diff --git a/lib/download.ts b/lib/download.ts
index df77dc08..f9e3e5cd 100644
--- a/lib/download.ts
+++ b/lib/download.ts
@@ -29,13 +29,20 @@ async function isValidVersion(version: string) {
 	return version === 'insiders' || validVersions.indexOf(version) !== -1;
 }
 
+/**
+ * Adapted from https://github.com/microsoft/TypeScript/issues/29729
+ * Since `string | 'foo'` doesn't offer auto completion
+ */
+type StringLiteralUnion<T extends U, U = string> = T | (U & {});
+export type DownloadVersion = StringLiteralUnion<'insiders'>;
+
 /**
  * Download a copy of VS Code archive to `.vscode-test`.
  *
  * @param version The version of VS Code to download such as '1.32.0'. You can also use
  * 'insiders' for downloading latest Insiders.
  */
-async function downloadVSCodeArchive(version: string): Promise<string> {
+async function downloadVSCodeArchive(version: DownloadVersion): Promise<string> {
 	if (!fs.existsSync(vscodeTestDir)) {
 		fs.mkdirSync(vscodeTestDir);
 	}
diff --git a/lib/runTest.ts b/lib/runTest.ts
index 3418c073..631af342 100644
--- a/lib/runTest.ts
+++ b/lib/runTest.ts
@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import * as cp from 'child_process';
-import { downloadAndUnzipVSCode } from './download';
+import { downloadAndUnzipVSCode, DownloadVersion } from './download';
 
 export interface TestOptions {
 	/**
@@ -22,7 +22,7 @@ export interface TestOptions {
 	 *
 	 * Defaults to latest stable version.
 	 */
-	version?: string;
+	version?: DownloadVersion;
 
 	/**
 	 * Absolute path to the extension root. Passed to `--extensionDevelopmentPath`.
@@ -98,19 +98,19 @@ async function innerRunTests(
 		const fullEnv = Object.assign({}, process.env, testRunnerEnv);
 		const cmd = cp.spawn(executable, args, { env: fullEnv });
 
-		cmd.stdout.on('data', function(data) {
+		cmd.stdout.on('data', function (data) {
 			console.log(data.toString());
 		});
 
-		cmd.stderr.on('data', function(data) {
+		cmd.stderr.on('data', function (data) {
 			console.error(data.toString());
 		});
 
-		cmd.on('error', function(data) {
+		cmd.on('error', function (data) {
 			console.log('Test error: ' + data.toString());
 		});
 
-		cmd.on('close', function(code) {
+		cmd.on('close', function (code) {
 			console.log(`Exit code:   ${code}`);
 
 			if (code !== 0) {