diff --git a/src/harness/unittests/projectErrors.ts b/src/harness/unittests/projectErrors.ts
index f250d4d9b36f4..4a76ed2c98c76 100644
--- a/src/harness/unittests/projectErrors.ts
+++ b/src/harness/unittests/projectErrors.ts
@@ -4,12 +4,12 @@
 
 namespace ts.projectSystem {
     describe("Project errors", () => {
-        function checkProjectErrors(projectFiles: server.ProjectFilesWithTSDiagnostics, expectedErrors: string[]) {
+        function checkProjectErrors(projectFiles: server.ProjectFilesWithTSDiagnostics, expectedErrors: ReadonlyArray<string>): void {
             assert.isTrue(projectFiles !== undefined, "missing project files");
             checkProjectErrorsWorker(projectFiles.projectErrors, expectedErrors);
         }
 
-        function checkProjectErrorsWorker(errors: Diagnostic[], expectedErrors: string[]) {
+        function checkProjectErrorsWorker(errors: ReadonlyArray<Diagnostic>, expectedErrors: ReadonlyArray<string>): void {
             assert.equal(errors ? errors.length : 0, expectedErrors.length, `expected ${expectedErrors.length} error in the list`);
             if (expectedErrors.length) {
                 for (let i = 0; i < errors.length; i++) {
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 5bb1da251eacb..1c6e5f36491de 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -22,7 +22,7 @@ namespace ts.server {
 
     export interface ConfigFileDiagEvent {
         eventName: typeof ConfigFileDiagEvent;
-        data: { triggerFile: string, configFileName: string, diagnostics: Diagnostic[] };
+        data: { triggerFile: string, configFileName: string, diagnostics: ReadonlyArray<Diagnostic> };
     }
 
     export interface ProjectLanguageServiceStateEvent {
@@ -200,7 +200,7 @@ namespace ts.server {
     /**
      * This helper function processes a list of projects and return the concatenated, sortd and deduplicated output of processing each project.
      */
-    export function combineProjectOutput<T>(projects: Project[], action: (project: Project) => T[], comparer?: (a: T, b: T) => number, areEqual?: (a: T, b: T) => boolean) {
+    export function combineProjectOutput<T>(projects: ReadonlyArray<Project>, action: (project: Project) => ReadonlyArray<T>, comparer?: (a: T, b: T) => number, areEqual?: (a: T, b: T) => boolean) {
         const result = flatMap(projects, action).sort(comparer);
         return projects.length > 1 ? deduplicate(result, areEqual) : result;
     }
@@ -220,14 +220,14 @@ namespace ts.server {
 
     interface OpenConfigFileResult {
         success: boolean;
-        errors?: Diagnostic[];
+        errors?: ReadonlyArray<Diagnostic>;
 
         project?: ConfiguredProject;
     }
 
     export interface OpenConfiguredProjectResult {
         configFileName?: NormalizedPath;
-        configFileErrors?: Diagnostic[];
+        configFileErrors?: ReadonlyArray<Diagnostic>;
     }
 
     interface FilePropertyReader<T> {
@@ -1102,18 +1102,18 @@ namespace ts.server {
             }
         }
 
-        private reportConfigFileDiagnostics(configFileName: string, diagnostics: Diagnostic[], triggerFile: string) {
+        private reportConfigFileDiagnostics(configFileName: string, diagnostics: ReadonlyArray<Diagnostic>, triggerFile: string) {
             if (!this.eventHandler) {
                 return;
             }
 
             this.eventHandler(<ConfigFileDiagEvent>{
                 eventName: ConfigFileDiagEvent,
-                data: { configFileName, diagnostics: diagnostics || [], triggerFile }
+                data: { configFileName, diagnostics: diagnostics || emptyArray, triggerFile }
             });
         }
 
-        private createAndAddConfiguredProject(configFileName: NormalizedPath, projectOptions: ProjectOptions, configFileErrors: Diagnostic[], clientFileName?: string) {
+        private createAndAddConfiguredProject(configFileName: NormalizedPath, projectOptions: ProjectOptions, configFileErrors: ReadonlyArray<Diagnostic>, clientFileName?: string) {
             const sizeLimitExceeded = this.exceededTotalSizeLimitForNonTsFiles(configFileName, projectOptions.compilerOptions, projectOptions.files, fileNamePropertyReader);
             const project = new ConfiguredProject(
                 configFileName,
@@ -1145,7 +1145,7 @@ namespace ts.server {
             }
         }
 
