Window: unhandledrejection event

Theunhandledrejection event is sent to the global scope of a script when a JavaScript Promise that has no rejection handler is rejected; typically, this is thewindow, but may also be aWorker.

This is useful for debugging and for providing fallback error handling for unexpected situations.

Syntax

Use the event name in methods likeaddEventListener(), or set an event handler property.

addEventListener("unhandledrejection", (event) => { })onunhandledrejection = (event) => { }

Event type

APromiseRejectionEvent. Inherits fromEvent.

Event properties

PromiseRejectionEvent.promiseRead only: The JavaScriptPromise that was rejected.
PromiseRejectionEvent.reasonRead only: A value orObject indicating why the promise was rejected, as passed toPromise.reject().

Event handler aliases

In addition to theWindow interface, the event handler propertyonunhandledrejection is also available on the following targets:

Allowing theunhandledrejection event to bubble will eventually result in an error message being output to the console. You can prevent this by callingpreventDefault() on thePromiseRejectionEvent; seePreventing default handling below for an example.

Because this event can leak data,Promise rejections that originate from a cross-origin script won't fire this event.

Examples

Basic error logging

This example logs information about the unhandled promise rejection to the console.

window.addEventListener("unhandledrejection", (event) => {  console.warn(`UNHANDLED PROMISE REJECTION: ${event.reason}`);});

You can also use theonunhandledrejection event handler property to set up the event listener:

window.onunhandledrejection = (event) => {  console.warn(`UNHANDLED PROMISE REJECTION: ${event.reason}`);};

Preventing default handling

Many environments (such asNode.js) report unhandled promise rejections to the console by default. You can prevent that from happening by adding a handler forunhandledrejection events that—in addition to any other tasks you wish to perform—callspreventDefault() to cancel the event, preventing it from bubbling up to be handled by the runtime's logging code. This works becauseunhandledrejection is cancelable.

window.addEventListener("unhandledrejection", (event) => {  // code for handling the unhandled rejection  // …  // Prevent the default handling (such as outputting the  // error to the console)  event.preventDefault();});