diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index e209041f9f..197dc4ccbd 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -28090,7 +28090,7 @@ namespace ts {
             return ts.typeHasCallOrConstructSignatures(type, checker);
         }
 
-        function getRootSymbols(symbol: Symbol): Symbol[] {
+        function getRootSymbols(symbol: Symbol): ReadonlyArray<Symbol> {
             const roots = getImmediateRootSymbols(symbol);
             return roots ? flatMap(roots, getRootSymbols) : [symbol];
         }
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 7e9813658f..6288f30b14 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -511,12 +511,27 @@ namespace ts {
      * @param array The array to map.
      * @param mapfn The callback used to map the result into one or more values.
      */
-    export function flatMap<T, U>(array: ReadonlyArray<T>, mapfn: (x: T, i: number) => U | ReadonlyArray<U> | undefined): U[];
-    export function flatMap<T, U>(array: ReadonlyArray<T> | undefined, mapfn: (x: T, i: number) => U | ReadonlyArray<U> | undefined): U[] | undefined;
-    export function flatMap<T, U>(array: ReadonlyArray<T> | undefined, mapfn: (x: T, i: number) => U | ReadonlyArray<U> | undefined): U[] | undefined {
+    export function flatMap<T, U>(array: ReadonlyArray<T> | undefined, mapfn: (x: T, i: number) => U | ReadonlyArray<U> | undefined): ReadonlyArray<U> {
         let result: U[] | undefined;
         if (array) {
-            result = [];
+            for (let i = 0; i < array.length; i++) {
+                const v = mapfn(array[i], i);
+                if (v) {
+                    if (isArray(v)) {
+                        result = addRange(result, v);
+                    }
+                    else {
+                        result = append(result, v);
+                    }
+                }
+            }
+        }
+        return result || emptyArray;
+    }
+
+    export function flatMapToMutable<T, U>(array: ReadonlyArray<T> | undefined, mapfn: (x: T, i: number) => U | ReadonlyArray<U> | undefined): U[] {
+        const result: U[] = [];
+        if (array) {
             for (let i = 0; i < array.length; i++) {
                 const v = mapfn(array[i], i);
                 if (v) {
diff --git a/src/compiler/transformers/declarations.ts b/src/compiler/transformers/declarations.ts
index 00e787c421..9ccca4732e 100644
--- a/src/compiler/transformers/declarations.ts
+++ b/src/compiler/transformers/declarations.ts
@@ -1043,7 +1043,7 @@ namespace ts {
                     const modifiers = createNodeArray(ensureModifiers(input, isPrivate));
                     const typeParameters = ensureTypeParams(input, input.typeParameters);
                     const ctor = getFirstConstructorWithBody(input);
-                    let parameterProperties: PropertyDeclaration[] | undefined;
+                    let parameterProperties: ReadonlyArray<PropertyDeclaration> | undefined;
                     if (ctor) {
                         const oldDiag = getSymbolAccessibilityDiagnostic;
                         parameterProperties = compact(flatMap(ctor.parameters, param => {
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index e7d20bfc24..aeb310671e 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2793,7 +2793,7 @@ namespace ts {
     export interface ParseConfigHost {
         useCaseSensitiveFileNames: boolean;
 
-        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): string[];
+        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): ReadonlyArray<string>;
 
         /**
          * Gets a value indicating whether the specified path exists and is a file.
@@ -3079,7 +3079,7 @@ namespace ts {
 
         getFullyQualifiedName(symbol: Symbol): string;
         getAugmentedPropertiesOfType(type: Type): Symbol[];
-        getRootSymbols(symbol: Symbol): Symbol[];
+        getRootSymbols(symbol: Symbol): ReadonlyArray<Symbol>;
         getContextualType(node: Expression): Type | undefined;
         /* @internal */ getContextualTypeForObjectLiteralElement(element: ObjectLiteralElementLike): Type | undefined;
         /* @internal */ getContextualTypeForArgumentAtIndex(call: CallLikeExpression, argIndex: number): Type | undefined;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index fa3885d4b7..96b9c3c2f6 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -3038,7 +3038,7 @@ namespace ts {
                 return fileDiagnostics.get(fileName) || [];
             }
 