-        private addFilesToProjectAndUpdateGraph<T>(project: ConfiguredProject | ExternalProject, files: T[], propertyReader: FilePropertyReader<T>, clientFileName: string, typeAcquisition: TypeAcquisition, configFileErrors: Diagnostic[]): void {
+        private addFilesToProjectAndUpdateGraph<T>(project: ConfiguredProject | ExternalProject, files: T[], propertyReader: FilePropertyReader<T>, clientFileName: string, typeAcquisition: TypeAcquisition, configFileErrors: ReadonlyArray<Diagnostic>): void {
             let errors: Diagnostic[];
             for (const f of files) {
                 const rootFilename = propertyReader.getFileName(f);
@@ -1458,7 +1458,7 @@ namespace ts.server {
 
         openClientFileWithNormalizedPath(fileName: NormalizedPath, fileContent?: string, scriptKind?: ScriptKind, hasMixedContent?: boolean, projectRootPath?: NormalizedPath): OpenConfiguredProjectResult {
             let configFileName: NormalizedPath;
-            let configFileErrors: Diagnostic[];
+            let configFileErrors: ReadonlyArray<Diagnostic>;
 
             let project: ConfiguredProject | ExternalProject = this.findContainingExternalProject(fileName);
             if (!project) {
diff --git a/src/server/project.ts b/src/server/project.ts
index 844ded761d802..c814f1bd78d9d 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -54,7 +54,7 @@ namespace ts.server {
 
     /* @internal */
     export interface ProjectFilesWithTSDiagnostics extends protocol.ProjectFiles {
-        projectErrors: Diagnostic[];
+        projectErrors: ReadonlyArray<Diagnostic>;
     }
 
     export class UnresolvedImportsMap {
@@ -147,7 +147,7 @@ namespace ts.server {
 
         private typingFiles: SortedReadonlyArray<string>;
 
-        protected projectErrors: Diagnostic[];
+        protected projectErrors: ReadonlyArray<Diagnostic>;
 
         public typesVersion = 0;
 
@@ -1045,7 +1045,7 @@ namespace ts.server {
             return getDirectoryPath(this.getConfigFilePath());
         }
 
-        setProjectErrors(projectErrors: Diagnostic[]) {
+        setProjectErrors(projectErrors: ReadonlyArray<Diagnostic>) {
             this.projectErrors = projectErrors;
         }
 
@@ -1189,7 +1189,7 @@ namespace ts.server {
             return this.typeAcquisition;
         }
 
-        setProjectErrors(projectErrors: Diagnostic[]) {
+        setProjectErrors(projectErrors: ReadonlyArray<Diagnostic>) {
             this.projectErrors = projectErrors;
         }
 
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index b6756a7d4ff11..a75c04764c036 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -914,7 +914,7 @@ namespace ts.server.protocol {
         /**
          * An array of span groups (one per file) that refer to the item to be renamed.
          */
-        locs: SpanGroup[];
+        locs: ReadonlyArray<SpanGroup>;
     }
 
     /**
diff --git a/src/server/session.ts b/src/server/session.ts
index d8e0f695deff5..a0c2b23959985 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -379,7 +379,7 @@ namespace ts.server {
             this.host.write(formatMessage(msg, this.logger, this.byteLength, this.host.newLine));
         }
 
-        public configFileDiagnosticEvent(triggerFile: string, configFile: string, diagnostics: Diagnostic[]) {
+        public configFileDiagnosticEvent(triggerFile: string, configFile: string, diagnostics: ReadonlyArray<Diagnostic>) {
             const bakedDiags = map(diagnostics, diagnostic => formatConfigFileDiag(diagnostic, /*includeFileName*/ true));
             const ev: protocol.ConfigFileDiagnosticEvent = {
                 seq: 0,
@@ -539,7 +539,7 @@ namespace ts.server {
                 );
         }
 
