diff --git a/src/Bundle.ts b/src/Bundle.ts
index 5969c4f2891..050af731d52 100644
--- a/src/Bundle.ts
+++ b/src/Bundle.ts
@@ -267,7 +267,7 @@ function getIncludedModules(modulesById: ReadonlyMap<string, Module | ExternalMo
 }
 
 function getAbsoluteEntryModulePaths(
-	includedModules: Module[],
+	includedModules: readonly Module[],
 	preserveModules: boolean
 ): string[] {
 	const absoluteEntryModulePaths: string[] = [];
diff --git a/src/ast/scopes/ClassBodyScope.ts b/src/ast/scopes/ClassBodyScope.ts
index 40392c24ee1..d92e63c4b08 100644
--- a/src/ast/scopes/ClassBodyScope.ts
+++ b/src/ast/scopes/ClassBodyScope.ts
@@ -6,8 +6,8 @@ import ChildScope from './ChildScope';
 import type Scope from './Scope';
 
 export default class ClassBodyScope extends ChildScope {
-	instanceScope: ChildScope;
-	thisVariable: LocalVariable;
+	readonly instanceScope: ChildScope;
+	readonly thisVariable: LocalVariable;
 
 	constructor(parent: Scope, classNode: ClassNode, context: AstContext) {
 		super(parent);
diff --git a/src/ast/scopes/ModuleScope.ts b/src/ast/scopes/ModuleScope.ts
index d1787db60ab..c976e3ff260 100644
--- a/src/ast/scopes/ModuleScope.ts
+++ b/src/ast/scopes/ModuleScope.ts
@@ -10,7 +10,7 @@ import ChildScope from './ChildScope';
 import type GlobalScope from './GlobalScope';
 
 export default class ModuleScope extends ChildScope {
-	context: AstContext;
+	readonly context: AstContext;
 	declare parent: GlobalScope;
 
 	constructor(parent: GlobalScope, context: AstContext) {
diff --git a/src/ast/scopes/ParameterScope.ts b/src/ast/scopes/ParameterScope.ts
index 3ac0a8c576f..9d26224f472 100644
--- a/src/ast/scopes/ParameterScope.ts
+++ b/src/ast/scopes/ParameterScope.ts
@@ -10,9 +10,8 @@ import type Scope from './Scope';
 
 export default class ParameterScope extends ChildScope {
 	readonly hoistedBodyVarScope: ChildScope;
-
 	protected parameters: readonly LocalVariable[][] = [];
-	private context: AstContext;
+	private readonly context: AstContext;
 	private hasRest = false;
 
 	constructor(parent: Scope, context: AstContext) {
diff --git a/src/ast/scopes/ReturnValueScope.ts b/src/ast/scopes/ReturnValueScope.ts
index dead794d212..be4a522f332 100644
--- a/src/ast/scopes/ReturnValueScope.ts
+++ b/src/ast/scopes/ReturnValueScope.ts
@@ -4,7 +4,7 @@ import ParameterScope from './ParameterScope';
 
 export default class ReturnValueScope extends ParameterScope {
 	private returnExpression: ExpressionEntity | null = null;
-	private returnExpressions: ExpressionEntity[] = [];
+	private readonly returnExpressions: ExpressionEntity[] = [];
 
 	addReturnExpression(expression: ExpressionEntity): void {
 		this.returnExpressions.push(expression);
diff --git a/src/ast/scopes/Scope.ts b/src/ast/scopes/Scope.ts
index cd1dfd5eaec..9aff5a2c811 100644
--- a/src/ast/scopes/Scope.ts
+++ b/src/ast/scopes/Scope.ts
@@ -7,8 +7,8 @@ import type Variable from '../variables/Variable';
 import type ChildScope from './ChildScope';
 
 export default class Scope {
-	children: ChildScope[] = [];
-	variables = new Map<string, Variable>();
+	readonly children: ChildScope[] = [];
+	readonly variables = new Map<string, Variable>();
 