-            const fileDiags: Diagnostic[] = flatMap(filesWithDiagnostics, f => fileDiagnostics.get(f));
+            const fileDiags: Diagnostic[] = flatMapToMutable(filesWithDiagnostics, f => fileDiagnostics.get(f));
             if (!nonFileDiagnostics.length) {
                 return fileDiags;
             }
@@ -5234,7 +5234,7 @@ namespace ts {
         }
         if (isJSDocTypeAlias(node)) {
             Debug.assert(node.parent.kind === SyntaxKind.JSDocComment);
-            return flatMap(node.parent.tags, tag => isJSDocTemplateTag(tag) ? tag.typeParameters : undefined) as ReadonlyArray<TypeParameterDeclaration>;
+            return flatMap(node.parent.tags, tag => isJSDocTemplateTag(tag) ? tag.typeParameters : undefined);
         }
         if (node.typeParameters) {
             return node.typeParameters;
@@ -7777,7 +7777,7 @@ namespace ts {
         return `^(${pattern})${terminator}`;
     }
 
-    export function getRegularExpressionsForWildcards(specs: ReadonlyArray<string> | undefined, basePath: string, usage: "files" | "directories" | "exclude"): string[] | undefined {
+    export function getRegularExpressionsForWildcards(specs: ReadonlyArray<string> | undefined, basePath: string, usage: "files" | "directories" | "exclude"): ReadonlyArray<string> | undefined {
         if (specs === undefined || specs.length === 0) {
             return undefined;
         }
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index 82ef60e7fb..a3721dd29f 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -491,7 +491,7 @@ namespace FourSlash {
             ];
         }
 
-        private getAllDiagnostics(): ts.Diagnostic[] {
+        private getAllDiagnostics(): ReadonlyArray<ts.Diagnostic> {
             return ts.flatMap(this.languageServiceAdapterHost.getFilenames(), fileName => {
                 if (!ts.isAnySupportedFileExtension(fileName)) {
                     return [];
@@ -532,7 +532,7 @@ namespace FourSlash {
                 predicate(start!, start! + length!, startMarker.position, endMarker === undefined ? undefined : endMarker.position)); // TODO: GH#18217
         }
 
-        private printErrorLog(expectErrors: boolean, errors: ts.Diagnostic[]) {
+        private printErrorLog(expectErrors: boolean, errors: ReadonlyArray<ts.Diagnostic>): void {
             if (expectErrors) {
                 Harness.IO.log("Expected error not found.  Error list is:");
             }
@@ -613,7 +613,7 @@ namespace FourSlash {
             this.verifyGoToX(arg0, endMarkerNames, () => this.getGoToDefinitionAndBoundSpan());
         }
 
-        private getGoToDefinition(): ts.DefinitionInfo[] {
+        private getGoToDefinition(): ReadonlyArray<ts.DefinitionInfo> {
             return this.languageService.getDefinitionAtPosition(this.activeFile.fileName, this.currentCaretPosition)!;
         }
 
@@ -626,7 +626,7 @@ namespace FourSlash {
                 this.languageService.getTypeDefinitionAtPosition(this.activeFile.fileName, this.currentCaretPosition));
         }
 
-        private verifyGoToX(arg0: any, endMarkerNames: ArrayOrSingle<string> | undefined, getDefs: () => ts.DefinitionInfo[] | ts.DefinitionInfoAndBoundSpan | undefined) {
+        private verifyGoToX(arg0: any, endMarkerNames: ArrayOrSingle<string> | undefined, getDefs: () => ReadonlyArray<ts.DefinitionInfo> | ts.DefinitionInfoAndBoundSpan | undefined) {
             if (endMarkerNames) {
                 this.verifyGoToXPlain(arg0, endMarkerNames, getDefs);
             }
@@ -646,7 +646,7 @@ namespace FourSlash {
             }
         }
 
