diff --git a/extensions/typescript-language-features/src/languageFeatures/documentHighlight.ts b/extensions/typescript-language-features/src/languageFeatures/documentHighlight.ts
index 3a7b43ed5eb..2758b4227b0 100644
--- a/extensions/typescript-language-features/src/languageFeatures/documentHighlight.ts
+++ b/extensions/typescript-language-features/src/languageFeatures/documentHighlight.ts
@@ -6,7 +6,6 @@
 import * as vscode from 'vscode';
 import type * as Proto from '../protocol';
 import { ITypeScriptServiceClient } from '../typescriptService';
-import { flatten } from '../utils/arrays';
 import { DocumentSelector } from '../utils/documentSelector';
 import * as typeConverters from '../utils/typeConverters';
 
@@ -34,10 +33,10 @@ class TypeScriptDocumentHighlightProvider implements vscode.DocumentHighlightPro
 			return [];
 		}
 
-		return flatten(
-			response.body
-				.filter(highlight => highlight.file === file)
-				.map(convertDocumentHighlight));
+		return response.body
+			.filter(highlight => highlight.file === file)
+			.map(convertDocumentHighlight)
+			.flat();
 	}
 }
 
diff --git a/extensions/typescript-language-features/src/languageFeatures/tsconfig.ts b/extensions/typescript-language-features/src/languageFeatures/tsconfig.ts
index 4e5eda244b6..b64ead7ca15 100644
--- a/extensions/typescript-language-features/src/languageFeatures/tsconfig.ts
+++ b/extensions/typescript-language-features/src/languageFeatures/tsconfig.ts
@@ -8,7 +8,7 @@ import { basename, dirname, join, posix } from 'path';
 import * as vscode from 'vscode';
 import * as nls from 'vscode-nls';
 import { Utils } from 'vscode-uri';
-import { coalesce, flatten } from '../utils/arrays';
+import { coalesce } from '../utils/arrays';
 import { exists } from '../utils/fs';
 
 function mapChildren<R>(node: jsonc.Node | undefined, f: (x: jsonc.Node) => R): R[] {
@@ -195,9 +195,9 @@ export function register() {
 
 	const languages = ['json', 'jsonc'];
 
-	const selector: vscode.DocumentSelector = flatten(
-		languages.map(language =>
-			patterns.map((pattern): vscode.DocumentFilter => ({ language, pattern }))));
+	const selector: vscode.DocumentSelector =
+		languages.map(language => patterns.map((pattern): vscode.DocumentFilter => ({ language, pattern })))
+			.flat();
 
 	return vscode.Disposable.from(
 		vscode.commands.registerCommand(openExtendsLinkCommandId, async ({ resourceUri, extendsValue, }: OpenExtendsLinkCommandArgs) => {
diff --git a/extensions/typescript-language-features/src/lazyClientHost.ts b/extensions/typescript-language-features/src/lazyClientHost.ts
index 1a7432674fc..8ad2efac84c 100644
--- a/extensions/typescript-language-features/src/lazyClientHost.ts
+++ b/extensions/typescript-language-features/src/lazyClientHost.ts
@@ -11,7 +11,6 @@ import { TsServerProcessFactory } from './tsServer/server';
 import { ITypeScriptVersionProvider } from './tsServer/versionProvider';
 import TypeScriptServiceClientHost from './typeScriptServiceClientHost';
 import { ActiveJsTsEditorTracker } from './utils/activeJsTsEditorTracker';
-import { flatten } from './utils/arrays';
 import { ServiceConfigurationProvider } from './utils/configuration';
 import * as fileSchemes from './utils/fileSchemes';
 import { standardLanguageDescriptions } from './utils/languageDescription';
@@ -55,10 +54,10 @@ export function lazilyActivateClient(
 ): vscode.Disposable {
 	const disposables: vscode.Disposable[] = [];
 
-	const supportedLanguage = flatten([
+	const supportedLanguage = [
 		...standardLanguageDescriptions.map(x => x.languageIds),
 		...pluginManager.plugins.map(x => x.languages)
-	]);
+	].flat();
 
