 @@
 import type {PokemonEventMethods, ConditionData} from './dex-conditions';
-import {BasicEffect, toID} from './dex-data';
+import {assignMissingFields, BasicEffect, toID} from './dex-data';
 import {Utils} from '../lib';
 
 interface AbilityEventMethods {
@@ -64,6 +64,7 @@ export class Ability extends BasicEffect implements Readonly<BasicEffect> {
 				this.gen = 3;
 			}
 		}
+		assignMissingFields(this, data);
 	}
 }
 
diff --git a/sim/dex-conditions.ts b/sim/dex-conditions.ts
index 9e95c69bd1fe..c7bd4be888d0 100644
--- a/sim/dex-conditions.ts
+++ b/sim/dex-conditions.ts
@@ -1,5 +1,5 @@
 import {Utils} from '../lib';
-import {BasicEffect, toID} from './dex-data';
+import {assignMissingFields, BasicEffect, toID} from './dex-data';
 import type {SecondaryEffect, MoveEventMethods} from './dex-moves';
 
 export interface EventMethods {
@@ -628,9 +628,8 @@ export class Condition extends BasicEffect implements
 
 	constructor(data: AnyObject) {
 		super(data);
-		// eslint-disable-next-line @typescript-eslint/no-this-alias
-		data = this;
 		this.effectType = (['Weather', 'Status'].includes(data.effectType) ? data.effectType : 'Condition');
+		assignMissingFields(this, data);
 	}
 }
 
diff --git a/sim/dex-data.ts b/sim/dex-data.ts
index e6eea3d3496f..c52b0e90d3b8 100644
--- a/sim/dex-data.ts
+++ b/sim/dex-data.ts
@@ -34,7 +34,19 @@ export function toID(text: any): ID {
 	/* eslint-enable @typescript-eslint/prefer-optional-chain */
 }
 
-export class BasicEffect implements EffectData {
+/**
+ * Like Object.assign but only assigns fields missing from self.
+ * Facilitates consistent field ordering in constructors.
+ * Modifies self in-place.
+ */
+export function assignMissingFields(self: AnyObject, data: AnyObject) {
+	for (const k in data) {
+		if (k in self) continue;
+		self[k] = data[k];
+	}
+}
+
+export abstract class BasicEffect implements EffectData {
 	/**
 	 * ID. This will be a lowercase version of the name with all the
 	 * non-alphanumeric characters removed. So, for instance, "Mr. Mime"
@@ -102,14 +114,11 @@ export class BasicEffect implements EffectData {
 	sourceEffect: string;
 
 	constructor(data: AnyObject) {
-		this.exists = true;
-		Object.assign(this, data);
-
 		this.name = Utils.getString(data.name).trim();
 		this.id = data.realMove ? toID(data.realMove) : toID(this.name); // Hidden Power hack
 		this.fullname = Utils.getString(data.fullname) || this.name;
 		this.effectType = Utils.getString(data.effectType) as EffectType || 'Condition';
-		this.exists = !!(this.exists && this.id);
+		this.exists = data.exists ?? !!this.id;
 		this.num = data.num || 0;
 		this.gen = data.gen || 0;
 		this.shortDesc = data.shortDesc || '';
@@ -134,14 +143,12 @@ export class Nature extends BasicEffect implements Readonly<BasicEffect & Nature
 	readonly minus?: StatIDExceptHP;
 	constructor(data: AnyObject) {
 		super(data);
-		// eslint-disable-next-line @typescript-eslint/no-this-alias
-		data = this;
-
 		this.fullname = `nature: ${this.name}`;
 		this.effectType = 'Nature';
 		this.gen = 3;
 		this.plus = data.plus || undefined;
 		this.minus = data.minus || undefined;
+		assignMissingFields(this, data);
 	}
 }
 