-        private verifyGoToXPlain(startMarkerNames: ArrayOrSingle<string>, endMarkerNames: ArrayOrSingle<string>, getDefs: () => ts.DefinitionInfo[] | ts.DefinitionInfoAndBoundSpan | undefined) {
+        private verifyGoToXPlain(startMarkerNames: ArrayOrSingle<string>, endMarkerNames: ArrayOrSingle<string>, getDefs: () => ReadonlyArray<ts.DefinitionInfo> | ts.DefinitionInfoAndBoundSpan | undefined) {
             for (const start of toArray(startMarkerNames)) {
                 this.verifyGoToXSingle(start, endMarkerNames, getDefs);
             }
@@ -1890,7 +1890,7 @@ Actual: ${stringify(fullActual)}`);
 
         public verifyRangesInImplementationList(markerName: string) {
             this.goToMarker(markerName);
-            const implementations: ImplementationLocationInformation[] = this.languageService.getImplementationAtPosition(this.activeFile.fileName, this.currentCaretPosition)!;
+            const implementations: ReadonlyArray<ImplementationLocationInformation> = this.languageService.getImplementationAtPosition(this.activeFile.fileName, this.currentCaretPosition)!;
             if (!implementations || !implementations.length) {
                 this.raiseError("verifyRangesInImplementationList failed - expected to find at least one implementation location but got 0");
             }
@@ -2383,7 +2383,7 @@ Actual: ${stringify(fullActual)}`);
          * Rerieves a codefix satisfying the parameters, or undefined if no such codefix is found.
          * @param fileName Path to file where error should be retrieved from.
          */
-        private getCodeFixes(fileName: string, errorCode?: number, preferences: ts.UserPreferences = ts.emptyOptions): ts.CodeFixAction[] {
+        private getCodeFixes(fileName: string, errorCode?: number, preferences: ts.UserPreferences = ts.emptyOptions): ReadonlyArray<ts.CodeFixAction> {
             const diagnosticsForCodeFix = this.getDiagnostics(fileName, /*includeSuggestions*/ true).map(diagnostic => ({
                 start: diagnostic.start,
                 length: diagnostic.length,
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 5896ff3a78..c19c1ab141 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -490,7 +490,7 @@ namespace Harness {
         getExecutingFilePath(): string;
         getWorkspaceRoot(): string;
         exit(exitCode?: number): void;
-        readDirectory(path: string, extension?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[];
+        readDirectory(path: string, extension?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): ReadonlyArray<string>;
         getAccessibleFileSystemEntries(dirname: string): ts.FileSystemEntries;
         tryEnableSourceMapsForHost?(): void;
         getEnvironmentVariable?(name: string): string;
diff --git a/src/server/project.ts b/src/server/project.ts
index b87ddc9fb7..ecf62b724e 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -548,7 +548,7 @@ namespace ts.server {
         }
 
         getExternalFiles(): SortedReadonlyArray<string> {
-            return toSortedArray(flatMap(this.plugins, plugin => {
+            return toSortedArray(flatMapToMutable(this.plugins, plugin => {
                 if (typeof plugin.getExternalFiles !== "function") return;
                 try {
                     return plugin.getExternalFiles(this);
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index 14b2fa7c43..81b7af72e5 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -1023,7 +1023,7 @@ namespace ts.server.protocol {
         /**
          * The file locations referencing the symbol.
          */
-        refs: ReferencesResponseItem[];
+        refs: ReadonlyArray<ReferencesResponseItem>;
 
