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. Syntax reason = PromiseRejectionEvent.reason Specifications Specification Status Comment WHATWG HTML Living StandardThe definition of 'PromiseRejectionEvent.reason' in that specification. Living Standard Initial definition. Browser compatibility Desktop Mobile Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari Basic support 49 No support No support No support No support Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Basic support No support No support No support No support No support See also Promise PromiseRejectionEvent rejectionhandled unhandledrejection