-        private convertToDiagnosticsWithLinePositionFromDiagnosticFile(diagnostics: Diagnostic[]) {
+        private convertToDiagnosticsWithLinePositionFromDiagnosticFile(diagnostics: ReadonlyArray<Diagnostic>): protocol.DiagnosticWithLinePosition[] {
             return diagnostics.map(d => <protocol.DiagnosticWithLinePosition>{
                 message: flattenDiagnosticMessageText(d.messageText, this.host.newLine),
                 start: d.start,
@@ -565,7 +565,7 @@ namespace ts.server {
             );
         }
 
-        private convertToDiagnosticsWithLinePosition(diagnostics: Diagnostic[], scriptInfo: ScriptInfo) {
+        private convertToDiagnosticsWithLinePosition(diagnostics: ReadonlyArray<Diagnostic>, scriptInfo: ScriptInfo): protocol.DiagnosticWithLinePosition[] {
             return diagnostics.map(d => <protocol.DiagnosticWithLinePosition>{
                 message: flattenDiagnosticMessageText(d.messageText, this.host.newLine),
                 start: d.start,
@@ -578,10 +578,12 @@ namespace ts.server {
             });
         }
 
-        private getDiagnosticsWorker(args: protocol.FileRequestArgs, isSemantic: boolean, selector: (project: Project, file: string) => Diagnostic[], includeLinePosition: boolean) {
+        private getDiagnosticsWorker(
+            args: protocol.FileRequestArgs, isSemantic: boolean, selector: (project: Project, file: string) => ReadonlyArray<Diagnostic>, includeLinePosition: boolean
+            ): ReadonlyArray<protocol.DiagnosticWithLinePosition> | ReadonlyArray<protocol.Diagnostic> {
             const { project, file } = this.getFileAndProject(args);
             if (isSemantic && isDeclarationFileInJSOnlyNonConfiguredProject(project, file)) {
-                return [];
+                return emptyArray;
             }
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const diagnostics = selector(project, file);
@@ -590,14 +592,14 @@ namespace ts.server {
                 : diagnostics.map(d => formatDiag(file, project, d));
         }
 
-        private getDefinition(args: protocol.FileLocationRequestArgs, simplifiedResult: boolean): protocol.FileSpan[] | DefinitionInfo[] {
+        private getDefinition(args: protocol.FileLocationRequestArgs, simplifiedResult: boolean): ReadonlyArray<protocol.FileSpan> | ReadonlyArray<DefinitionInfo> {
             const { file, project } = this.getFileAndProject(args);
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, scriptInfo);
 
             const definitions = project.getLanguageService().getDefinitionAtPosition(file, position);
             if (!definitions) {
-                return undefined;
+                return emptyArray;
             }
 
             if (simplifiedResult) {
@@ -615,7 +617,7 @@ namespace ts.server {
             }
         }
 
-        private getTypeDefinition(args: protocol.FileLocationRequestArgs): protocol.FileSpan[] {
+        private getTypeDefinition(args: protocol.FileLocationRequestArgs): ReadonlyArray<protocol.FileSpan> {
             const { file, project } = this.getFileAndProject(args);
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, scriptInfo);
@@ -635,12 +637,12 @@ namespace ts.server {
             });
         }
 
-        private getImplementation(args: protocol.FileLocationRequestArgs, simplifiedResult: boolean): protocol.FileSpan[] | ImplementationLocation[] {
+        private getImplementation(args: protocol.FileLocationRequestArgs, simplifiedResult: boolean): ReadonlyArray<protocol.FileSpan> | ReadonlyArray<ImplementationLocation> {
             const { file, project } = this.getFileAndProject(args);
             const position = this.getPosition(args, project.getScriptInfoForNormalizedPath(file));
             const implementations = project.getLanguageService().getImplementationAtPosition(file, position);
             if (!implementations) {
-                return [];
+                return emptyArray;
             }
             if (simplifiedResult) {
                 return implementations.map(({ fileName, textSpan }) => {
@@ -657,7 +659,7 @@ namespace ts.server {
             }
         }
 