         /**
          * The name of the symbol.
diff --git a/src/server/session.ts b/src/server/session.ts
index efd369edb6..54caf953ee 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -267,7 +267,7 @@ namespace ts.server {
         projects: Projects,
         action: (project: Project, value: T) => ReadonlyArray<U> | U | undefined,
     ): U[] {
-        const outputs = flatMap(isArray(projects) ? projects : projects.projects, project => action(project, defaultValue));
+        const outputs = flatMapToMutable(isArray(projects) ? projects : projects.projects, project => action(project, defaultValue));
         if (!isArray(projects) && projects.symLinkedProjects) {
             projects.symLinkedProjects.forEach((projects, path) => {
                 const value = getValue(path as Path);
@@ -1230,7 +1230,7 @@ namespace ts.server {
                 const nameSpan = nameInfo && nameInfo.textSpan;
                 const symbolStartOffset = nameSpan ? scriptInfo.positionToLineOffset(nameSpan.start).offset : 0;
                 const symbolName = nameSpan ? scriptInfo.getSnapshot().getText(nameSpan.start, textSpanEnd(nameSpan)) : "";
-                const refs: protocol.ReferencesResponseItem[] = flatMap(references, referencedSymbol =>
+                const refs: ReadonlyArray<protocol.ReferencesResponseItem> = flatMap(references, referencedSymbol =>
                     referencedSymbol.references.map(({ fileName, textSpan, isWriteAccess, isDefinition }): protocol.ReferencesResponseItem => {
                         const scriptInfo = Debug.assertDefined(this.projectService.getScriptInfo(fileName));
                         const start = scriptInfo.positionToLineOffset(textSpan.start);
diff --git a/src/services/codeFixProvider.ts b/src/services/codeFixProvider.ts
index ad4e37974b..8d42fdcc6c 100644
--- a/src/services/codeFixProvider.ts
+++ b/src/services/codeFixProvider.ts
@@ -62,7 +62,7 @@ namespace ts {
             return arrayFrom(errorCodeToFixes.keys());
         }
 
-        export function getFixes(context: CodeFixContext): CodeFixAction[] {
+        export function getFixes(context: CodeFixContext): ReadonlyArray<CodeFixAction> {
             return flatMap(errorCodeToFixes.get(String(context.errorCode)) || emptyArray, f => f.getCodeActions(context));
         }
 
diff --git a/src/services/codefixes/importFixes.ts b/src/services/codefixes/importFixes.ts
index 2f6d68c52f..14bfc8b276 100644
--- a/src/services/codefixes/importFixes.ts
+++ b/src/services/codefixes/importFixes.ts
@@ -289,7 +289,7 @@ namespace ts.codefix {
                 // `position` should only be undefined at a missing jsx namespace, in which case we shouldn't be looking for pure types.
                 exportedSymbolIsTypeOnly && isJs ? { kind: ImportFixKind.ImportType, moduleSpecifier, position: Debug.assertDefined(position) } : { kind: ImportFixKind.AddNew, moduleSpecifier, importKind }));
         // Sort to keep the shortest paths first
-        return choicesForEachExportingModule.sort((a, b) => a.moduleSpecifier.length - b.moduleSpecifier.length);
+        return sort(choicesForEachExportingModule, (a, b) => a.moduleSpecifier.length - b.moduleSpecifier.length);
     }
 
     function getFixesForAddImport(
diff --git a/src/services/codefixes/inferFromUsage.ts b/src/services/codefixes/inferFromUsage.ts
index cef820bbf5..997f0988c3 100644
--- a/src/services/codefixes/inferFromUsage.ts
+++ b/src/services/codefixes/inferFromUsage.ts
@@ -228,7 +228,7 @@ namespace ts.codefix {
 
     function addJSDocTags(changes: textChanges.ChangeTracker, sourceFile: SourceFile, parent: HasJSDoc, newTags: ReadonlyArray<JSDocTag>): void {
         const comments = mapDefined(parent.jsDoc, j => j.comment);
-        const oldTags = flatMap(parent.jsDoc, j => j.tags);
+        const oldTags = flatMapToMutable(parent.jsDoc, j => j.tags);
         const unmergedNewTags = newTags.filter(newTag => !oldTags || !oldTags.some((tag, i) => {
             const merged = tryMergeJsdocTags(tag, newTag);
             if (merged) oldTags[i] = merged;
diff --git a/src/services/findAllReferences.ts b/src/services/findAllReferences.ts
index 41aae6ae82..b7ded40c28 100644
--- a/src/services/findAllReferences.ts
+++ b/src/services/findAllReferences.ts
@@ -1,8 +1,8 @@
 /* @internal */
 namespace ts.FindAllReferences {
     export interface SymbolAndEntries {
-        definition: Definition | undefined;
-        references: Entry[];
+        readonly definition: Definition | undefined;
+        readonly references: ReadonlyArray<Entry>;
     }
 
     export const enum DefinitionKind { Symbol, Label, Keyword, This, String }
@@ -60,7 +60,7 @@ namespace ts.FindAllReferences {
         return map(referenceEntries, entry => toImplementationLocation(entry, checker));
     }
 