 	let hasActivated = false;
 	const maybeActivate = (textDocument: vscode.TextDocument): boolean => {
diff --git a/extensions/typescript-language-features/src/task/taskProvider.ts b/extensions/typescript-language-features/src/task/taskProvider.ts
index 75ca51278dd..85484f6d729 100644
--- a/extensions/typescript-language-features/src/task/taskProvider.ts
+++ b/extensions/typescript-language-features/src/task/taskProvider.ts
@@ -9,7 +9,7 @@ import * as vscode from 'vscode';
 import * as nls from 'vscode-nls';
 import { wait } from '../test/testUtils';
 import { ITypeScriptServiceClient, ServerResponse } from '../typescriptService';
-import { coalesce, flatten } from '../utils/arrays';
+import { coalesce } from '../utils/arrays';
 import { Disposable } from '../utils/dispose';
 import { exists } from '../utils/fs';
 import { isTsConfigFileName } from '../utils/languageDescription';
@@ -98,10 +98,10 @@ class TscTaskProvider extends Disposable implements vscode.TaskProvider {
 	}
 
 	private async getAllTsConfigs(token: vscode.CancellationToken): Promise<TSConfig[]> {
-		const configs = flatten(await Promise.all([
+		const configs = (await Promise.all([
 			this.getTsConfigForActiveFile(token),
 			this.getTsConfigsInWorkspace(token),
-		]));
+		])).flat();
 
 		return Promise.all(
 			configs.map(async config => await exists(config.uri) ? config : undefined),
diff --git a/extensions/typescript-language-features/src/typeScriptServiceClientHost.ts b/extensions/typescript-language-features/src/typeScriptServiceClientHost.ts
index 0543a6ae847..c8635b2f668 100644
--- a/extensions/typescript-language-features/src/typeScriptServiceClientHost.ts
+++ b/extensions/typescript-language-features/src/typeScriptServiceClientHost.ts
@@ -23,7 +23,7 @@ import TypeScriptServiceClient from './typescriptServiceClient';
 import { IntellisenseStatus } from './ui/intellisenseStatus';
 import { VersionStatus } from './ui/versionStatus';
 import { ActiveJsTsEditorTracker } from './utils/activeJsTsEditorTracker';
-import { coalesce, flatten } from './utils/arrays';
+import { coalesce } from './utils/arrays';
 import { ServiceConfigurationProvider } from './utils/configuration';
 import { Disposable } from './utils/dispose';
 import * as errorCodes from './utils/errorCodes';
@@ -165,11 +165,10 @@ export default class TypeScriptServiceClientHost extends Disposable {
 	}
 
 	private getAllModeIds(descriptions: LanguageDescription[], pluginManager: PluginManager) {
-		const allModeIds = flatten([
+		return [
 			...descriptions.map(x => x.languageIds),
 			...pluginManager.plugins.map(x => x.languages)
-		]);
-		return allModeIds;
+		].flat();
 	}
 
 	public get serviceClient(): TypeScriptServiceClient {
diff --git a/extensions/typescript-language-features/src/utils/arrays.ts b/extensions/typescript-language-features/src/utils/arrays.ts
index 3619d828dc3..a25bbe54733 100644
--- a/extensions/typescript-language-features/src/utils/arrays.ts
+++ b/extensions/typescript-language-features/src/utils/arrays.ts
@@ -19,10 +19,6 @@ export function equals<T>(
 	return a.every((x, i) => itemEquals(x, b[i]));
 }
 
-export function flatten<T>(array: ReadonlyArray<T>[]): T[] {
-	return Array.prototype.concat.apply([], array);
-}
-
 export function coalesce<T>(array: ReadonlyArray<T | undefined>): T[] {
 	return <T[]>array.filter(e => !!e);
 }
diff --git a/extensions/typescript-language-features/src/utils/plugins.ts b/extensions/typescript-language-features/src/utils/plugins.ts
index 0716aba36cb..92d61e3e235 100644
--- a/extensions/typescript-language-features/src/utils/plugins.ts
+++ b/extensions/typescript-language-features/src/utils/plugins.ts
@@ -37,7 +37,7 @@ export class PluginManager extends Disposable {
 				return;
 			}
 			const newPlugins = this.readPlugins();
-			if (!arrays.equals(arrays.flatten(Array.from(this._plugins.values())), arrays.flatten(Array.from(newPlugins.values())), TypeScriptServerPlugin.equals)) {
+			if (!arrays.equals(Array.from(this._plugins.values()).flat(), Array.from(newPlugins.values()).flat(), TypeScriptServerPlugin.equals)) {
 				this._plugins = newPlugins;
 				this._onDidUpdatePlugins.fire(this);
 			}
@@ -48,7 +48,7 @@ export class PluginManager extends Disposable {
 		if (!this._plugins) {
 			this._plugins = this.readPlugins();
 		}
-		return arrays.flatten(Array.from(this._plugins.values()));
+		return Array.from(this._plugins.values()).flat();
 	}
 
 	private readonly _onDidUpdatePlugins = this._register(new vscode.EventEmitter<this>());