 	addDeclaration(
 		identifier: Identifier,
diff --git a/src/ast/scopes/TrackingScope.ts b/src/ast/scopes/TrackingScope.ts
index f1f40b878c0..f90cf4d42c8 100644
--- a/src/ast/scopes/TrackingScope.ts
+++ b/src/ast/scopes/TrackingScope.ts
@@ -5,7 +5,7 @@ import type LocalVariable from '../variables/LocalVariable';
 import BlockScope from './BlockScope';
 
 export default class TrackingScope extends BlockScope {
-	hoistedDeclarations: Identifier[] = [];
+	readonly hoistedDeclarations: Identifier[] = [];
 
 	addDeclaration(
 		identifier: Identifier,
diff --git a/src/ast/variables/ExportShimVariable.ts b/src/ast/variables/ExportShimVariable.ts
index ef7ebe5efff..f473c49c875 100644
--- a/src/ast/variables/ExportShimVariable.ts
+++ b/src/ast/variables/ExportShimVariable.ts
@@ -3,7 +3,7 @@ import { MISSING_EXPORT_SHIM_VARIABLE } from '../../utils/variableNames';
 import Variable from './Variable';
 
 export default class ExportShimVariable extends Variable {
-	module: Module;
+	readonly module: Module;
 
 	constructor(module: Module) {
 		super(MISSING_EXPORT_SHIM_VARIABLE);
diff --git a/src/ast/variables/ExternalVariable.ts b/src/ast/variables/ExternalVariable.ts
index 8119a3d29d1..ec2d465746c 100644
--- a/src/ast/variables/ExternalVariable.ts
+++ b/src/ast/variables/ExternalVariable.ts
@@ -6,8 +6,8 @@ import type { ObjectPath } from '../utils/PathTracker';
 import Variable from './Variable';
 
 export default class ExternalVariable extends Variable {
-	isNamespace: boolean;
-	module: ExternalModule;
+	readonly isNamespace: boolean;
+	readonly module: ExternalModule;
 	referenced = false;
 
 	constructor(module: ExternalModule, name: string) {
diff --git a/src/ast/variables/NamespaceVariable.ts b/src/ast/variables/NamespaceVariable.ts
index e82e3543973..a618346ff4c 100644
--- a/src/ast/variables/NamespaceVariable.ts
+++ b/src/ast/variables/NamespaceVariable.ts
@@ -15,9 +15,9 @@ import { SymbolToStringTag, UNKNOWN_PATH } from '../utils/PathTracker';
 import Variable from './Variable';
 
 export default class NamespaceVariable extends Variable {
-	context: AstContext;
+	readonly context: AstContext;
 	declare isNamespace: true;
-	module: Module;
+	readonly module: Module;
 
 	private memberVariables: { [name: string]: Variable } | null = null;
 	private mergedNamespaces: readonly Variable[] = [];
diff --git a/src/ast/variables/SyntheticNamedExportVariable.ts b/src/ast/variables/SyntheticNamedExportVariable.ts
index f7bcc50dc2c..84384c54010 100644
--- a/src/ast/variables/SyntheticNamedExportVariable.ts
+++ b/src/ast/variables/SyntheticNamedExportVariable.ts
@@ -4,9 +4,9 @@ import ExportDefaultVariable from './ExportDefaultVariable';
 import Variable from './Variable';
 
 export default class SyntheticNamedExportVariable extends Variable {
-	context: AstContext;
-	module: Module;
-	syntheticNamespace: Variable;
+	readonly context: AstContext;
+	readonly module: Module;
+	readonly syntheticNamespace: Variable;
 
 	private baseVariable: Variable | null = null;
 
diff --git a/src/finalisers/cjs.ts b/src/finalisers/cjs.ts
index 093cfed9ba4..28fc9382edf 100644
--- a/src/finalisers/cjs.ts
+++ b/src/finalisers/cjs.ts
@@ -73,7 +73,7 @@ export default function cjs(
 }
 
 function getImportBlock(
-	dependencies: ChunkDependency[],
+	dependencies: readonly ChunkDependency[],
 	{ _, cnst, n }: GenerateCodeSnippets,
 	compact: boolean
 ): string {
diff --git a/src/finalisers/es.ts b/src/finalisers/es.ts
index f40cf512b7d..ac49a9113c6 100644
--- a/src/finalisers/es.ts
+++ b/src/finalisers/es.ts
@@ -32,7 +32,10 @@ export default function es(
 	magicString.trim();
 }
 
