The PromiseRejectionEvent interface's promise read-only property indicates the JavaScript Promise which was rejected. You can examine the event's PromiseRejectionEvent.reason property to learn why the promise was rejected.

The JavaScript Promise which was rejected, and whose rejection went unhandled.

This example listens for unhandled promises and, if the reason is an object with a code field containing the text "Module not ready", it sets up an idle callback that will retry the task that failed to execute correctly.

event.preventDefault() is called to indicate that the promise has now been handled.

window.onunhandledrejection = (event) => {
  if (event.reason?.code === "Module not ready") {
    requestIdleCallback((deadline) => {
      loadModule(event.reason.moduleName).then(performStartup);
    });
    event.preventDefault();
  }
};

• Promise rejection events
• Promise
• PromiseRejectionEvent
• rejectionhandled
• unhandledrejection