@@ -257,18 +264,16 @@ export class TypeInfo implements Readonly<TypeData> {
 	readonly HPdvs: SparseStatsTable;
 
 	constructor(data: AnyObject) {
-		this.exists = true;
-		Object.assign(this, data);
-
 		this.name = data.name;
 		this.id = data.id;
 		this.effectType = Utils.getString(data.effectType) as TypeInfoEffectType || 'Type';
-		this.exists = !!(this.exists && this.id);
+		this.exists = data.exists ?? !!this.id;
 		this.gen = data.gen || 0;
 		this.isNonstandard = data.isNonstandard || null;
 		this.damageTaken = data.damageTaken || {};
 		this.HPivs = data.HPivs || {};
 		this.HPdvs = data.HPdvs || {};
+		assignMissingFields(this, data);
 	}
 
 	toString() {
diff --git a/sim/dex-formats.ts b/sim/dex-formats.ts
index a70619fa877b..2e160117f2c4 100644
--- a/sim/dex-formats.ts
+++ b/sim/dex-formats.ts
@@ -1,5 +1,5 @@
 import {Utils} from '../lib';
-import {toID, BasicEffect} from './dex-data';
+import {assignMissingFields, toID, BasicEffect} from './dex-data';
 import {EventMethods} from './dex-conditions';
 import {SpeciesData} from './dex-species';
 import {Tags} from '../data/tags';
@@ -478,8 +478,6 @@ export class Format extends BasicEffect implements Readonly<BasicEffect> {
 
 	constructor(data: AnyObject) {
 		super(data);
-		// eslint-disable-next-line @typescript-eslint/no-this-alias
-		data = this;
 
 		this.mod = Utils.getString(data.mod) || 'gen9';
 		this.effectType = Utils.getString(data.effectType) as FormatEffectType || 'Format';
@@ -496,6 +494,7 @@ export class Format extends BasicEffect implements Readonly<BasicEffect> {
 		this.onBegin = data.onBegin || undefined;
 		this.noLog = !!data.noLog;
 		this.playerCount = (this.gameType === 'multi' || this.gameType === 'freeforall' ? 4 : 2);
+		assignMissingFields(this, data);
 	}
 }
 
diff --git a/sim/dex-items.ts b/sim/dex-items.ts
index 828a44f4f8d3..9c175d72fde8 100644
--- a/sim/dex-items.ts
+++ b/sim/dex-items.ts
@@ -1,5 +1,5 @@
 import type {PokemonEventMethods, ConditionData} from './dex-conditions';
-import {BasicEffect, toID} from './dex-data';
+import {assignMissingFields, BasicEffect, toID} from './dex-data';
 import {Utils} from '../lib';
 
 interface FlingData {
@@ -107,8 +107,6 @@ export class Item extends BasicEffect implements Readonly<BasicEffect> {
 
 	constructor(data: AnyObject) {
 		super(data);
-		// eslint-disable-next-line @typescript-eslint/no-this-alias
-		data = this;
 
 		this.fullname = `item: ${this.name}`;
 		this.effectType = 'Item';
@@ -152,6 +150,8 @@ export class Item extends BasicEffect implements Readonly<BasicEffect> {
 		if (this.onDrive) this.fling = {basePower: 70};
 		if (this.megaStone) this.fling = {basePower: 80};
 		if (this.onMemory) this.fling = {basePower: 50};
+
+		assignMissingFields(this, data);
 	}
 }
 
diff --git a/sim/dex-moves.ts b/sim/dex-moves.ts
index dcd66b11039a..7853c5e2135d 100644
--- a/sim/dex-moves.ts
+++ b/sim/dex-moves.ts
@@ -1,6 +1,6 @@
 import {Utils} from '../lib';
 import type {ConditionData} from './dex-conditions';
-import {BasicEffect, toID} from './dex-data';
+import {assignMissingFields, BasicEffect, toID} from './dex-data';
 
 /**
  * Describes the acceptable target(s) of a move.
@@ -481,8 +481,6 @@ export class DataMove extends BasicEffect implements Readonly<BasicEffect & Move
 
 	constructor(data: AnyObject) {
 		super(data);
-		// eslint-disable-next-line @typescript-eslint/no-this-alias
-		data = this;
 
 		this.fullname = `move: ${this.name}`;
 		this.effectType = 'Move';
@@ -520,7 +518,7 @@ export class DataMove extends BasicEffect implements Readonly<BasicEffect & Move
 		this.forceSTAB = !!data.forceSTAB;
 		this.volatileStatus = typeof data.volatileStatus === 'string' ? (data.volatileStatus as ID) : undefined;
 
