perf: make ast fields optional instead of null

With esTreeNode, optional branches of the AST are represented by
explicit null values, and this was reflected in the Rollup AST nodes as
well since we copy these null values from the esTreeNode to the Rollup
AST node during parseNode.

This is potentially memory inefficient as nodes frequent optional fields
will take up 4 bytes of memory on each instance of that node. In
comparison, if we can avoid setting these values at all on our nodes,
we can save some additional memory.

This has the potential to create more hidden classes as each variation
of the node will have a different set of properties, but performance
does not really appear to regress much from what I can tell.

src/ast/nodes/ArrayExpression.ts

@@ -18,7 +18,7 @@ import { ObjectEntity, type ObjectProperty } from './shared/ObjectEntity';
 export default class ArrayExpression extends NodeBase {
 	declare elements: readonly (ExpressionNode | SpreadElement | null)[];
 	declare type: NodeType.tArrayExpression;
-	private objectEntity: ObjectEntity | null = null;
+	private objectEntity?: ObjectEntity = undefined;
 
 	deoptimizeArgumentsOnInteractionAtPath(
 		interaction: NodeInteraction,
@@ -80,7 +80,7 @@ export default class ArrayExpression extends NodeBase {
 	}
 
 	private getObjectEntity(): ObjectEntity {
-		if (this.objectEntity !== null) {
+		if (this.objectEntity !== undefined) {
 			return this.objectEntity;
 		}
 		const properties: ObjectProperty[] = [

src/ast/nodes/ArrowFunctionExpression.ts

@@ -19,7 +19,7 @@ export default class ArrowFunctionExpression extends FunctionBase {
 	declare preventChildBlockScope: true;
 	declare scope: ReturnValueScope;
 	declare type: NodeType.tArrowFunctionExpression;
-	protected objectEntity: ObjectEntity | null = null;
+	protected objectEntity?: ObjectEntity;
 
 	createScope(parentScope: Scope): void {
 		this.scope = new ReturnValueScope(parentScope, this.context);
@@ -69,7 +69,7 @@ export default class ArrowFunctionExpression extends FunctionBase {
 	}
 
 	protected getObjectEntity(): ObjectEntity {
-		if (this.objectEntity !== null) {
+		if (this.objectEntity !== undefined) {
 			return this.objectEntity;
 		}
 		return (this.objectEntity = new ObjectEntity([], OBJECT_PROTOTYPE));

src/ast/nodes/BreakStatement.ts

@@ -4,7 +4,7 @@ import type * as NodeType from './NodeType';
 import { StatementBase } from './shared/Node';
 
 export default class BreakStatement extends StatementBase {
-	declare label: Identifier | null;
+	declare label?: Identifier;
 	declare type: NodeType.tBreakStatement;
 
 	hasEffects(context: HasEffectsContext): boolean {

src/ast/nodes/CallExpression.ts

@@ -127,7 +127,7 @@ export default class CallExpression
 	protected getReturnExpression(
 		recursionTracker: PathTracker = SHARED_RECURSION_TRACKER
 	): [expression: ExpressionEntity, isPure: boolean] {
-		if (this.returnExpression === null) {
+		if (this.returnExpression === undefined) {
 			this.returnExpression = UNKNOWN_RETURN_EXPRESSION;
 			return (this.returnExpression = this.callee.getReturnExpressionWhenCalledAtPath(
 				EMPTY_PATH,

src/ast/nodes/CatchClause.ts

@@ -8,7 +8,7 @@ import type { PatternNode } from './shared/Pattern';
 
 export default class CatchClause extends NodeBase {
 	declare body: BlockStatement;
-	declare param: PatternNode | null;
+	declare param?: PatternNode;
 	declare preventChildBlockScope: true;
 	declare scope: CatchScope;
 	declare type: NodeType.tCatchClause;

src/ast/nodes/ClassDeclaration.ts

@@ -9,18 +9,18 @@ import ClassNode from './shared/ClassNode';
 import type { GenericEsTreeNode } from './shared/Node';
 
 export default class ClassDeclaration extends ClassNode {
-	declare id: IdentifierWithVariable | null;
+	declare id?: IdentifierWithVariable;
 	declare type: NodeType.tClassDeclaration;
 
 	initialise(): void {
 		super.initialise();
-		if (this.id !== null) {
+		if (this.id != undefined) {
 			this.id.variable.isId = true;
 		}
 	}
 
 	parseNode(esTreeNode: GenericEsTreeNode): void {
-		if (esTreeNode.id !== null) {
+		if (esTreeNode.id != undefined) {
 			this.id = new Identifier(
 				esTreeNode.id,
 				this,

