The PromiseRejectionEvent interface represents events which are fired when JavaScript Promises are rejected. These events are particularly useful for telemetry and debugging purposes.
The PromiseRejectionEvent() constructor returns a newly created PromiseRejectionEvent, which represents events fired when a JavaScript Promise is rejected.
The PromiseRejectionEvent.promise read-only property indicates the JavaScript Promise which has been rejected. You can examine the event's PromiseRejectionEvent.reason property to learn why the promise was rejected.
The PromiseRejectionEvent.reason read-only property is any JavaScript value or Object which provides the reason passed into Promise.reject(). This in theory provides information about why the promise was rejected.