-    function getImplementationReferenceEntries(program: Program, cancellationToken: CancellationToken, sourceFiles: ReadonlyArray<SourceFile>, node: Node, position: number): Entry[] | undefined {
+    function getImplementationReferenceEntries(program: Program, cancellationToken: CancellationToken, sourceFiles: ReadonlyArray<SourceFile>, node: Node, position: number): ReadonlyArray<Entry> | undefined {
         if (node.kind === SyntaxKind.SourceFile) {
             return undefined;
         }
@@ -94,11 +94,19 @@ namespace ts.FindAllReferences {
 
     export type ToReferenceOrRenameEntry<T> = (entry: Entry, originalNode: Node) => T;
 
-    export function getReferenceEntriesForNode(position: number, node: Node, program: Program, sourceFiles: ReadonlyArray<SourceFile>, cancellationToken: CancellationToken, options: Options = {}, sourceFilesSet: ReadonlyMap<true> = arrayToSet(sourceFiles, f => f.fileName)): Entry[] | undefined {
+    export function getReferenceEntriesForNode(
+        position: number,
+        node: Node,
+        program: Program,
+        sourceFiles: ReadonlyArray<SourceFile>,
+        cancellationToken: CancellationToken,
+        options: Options = {},
+        sourceFilesSet: ReadonlyMap<true> = arrayToSet(sourceFiles, f => f.fileName),
+    ): ReadonlyArray<Entry> | undefined {
         return flattenEntries(Core.getReferencedSymbolsForNode(position, node, program, sourceFiles, cancellationToken, options, sourceFilesSet));
     }
 
-    function flattenEntries(referenceSymbols: SymbolAndEntries[] | undefined): Entry[] | undefined {
+    function flattenEntries(referenceSymbols: SymbolAndEntries[] | undefined): ReadonlyArray<Entry> | undefined {
         return referenceSymbols && flatMap(referenceSymbols, r => r.references);
     }
 
diff --git a/src/services/goToDefinition.ts b/src/services/goToDefinition.ts
index 02d590bb8b..8f495aadfe 100644
--- a/src/services/goToDefinition.ts
+++ b/src/services/goToDefinition.ts
@@ -1,6 +1,6 @@
 /* @internal */
 namespace ts.GoToDefinition {
-    export function getDefinitionAtPosition(program: Program, sourceFile: SourceFile, position: number): DefinitionInfo[] | undefined {
+    export function getDefinitionAtPosition(program: Program, sourceFile: SourceFile, position: number): ReadonlyArray<DefinitionInfo> | undefined {
         const reference = getReferenceAtPosition(sourceFile, position, program);
         if (reference) {
             return [getDefinitionInfoForFileReference(reference.fileName, reference.file.fileName)];
@@ -129,7 +129,7 @@ namespace ts.GoToDefinition {
     }
 
     /// Goto type
-    export function getTypeDefinitionAtPosition(typeChecker: TypeChecker, sourceFile: SourceFile, position: number): DefinitionInfo[] | undefined {
+    export function getTypeDefinitionAtPosition(typeChecker: TypeChecker, sourceFile: SourceFile, position: number): ReadonlyArray<DefinitionInfo> | undefined {
         const node = getTouchingPropertyName(sourceFile, position);
         if (node === sourceFile) {
             return undefined;
@@ -145,7 +145,7 @@ namespace ts.GoToDefinition {
         return fromReturnType && fromReturnType.length !== 0 ? fromReturnType : definitionFromType(typeAtLocation, typeChecker, node);
     }
 
-    function definitionFromType(type: Type, checker: TypeChecker, node: Node): DefinitionInfo[] {
+    function definitionFromType(type: Type, checker: TypeChecker, node: Node): ReadonlyArray<DefinitionInfo> {
         return flatMap(type.isUnion() && !(type.flags & TypeFlags.Enum) ? type.types : [type], t =>
             t.symbol && getDefinitionFromSymbol(checker, t.symbol, node));
     }
diff --git a/src/services/services.ts b/src/services/services.ts
index 725decf18f..a91836e668 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1496,7 +1496,7 @@ namespace ts {
         }
 