src/ast/nodes/ConditionalExpression.ts

@@ -35,7 +35,7 @@ export default class ConditionalExpression extends NodeBase implements Deoptimiz
 	}
 
 	private expressionsToBeDeoptimized: DeoptimizableEntity[] = [];
-	private usedBranch: ExpressionNode | null = null;
+	private usedBranch?: ExpressionNode;
 
 	deoptimizeArgumentsOnInteractionAtPath(
 		interaction: NodeInteraction,
@@ -47,9 +47,9 @@ export default class ConditionalExpression extends NodeBase implements Deoptimiz
 	}
 
 	deoptimizeCache(): void {
-		if (this.usedBranch !== null) {
+		if (this.usedBranch !== undefined) {
 			const unusedBranch = this.usedBranch === this.consequent ? this.alternate : this.consequent;
-			this.usedBranch = null;
+			this.usedBranch = undefined;
 			unusedBranch.deoptimizePath(UNKNOWN_PATH);
 			const { expressionsToBeDeoptimized } = this;
 			this.expressionsToBeDeoptimized = EMPTY_ARRAY as unknown as DeoptimizableEntity[];
@@ -141,7 +141,11 @@ export default class ConditionalExpression extends NodeBase implements Deoptimiz
 	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren): void {
 		this.included = true;
 		const usedBranch = this.getUsedBranch();
-		if (includeChildrenRecursively || this.test.shouldBeIncluded(context) || usedBranch === null) {
+		if (
+			includeChildrenRecursively ||
+			this.test.shouldBeIncluded(context) ||
+			usedBranch === undefined
+		) {
 			this.test.include(context, includeChildrenRecursively);
 			this.consequent.include(context, includeChildrenRecursively);
 			this.alternate.include(context, includeChildrenRecursively);
@@ -210,7 +214,7 @@ export default class ConditionalExpression extends NodeBase implements Deoptimiz
 		this.isBranchResolutionAnalysed = true;
 		const testValue = this.test.getLiteralValueAtPath(EMPTY_PATH, SHARED_RECURSION_TRACKER, this);
 		return typeof testValue === 'symbol'
-			? null
+			? undefined
 			: (this.usedBranch = testValue ? this.consequent : this.alternate);
 	}
 }

src/ast/nodes/ContinueStatement.ts

@@ -4,7 +4,7 @@ import type * as NodeType from './NodeType';
 import { StatementBase } from './shared/Node';
 
 export default class ContinueStatement extends StatementBase {
-	declare label: Identifier | null;
+	declare label?: Identifier;
 	declare type: NodeType.tContinueStatement;
 
 	hasEffects(context: HasEffectsContext): boolean {

src/ast/nodes/ExportAllDeclaration.ts

@@ -8,7 +8,7 @@ import { NodeBase } from './shared/Node';
 
 export default class ExportAllDeclaration extends NodeBase {
 	declare assertions: ImportAttribute[];
-	declare exported: Identifier | null;
+	declare exported?: Identifier;
 	declare needsBoundaries: true;
 	declare source: Literal<string>;
 	declare type: NodeType.tExportAllDeclaration;

src/ast/nodes/ExportDefaultDeclaration.ts

@@ -68,7 +68,7 @@ export default class ExportDefaultDeclaration extends NodeBase {
 			this.renderNamedDeclaration(
 				code,
 				declarationStart,
-				this.declaration.id === null
+				this.declaration.id === undefined
 					? getFunctionIdInsertPosition(code.original, declarationStart)
 					: null,
 				options
@@ -77,7 +77,7 @@ export default class ExportDefaultDeclaration extends NodeBase {
 			this.renderNamedDeclaration(
 				code,
 				declarationStart,
-				this.declaration.id === null
+				this.declaration.id === undefined
 					? findFirstOccurrenceOutsideComment(code.original, 'class', start) + 'class'.length
 					: null,
 				options

src/ast/nodes/ExportNamedDeclaration.ts

@@ -12,9 +12,9 @@ import { type Node, NodeBase } from './shared/Node';
 
 export default class ExportNamedDeclaration extends NodeBase {
 	declare assertions: ImportAttribute[];
-	declare declaration: FunctionDeclaration | ClassDeclaration | VariableDeclaration | null;
+	declare declaration?: FunctionDeclaration | ClassDeclaration | VariableDeclaration;
 	declare needsBoundaries: true;
-	declare source: Literal<string> | null;
+	declare source?: Literal<string>;
 	declare specifiers: readonly ExportSpecifier[];
 	declare type: NodeType.tExportNamedDeclaration;
 
