use Object#setPrototypeOf, #49386

src/vs/workbench/api/node/extHostTypes.ts

@@ -1889,6 +1889,12 @@ export class FileSystemError extends Error {
 		super(URI.isUri(uriOrMessage) ? uriOrMessage.toString(true) : uriOrMessage);
 		this.name = code ? `${code} (FileSystemError)` : `FileSystemError`;
 
+		// workaround when extending builtin objects and when compiling to ES5, see:
+		// https://github.com/Microsoft/TypeScript-wiki/blob/master/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work
+		if (typeof (<any>Object).setPrototypeOf === 'function') {
+			(<any>Object).setPrototypeOf(this, FileSystemError.prototype);
+		}
+
 		if (typeof Error.captureStackTrace === 'function' && typeof terminator === 'function') {
 			// nice stack traces
 			Error.captureStackTrace(this, terminator);

src/vs/workbench/test/electron-browser/api/extHostTypes.test.ts

@@ -513,4 +513,10 @@ suite('ExtHostTypes', function () {
 		assert.equal(string.value, '${BAR}');
 
 	});
+
+	test('instanceof doesn\'t work for FileSystemError #49386', function () {
+		const error = types.FileSystemError.Unavailable('foo');
+		assert.ok(error instanceof Error);
+		assert.ok(error instanceof types.FileSystemError);
+	});
 });