         /// Goto definition
-        function getDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined {
+        function getDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined {
             synchronizeHostData();
             return GoToDefinition.getDefinitionAtPosition(program, getValidSourceFile(fileName), position);
         }
@@ -1506,7 +1506,7 @@ namespace ts {
             return GoToDefinition.getDefinitionAndBoundSpan(program, getValidSourceFile(fileName), position);
         }
 
-        function getTypeDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined {
+        function getTypeDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined {
             synchronizeHostData();
             return GoToDefinition.getTypeDefinitionAtPosition(program.getTypeChecker(), getValidSourceFile(fileName), position);
         }
@@ -1519,7 +1519,7 @@ namespace ts {
         }
 
         /// References and Occurrences
-        function getOccurrencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined {
+        function getOccurrencesAtPosition(fileName: string, position: number): ReadonlyArray<ReferenceEntry> | undefined {
             return flatMap(getDocumentHighlights(fileName, position, [fileName]), entry => entry.highlightSpans.map<ReferenceEntry>(highlightSpan => ({
                 fileName: entry.fileName,
                 textSpan: highlightSpan.textSpan,
diff --git a/src/services/types.ts b/src/services/types.ts
index 21ce83070d..a43da2b646 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -289,19 +289,19 @@ namespace ts {
         getSignatureHelpItems(fileName: string, position: number, options: SignatureHelpItemsOptions | undefined): SignatureHelpItems | undefined;
 
         getRenameInfo(fileName: string, position: number): RenameInfo;
-        findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): RenameLocation[] | undefined;
+        findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): ReadonlyArray<RenameLocation> | undefined;
 
-        getDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined;
+        getDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined;
         getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan | undefined;
-        getTypeDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined;
-        getImplementationAtPosition(fileName: string, position: number): ImplementationLocation[] | undefined;
+        getTypeDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined;
+        getImplementationAtPosition(fileName: string, position: number): ReadonlyArray<ImplementationLocation> | undefined;
 
         getReferencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
         findReferences(fileName: string, position: number): ReferencedSymbol[] | undefined;
         getDocumentHighlights(fileName: string, position: number, filesToSearch: string[]): DocumentHighlights[] | undefined;
 
         /** @deprecated */
-        getOccurrencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
+        getOccurrencesAtPosition(fileName: string, position: number): ReadonlyArray<ReferenceEntry> | undefined;
 