-        private getOccurrences(args: protocol.FileLocationRequestArgs): protocol.OccurrencesResponseItem[] {
+        private getOccurrences(args: protocol.FileLocationRequestArgs): ReadonlyArray<protocol.OccurrencesResponseItem> {
             const { file, project } = this.getFileAndProject(args);
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, scriptInfo);
@@ -665,7 +667,7 @@ namespace ts.server {
             const occurrences = project.getLanguageService().getOccurrencesAtPosition(file, position);
 
             if (!occurrences) {
-                return undefined;
+                return emptyArray;
             }
 
             return occurrences.map(occurrence => {
@@ -687,17 +689,17 @@ namespace ts.server {
             });
         }
 
-        private getSyntacticDiagnosticsSync(args: protocol.SyntacticDiagnosticsSyncRequestArgs): protocol.Diagnostic[] | protocol.DiagnosticWithLinePosition[] {
+        private getSyntacticDiagnosticsSync(args: protocol.SyntacticDiagnosticsSyncRequestArgs): ReadonlyArray<protocol.Diagnostic> | ReadonlyArray<protocol.DiagnosticWithLinePosition> {
             const { configFile } = this.getConfigFileAndProject(args);
             if (configFile) {
                 // all the config file errors are reported as part of semantic check so nothing to report here
-                return [];
+                return emptyArray;
             }
 
             return this.getDiagnosticsWorker(args, /*isSemantic*/ false, (project, file) => project.getLanguageService().getSyntacticDiagnostics(file), args.includeLinePosition);
         }
 
-        private getSemanticDiagnosticsSync(args: protocol.SemanticDiagnosticsSyncRequestArgs): protocol.Diagnostic[] | protocol.DiagnosticWithLinePosition[] {
+        private getSemanticDiagnosticsSync(args: protocol.SemanticDiagnosticsSyncRequestArgs): ReadonlyArray<protocol.Diagnostic> | ReadonlyArray<protocol.DiagnosticWithLinePosition> {
             const { configFile, project } = this.getConfigFileAndProject(args);
             if (configFile) {
                 return this.getConfigFileDiagnostics(configFile, project, args.includeLinePosition);
@@ -705,7 +707,7 @@ namespace ts.server {
             return this.getDiagnosticsWorker(args, /*isSemantic*/ true, (project, file) => project.getLanguageService().getSemanticDiagnostics(file), args.includeLinePosition);
         }
 
-        private getDocumentHighlights(args: protocol.DocumentHighlightsRequestArgs, simplifiedResult: boolean): protocol.DocumentHighlightsItem[] | DocumentHighlights[] {
+        private getDocumentHighlights(args: protocol.DocumentHighlightsRequestArgs, simplifiedResult: boolean): ReadonlyArray<protocol.DocumentHighlightsItem> | ReadonlyArray<DocumentHighlights> {
             const { file, project } = this.getFileAndProject(args);
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, scriptInfo);
@@ -796,7 +798,7 @@ namespace ts.server {
             return info.getDefaultProject();
         }
 
-        private getRenameLocations(args: protocol.RenameRequestArgs, simplifiedResult: boolean): protocol.RenameResponseBody | RenameLocation[] {
+        private getRenameLocations(args: protocol.RenameRequestArgs, simplifiedResult: boolean): protocol.RenameResponseBody | ReadonlyArray<RenameLocation> {
             const file = toNormalizedPath(args.file);
             const info = this.projectService.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, info);
@@ -813,7 +815,7 @@ namespace ts.server {
                 if (!renameInfo.canRename) {
                     return {
                         info: renameInfo,
-                        locs: []
+                        locs: emptyArray
                     };
                 }
 
@@ -822,7 +824,7 @@ namespace ts.server {
                     (project: Project) => {
                         const renameLocations = project.getLanguageService().findRenameLocations(file, position, args.findInStrings, args.findInComments);
                         if (!renameLocations) {
-                            return [];
+                            return emptyArray;
                         }
 
                         return renameLocations.map(location => {
@@ -899,7 +901,7 @@ namespace ts.server {
             }
         }
 
-        private getReferences(args: protocol.FileLocationRequestArgs, simplifiedResult: boolean): protocol.ReferencesResponseBody | ReferencedSymbol[] {
+        private getReferences(args: protocol.FileLocationRequestArgs, simplifiedResult: boolean): protocol.ReferencesResponseBody | ReadonlyArray<ReferencedSymbol> {
             const file = toNormalizedPath(args.file);
             const projects = this.getProjects(args);
 