-		if (this.category !== 'Status' && !this.maxMove && this.id !== 'struggle') {
+		if (this.category !== 'Status' && !data.maxMove && this.id !== 'struggle') {
 			this.maxMove = {basePower: 1};
 			if (this.isMax || this.isZ) {
 				// already initialized to 1
@@ -560,10 +558,10 @@ export class DataMove extends BasicEffect implements Readonly<BasicEffect & Move
 				}
 			}
 		}
-		if (this.category !== 'Status' && !this.zMove && !this.isZ && !this.isMax && this.id !== 'struggle') {
+		if (this.category !== 'Status' && !data.zMove && !this.isZ && !this.isMax && this.id !== 'struggle') {
 			let basePower = this.basePower;
 			this.zMove = {};
-			if (Array.isArray(this.multihit)) basePower *= 3;
+			if (Array.isArray(data.multihit)) basePower *= 3;
 			if (!basePower) {
 				this.zMove.basePower = 100;
 			} else if (basePower >= 140) {
@@ -611,6 +609,7 @@ export class DataMove extends BasicEffect implements Readonly<BasicEffect & Move
 				this.gen = 1;
 			}
 		}
+		assignMissingFields(this, data);
 	}
 }
 
diff --git a/sim/dex-species.ts b/sim/dex-species.ts
index 297cdc45db01..ebfbf834fedf 100644
--- a/sim/dex-species.ts
+++ b/sim/dex-species.ts
@@ -1,5 +1,5 @@
+import {assignMissingFields, BasicEffect, toID} from './dex-data';
 import {Utils} from '../lib';
-import {toID, BasicEffect} from './dex-data';
 
 interface SpeciesAbility {
 	0: string;
@@ -273,8 +273,6 @@ export class Species extends BasicEffect implements Readonly<BasicEffect & Speci
 
 	constructor(data: AnyObject) {
 		super(data);
-		// eslint-disable-next-line @typescript-eslint/no-this-alias
-		data = this;
 
 		this.fullname = `pokemon: ${data.name}`;
 		this.effectType = 'Pokemon';
@@ -306,7 +304,7 @@ export class Species extends BasicEffect implements Readonly<BasicEffect & Speci
 			this.gender === 'N' ? {M: 0, F: 0} :
 			{M: 0.5, F: 0.5});
 		this.requiredItem = data.requiredItem || undefined;
-		this.requiredItems = this.requiredItems || (this.requiredItem ? [this.requiredItem] : undefined);
+		this.requiredItems = data.requiredItems || (this.requiredItem ? [this.requiredItem] : undefined);
 		this.baseStats = data.baseStats || {hp: 0, atk: 0, def: 0, spa: 0, spd: 0, spe: 0};
 		this.bst = this.baseStats.hp + this.baseStats.atk + this.baseStats.def +
 			this.baseStats.spa + this.baseStats.spd + this.baseStats.spe;
@@ -325,8 +323,8 @@ export class Species extends BasicEffect implements Readonly<BasicEffect & Speci
 		this.battleOnly = data.battleOnly || (this.isMega ? this.baseSpecies : undefined);
 		this.changesFrom = data.changesFrom ||
 			(this.battleOnly !== this.baseSpecies ? this.battleOnly : this.baseSpecies);
+		if (Array.isArray(this.changesFrom)) this.changesFrom = this.changesFrom[0];
 		this.pokemonGoData = data.pokemonGoData || undefined;
-		if (Array.isArray(data.changesFrom)) this.changesFrom = data.changesFrom[0];
 
 		if (!this.gen && this.num >= 1) {
 			if (this.num >= 906 || this.forme.includes('Paldea')) {
@@ -353,6 +351,7 @@ export class Species extends BasicEffect implements Readonly<BasicEffect & Speci
 				this.gen = 1;
 			}
 		}
+		assignMissingFields(this, data);
 	}
 }