         getNavigateToItems(searchValue: string, maxResultCount?: number, fileName?: string, excludeDtsFiles?: boolean): NavigateToItem[];
         getNavigationBarItems(fileName: string): NavigationBarItem[];
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 15606263db..87bbf1a1cd 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -1755,7 +1755,7 @@ declare namespace ts {
     }
     interface ParseConfigHost {
         useCaseSensitiveFileNames: boolean;
-        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): string[];
+        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): ReadonlyArray<string>;
         /**
          * Gets a value indicating whether the specified path exists and is a file.
          * @param path The path to test.
@@ -1925,7 +1925,7 @@ declare namespace ts {
         typePredicateToString(predicate: TypePredicate, enclosingDeclaration?: Node, flags?: TypeFormatFlags): string;
         getFullyQualifiedName(symbol: Symbol): string;
         getAugmentedPropertiesOfType(type: Type): Symbol[];
-        getRootSymbols(symbol: Symbol): Symbol[];
+        getRootSymbols(symbol: Symbol): ReadonlyArray<Symbol>;
         getContextualType(node: Expression): Type | undefined;
         /**
          * returns unknownSignature in the case of an error.
@@ -4696,16 +4696,16 @@ declare namespace ts {
         getBreakpointStatementAtPosition(fileName: string, position: number): TextSpan | undefined;
         getSignatureHelpItems(fileName: string, position: number, options: SignatureHelpItemsOptions | undefined): SignatureHelpItems | undefined;
         getRenameInfo(fileName: string, position: number): RenameInfo;
-        findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): RenameLocation[] | undefined;
-        getDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined;
+        findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): ReadonlyArray<RenameLocation> | undefined;
+        getDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined;
         getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan | undefined;
-        getTypeDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined;
-        getImplementationAtPosition(fileName: string, position: number): ImplementationLocation[] | undefined;
+        getTypeDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined;
+        getImplementationAtPosition(fileName: string, position: number): ReadonlyArray<ImplementationLocation> | undefined;
         getReferencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
         findReferences(fileName: string, position: number): ReferencedSymbol[] | undefined;
         getDocumentHighlights(fileName: string, position: number, filesToSearch: string[]): DocumentHighlights[] | undefined;
         /** @deprecated */
-        getOccurrencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
+        getOccurrencesAtPosition(fileName: string, position: number): ReadonlyArray<ReferenceEntry> | undefined;
         getNavigateToItems(searchValue: string, maxResultCount?: number, fileName?: string, excludeDtsFiles?: boolean): NavigateToItem[];
         getNavigationBarItems(fileName: string): NavigationBarItem[];
         getNavigationTree(fileName: string): NavigationTree;
@@ -6405,7 +6405,7 @@ declare namespace ts.server.protocol {
         /**
          * The file locations referencing the symbol.
          */
-        refs: ReferencesResponseItem[];
+        refs: ReadonlyArray<ReferencesResponseItem>;
         /**
          * The name of the symbol.
          */
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index ad934eef56..fd4f575261 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -1755,7 +1755,7 @@ declare namespace ts {
     }
     interface ParseConfigHost {
         useCaseSensitiveFileNames: boolean;
-        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): string[];
+        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string> | undefined, includes: ReadonlyArray<string>, depth?: number): ReadonlyArray<string>;
         /**
          * Gets a value indicating whether the specified path exists and is a file.
          * @param path The path to test.
@@ -1925,7 +1925,7 @@ declare namespace ts {
         typePredicateToString(predicate: TypePredicate, enclosingDeclaration?: Node, flags?: TypeFormatFlags): string;
         getFullyQualifiedName(symbol: Symbol): string;
         getAugmentedPropertiesOfType(type: Type): Symbol[];
-        getRootSymbols(symbol: Symbol): Symbol[];
+        getRootSymbols(symbol: Symbol): ReadonlyArray<Symbol>;
         getContextualType(node: Expression): Type | undefined;
         /**
          * returns unknownSignature in the case of an error.
@@ -4696,16 +4696,16 @@ declare namespace ts {
         getBreakpointStatementAtPosition(fileName: string, position: number): TextSpan | undefined;
         getSignatureHelpItems(fileName: string, position: number, options: SignatureHelpItemsOptions | undefined): SignatureHelpItems | undefined;
         getRenameInfo(fileName: string, position: number): RenameInfo;
-        findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): RenameLocation[] | undefined;
-        getDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined;
+        findRenameLocations(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): ReadonlyArray<RenameLocation> | undefined;
+        getDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined;
         getDefinitionAndBoundSpan(fileName: string, position: number): DefinitionInfoAndBoundSpan | undefined;
-        getTypeDefinitionAtPosition(fileName: string, position: number): DefinitionInfo[] | undefined;
-        getImplementationAtPosition(fileName: string, position: number): ImplementationLocation[] | undefined;
+        getTypeDefinitionAtPosition(fileName: string, position: number): ReadonlyArray<DefinitionInfo> | undefined;
+        getImplementationAtPosition(fileName: string, position: number): ReadonlyArray<ImplementationLocation> | undefined;
         getReferencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
         findReferences(fileName: string, position: number): ReferencedSymbol[] | undefined;
         getDocumentHighlights(fileName: string, position: number, filesToSearch: string[]): DocumentHighlights[] | undefined;
         /** @deprecated */
-        getOccurrencesAtPosition(fileName: string, position: number): ReferenceEntry[] | undefined;
+        getOccurrencesAtPosition(fileName: string, position: number): ReadonlyArray<ReferenceEntry> | undefined;
         getNavigateToItems(searchValue: string, maxResultCount?: number, fileName?: string, excludeDtsFiles?: boolean): NavigateToItem[];
         getNavigationBarItems(fileName: string): NavigationBarItem[];
         getNavigationTree(fileName: string): NavigationTree;