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

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

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

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

A PromiseRejectionEvent. Inherits from Event.

PromiseRejectionEvent.promise Read only

The JavaScript Promise that was rejected.

PromiseRejectionEvent.reason Read only

A value or Object indicating why the promise was rejected, as passed to Promise.reject().

In addition to the Window interface, the event handler property onunhandledrejection is also available on the following targets:

• HTMLBodyElement
• HTMLFrameSetElement
• SVGSVGElement

Allowing the unhandledrejection event to bubble will eventually result in an error message being output to the console. You can prevent this by calling preventDefault() on the PromiseRejectionEvent; see Preventing 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.

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 the onunhandledrejection event handler property to set up the event listener:

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

Many environments (such as Node.js) report unhandled promise rejections to the console by default. You can prevent that from happening by adding a handler for unhandledrejection events that—in addition to any other tasks you wish to perform—calls preventDefault() to cancel the event, preventing it from bubbling up to be handled by the runtime's logging code. This works because unhandledrejection 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();
});

• Promise rejection events
• rejectionhandled event
• Promise