Bug 135866 – Built-in Promise API does not work with headless JSContexts

• Component: JavaScriptCore
• Status: RESOLVED FIXED

The built-in Promise implementation does not work with a JSContext that is not associated with a WebView/UIWebView/WKWebView and prints out the error, "Event loop not supported". For example, fulfilling a Promise calls "globalObject->queueMicrotask" which fails to invoke subscribed callbacks.

Sample code:

JSContext *context = [[JSContext alloc] init];
[context evaluateScript:@"Promise.resolve(42).then(function(result) { /*...*/ })"];
// Prints "ERROR: Event loop not supported."

This appears to have been fixed in iOS 10.0. The original sample code behaves as expected and also doesn't print an error message.

In iOS 10.3, async functions work too:
[_context evaluateScript:@"(async () => 42)().then(result => { /*...*/ })"];