-function getImportBlock(dependencies: ChunkDependency[], { _ }: GenerateCodeSnippets): string[] {
+function getImportBlock(
+	dependencies: readonly ChunkDependency[],
+	{ _ }: GenerateCodeSnippets
+): string[] {
 	const importBlock: string[] = [];
 	for (const { importPath, reexports, imports, name, assertions } of dependencies) {
 		const assertion = assertions ? `${_}assert${_}${assertions}` : '';
diff --git a/src/finalisers/shared/getCompleteAmdId.ts b/src/finalisers/shared/getCompleteAmdId.ts
index fec3d235300..c6fb4d0074e 100644
--- a/src/finalisers/shared/getCompleteAmdId.ts
+++ b/src/finalisers/shared/getCompleteAmdId.ts
@@ -9,5 +9,5 @@ export default function getCompleteAmdId(
 		return `${options.basePath ? options.basePath + '/' : ''}${removeJsExtension(chunkId)}`;
 	}
 
-	return options.id || '';
+	return options.id ?? '';
 }
diff --git a/src/finalisers/shared/getExportBlock.ts b/src/finalisers/shared/getExportBlock.ts
index 2fc93be2738..9b6a363f2c8 100644
--- a/src/finalisers/shared/getExportBlock.ts
+++ b/src/finalisers/shared/getExportBlock.ts
@@ -10,7 +10,7 @@ import {
 
 export function getExportBlock(
 	exports: ChunkExports,
-	dependencies: ChunkDependency[],
+	dependencies: readonly ChunkDependency[],
 	namedExportsMode: boolean,
 	interop: GetInterop,
 	snippets: GenerateCodeSnippets,
@@ -117,7 +117,7 @@ export function getExportBlock(
 
 function getSingleDefaultExport(
 	exports: ChunkExports,
-	dependencies: ChunkDependency[],
+	dependencies: readonly ChunkDependency[],
 	interop: GetInterop,
 	externalLiveBindings: boolean,
 	getPropertyAccess: (name: string) => string
diff --git a/src/utils/FileEmitter.ts b/src/utils/FileEmitter.ts
index 9faa59d5c5f..bcb7938fbcf 100644
--- a/src/utils/FileEmitter.ts
+++ b/src/utils/FileEmitter.ts
@@ -246,8 +246,11 @@ export class FileEmitter {
 		bundle: OutputBundleWithPlaceholders,
 		outputOptions: NormalizedOutputOptions
 	): void => {
-		const fileNamesBySource = new Map();
-		const output = (this.output = { bundle, fileNamesBySource, outputOptions });
+		const output = (this.output = {
+			bundle,
+			fileNamesBySource: new Map<string, string>(),
+			outputOptions
+		});
 		for (const emittedFile of this.filesByReferenceId.values()) {
 			if (emittedFile.fileName) {
 				reserveFileNameInBundle(emittedFile.fileName, output, this.options.onwarn);
@@ -265,16 +268,13 @@ export class FileEmitter {
 	}
 
 	private assignReferenceId(file: ConsumedFile, idBase: string): string {
-		let referenceId: string | undefined;
+		let referenceId = idBase;
 
 		do {
-			referenceId = createHash()
-				.update(referenceId || idBase)
-				.digest('hex')
-				.slice(0, 8);
+			referenceId = createHash().update(referenceId).digest('hex').slice(0, 8);
 		} while (
 			this.filesByReferenceId.has(referenceId) ||
-			this.outputFileEmitters.some(({ filesByReferenceId }) => filesByReferenceId.has(referenceId!))
+			this.outputFileEmitters.some(({ filesByReferenceId }) => filesByReferenceId.has(referenceId))
 		);
 		this.filesByReferenceId.set(referenceId, file);
 		for (const { filesByReferenceId } of this.outputFileEmitters) {