@@ -909,7 +911,7 @@ namespace ts.server {
             if (simplifiedResult) {
                 const nameInfo = defaultProject.getLanguageService().getQuickInfoAtPosition(file, position);
                 if (!nameInfo) {
-                    return undefined;
+                    return emptyArray;
                 }
 
                 const displayString = displayPartsToString(nameInfo.displayParts);
@@ -921,7 +923,7 @@ namespace ts.server {
                     (project: Project) => {
                         const references = project.getLanguageService().getReferencesAtPosition(file, position);
                         if (!references) {
-                            return [];
+                            return emptyArray;
                         }
 
                         return references.map(ref => {
@@ -978,7 +980,7 @@ namespace ts.server {
             if (this.eventHandler) {
                 this.eventHandler({
                     eventName: "configFileDiag",
-                    data: { triggerFile: fileName, configFileName, diagnostics: configFileErrors || [] }
+                    data: { triggerFile: fileName, configFileName, diagnostics: configFileErrors || emptyArray }
                 });
             }
         }
@@ -1163,7 +1165,7 @@ namespace ts.server {
             });
         }
 
-        private getCompletions(args: protocol.CompletionsRequestArgs, simplifiedResult: boolean): protocol.CompletionEntry[] | CompletionInfo {
+        private getCompletions(args: protocol.CompletionsRequestArgs, simplifiedResult: boolean): ReadonlyArray<protocol.CompletionEntry> | CompletionInfo {
             const prefix = args.prefix || "";
             const { file, project } = this.getFileAndProject(args);
 
@@ -1172,7 +1174,7 @@ namespace ts.server {
 
             const completions = project.getLanguageService().getCompletionsAtPosition(file, position);
             if (!completions) {
-                return undefined;
+                return emptyArray;
             }
             if (simplifiedResult) {
                 return mapDefined(completions.entries, entry => {
@@ -1188,7 +1190,7 @@ namespace ts.server {
             }
         }
 
-        private getCompletionEntryDetails(args: protocol.CompletionDetailsRequestArgs): protocol.CompletionEntryDetails[] {
+        private getCompletionEntryDetails(args: protocol.CompletionDetailsRequestArgs): ReadonlyArray<protocol.CompletionEntryDetails> {
             const { file, project } = this.getFileAndProject(args);
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, scriptInfo);
@@ -1197,12 +1199,12 @@ namespace ts.server {
                 project.getLanguageService().getCompletionEntryDetails(file, position, entryName));
         }
 
-        private getCompileOnSaveAffectedFileList(args: protocol.FileRequestArgs): protocol.CompileOnSaveAffectedFileListSingleProject[] {
+        private getCompileOnSaveAffectedFileList(args: protocol.FileRequestArgs): ReadonlyArray<protocol.CompileOnSaveAffectedFileListSingleProject> {
             const info = this.projectService.getScriptInfo(args.file);
             const result: protocol.CompileOnSaveAffectedFileListSingleProject[] = [];
 
             if (!info) {
-                return [];
+                return emptyArray;
             }
 
             // if specified a project, we only return affected file list in this project
@@ -1360,7 +1362,7 @@ namespace ts.server {
                     : tree;
         }
 
-        private getNavigateToItems(args: protocol.NavtoRequestArgs, simplifiedResult: boolean): protocol.NavtoItem[] | NavigateToItem[] {
+        private getNavigateToItems(args: protocol.NavtoRequestArgs, simplifiedResult: boolean): ReadonlyArray<protocol.NavtoItem> | ReadonlyArray<NavigateToItem> {
             const projects = this.getProjects(args);
 
             const fileName = args.currentFileOnly ? args.file && normalizeSlashes(args.file) : undefined;
@@ -1370,7 +1372,7 @@ namespace ts.server {
                     project => {
                         const navItems = project.getLanguageService().getNavigateToItems(args.searchValue, args.maxResultCount, fileName, /*excludeDts*/ project.isNonTsProject());
                         if (!navItems) {
-                            return [];
+                            return emptyArray;
                         }
 
                         return navItems.map((navItem) => {