@@ -33,7 +33,7 @@ export default class ExportNamedDeclaration extends NodeBase {
 
 	render(code: MagicString, options: RenderOptions, nodeRenderOptions?: NodeRenderOptions): void {
 		const { start, end } = nodeRenderOptions as { end: number; start: number };
-		if (this.declaration === null) {
+		if (this.declaration === undefined) {
 			code.remove(start, end);
 		} else {
 			code.remove(this.start, this.declaration.start);

src/ast/nodes/ForStatement.ts

@@ -15,10 +15,10 @@ import { hasLoopBodyEffects, includeLoopBody } from './shared/loops';
 
 export default class ForStatement extends StatementBase {
 	declare body: StatementNode;
-	declare init: VariableDeclaration | ExpressionNode | null;
-	declare test: ExpressionNode | null;
+	declare init?: VariableDeclaration | ExpressionNode;
+	declare test?: ExpressionNode;
 	declare type: NodeType.tForStatement;
-	declare update: ExpressionNode | null;
+	declare update?: ExpressionNode;
 
 	createScope(parentScope: ChildScope): void {
 		this.scope = new BlockScope(parentScope, parentScope.context);

src/ast/nodes/FunctionDeclaration.ts

@@ -9,13 +9,13 @@ export default class FunctionDeclaration extends FunctionNode {
 
 	initialise(): void {
 		super.initialise();
-		if (this.id !== null) {
+		if (this.id != null) {
 			this.id.variable.isId = true;
 		}
 	}
 
 	parseNode(esTreeNode: GenericEsTreeNode): void {
-		if (esTreeNode.id !== null) {
+		if (esTreeNode.id != null) {
 			this.id = new Identifier(
 				esTreeNode.id,
 				this,

src/ast/nodes/Identifier.ts

@@ -43,21 +43,17 @@ const tdzVariableKinds = {
 export default class Identifier extends NodeBase implements PatternNode {
 	declare name: string;
 	declare type: NodeType.tIdentifier;
-	variable: Variable | null = null;
+	variable?: Variable;
 
 	private get isTDZAccess(): boolean | null {
 		if (!isFlagSet(this.flags, Flag.tdzAccessDefined)) {
 			return null;
 		}
 		return isFlagSet(this.flags, Flag.tdzAccess);
 	}
-	private set isTDZAccess(value: boolean | null) {
-		if (value === null) {
-			setFlag(this.flags, Flag.tdzAccessDefined, false);
-		} else {
-			setFlag(this.flags, Flag.tdzAccessDefined, true);
-			setFlag(this.flags, Flag.tdzAccess, value);
-		}
+	private set isTDZAccess(value: boolean) {
+		setFlag(this.flags, Flag.tdzAccessDefined, true);
+		setFlag(this.flags, Flag.tdzAccess, value);
 	}
 
 	addExportedVariables(
@@ -179,7 +175,7 @@ export default class Identifier extends NodeBase implements PatternNode {
 		switch (interaction.type) {
 			case INTERACTION_ACCESSED: {
 				return (
-					this.variable !== null &&
+					this.variable !== undefined &&
 					!this.isPureFunction(path) &&
 					this.getVariableRespectingTDZ()!.hasEffectsOnInteractionAtPath(path, interaction, context)
 				);
@@ -202,7 +198,7 @@ export default class Identifier extends NodeBase implements PatternNode {
 		if (!this.deoptimized) this.applyDeoptimizations();
 		if (!this.included) {
 			this.included = true;
-			if (this.variable !== null) {
+			if (this.variable !== undefined) {
 				this.context.includeVariableInModule(this.variable);
 			}
 		}
@@ -301,7 +297,7 @@ export default class Identifier extends NodeBase implements PatternNode {
 		);
 	}
 
-	private getVariableRespectingTDZ(): ExpressionEntity | null {
+	private getVariableRespectingTDZ(): ExpressionEntity | undefined {
 		if (this.isPossibleTDZ()) {
 			return UNKNOWN_EXPRESSION;
 		}

src/ast/nodes/IfStatement.ts

@@ -19,7 +19,7 @@ import {
 const unset = Symbol('unset');
 
 export default class IfStatement extends StatementBase implements DeoptimizableEntity {
-	declare alternate: StatementNode | null;
+	declare alternate?: StatementNode;
 	declare consequent: StatementNode;
 	declare test: ExpressionNode;
 	declare type: NodeType.tIfStatement;
@@ -43,7 +43,7 @@ export default class IfStatement extends StatementBase implements DeoptimizableE
 			// eslint-disable-next-line unicorn/consistent-destructuring
 			const consequentBrokenFlow = context.brokenFlow;
 			context.brokenFlow = brokenFlow;
-			if (this.alternate === null) return false;
+			if (this.alternate === undefined) return false;
 			if (this.alternate.hasEffects(context)) return true;
 			// eslint-disable-next-line unicorn/consistent-destructuring
 			context.brokenFlow = context.brokenFlow && consequentBrokenFlow;

src/ast/nodes/ImportExpression.ts

@@ -40,15 +40,15 @@ interface DynamicImportMechanism {
 //  default, keys.ts should be updated
 export default class ImportExpression extends NodeBase {
 	declare arguments: ObjectExpression[] | undefined;
-	inlineNamespace: NamespaceVariable | null = null;
+	inlineNamespace?: NamespaceVariable;
 	declare source: ExpressionNode;
 	declare type: NodeType.tImportExpression;
 
-	private assertions: string | null | true = null;
+	private assertions?: string | true;
 	private mechanism: DynamicImportMechanism | null = null;
-	private namespaceExportName: string | false | undefined = undefined;
-	private resolution: Module | ExternalModule | string | null = null;
-	private resolutionString: string | null = null;
+	private namespaceExportName?: string | false;
+	private resolution?: Module | ExternalModule | string;
+	private resolutionString?: string;
 
 	// Do not bind assertions
 	bind(): void {
@@ -226,30 +226,30 @@ export default class ImportExpression extends NodeBase {
 
 	setExternalResolution(
 		exportMode: 'none' | 'named' | 'default' | 'external',
-		resolution: Module | ExternalModule | string | null,
+		resolution: Module | ExternalModule | string | null | undefined,
 		options: NormalizedOutputOptions,
 		snippets: GenerateCodeSnippets,
 		pluginDriver: PluginDriver,
 		accessedGlobalsByScope: Map<ChildScope, Set<string>>,
 		resolutionString: string,
-		namespaceExportName: string | false | undefined,
-		assertions: string | null | true
+		namespaceExportName: string | false | null | undefined,
+		assertions: string | null | undefined | true
 	): void {
 		const { format } = options;
-		this.inlineNamespace = null;
-		this.resolution = resolution;
+		this.resolution = resolution ?? undefined;
 		this.resolutionString = resolutionString;
-		this.namespaceExportName = namespaceExportName;
-		this.assertions = assertions;
+		this.namespaceExportName = namespaceExportName ?? undefined;
+		this.assertions = assertions ?? undefined;
 		const accessedGlobals = [...(accessedImportGlobals[format] || [])];
-		let helper: string | null;
+		let helper: string | null = null;
 		({ helper, mechanism: this.mechanism } = this.getDynamicImportMechanismAndHelper(
-			resolution,
+			resolution ?? undefined,
 			exportMode,
 			options,
 			snippets,
 			pluginDriver
 		));
+
 		if (helper) {
 			accessedGlobals.push(helper);
 		}
@@ -265,7 +265,7 @@ export default class ImportExpression extends NodeBase {
 	protected applyDeoptimizations() {}
 
 	private getDynamicImportMechanismAndHelper(
-		resolution: Module | ExternalModule | string | null,
+		resolution: Module | ExternalModule | string | undefined,
 		exportMode: 'none' | 'named' | 'default' | 'external',
 		{
 			compact,
@@ -382,7 +382,7 @@ export default class ImportExpression extends NodeBase {
 }
 
 function getInteropHelper(
-	resolution: Module | ExternalModule | string | null,
+	resolution: Module | ExternalModule | string | undefined,
 	exportMode: 'none' | 'named' | 'default' | 'external',
 	interop: GetInterop
 ): string | null {

src/ast/nodes/Literal.ts

@@ -40,7 +40,7 @@ export default class Literal<T extends LiteralValue = LiteralValue> extends Node
 		if (
 			path.length > 0 ||
 			// unknown literals can also be null but do not start with an "n"
-			(this.value === null && this.context.code.charCodeAt(this.start) !== 110) ||
+			(this.value == null && this.context.code.charCodeAt(this.start) !== 110) ||
 			typeof this.value === 'bigint' ||
 			// to support shims for regular expressions
 			this.context.code.charCodeAt(this.start) === 47
@@ -64,7 +64,7 @@ export default class Literal<T extends LiteralValue = LiteralValue> extends Node
 	): boolean {
 		switch (interaction.type) {
 			case INTERACTION_ACCESSED: {
-				return path.length > (this.value === null ? 0 : 1);
+				return path.length > (this.value == null ? 0 : 1);
 			}
 			case INTERACTION_ASSIGNED: {
 				return true;