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 => { };
In addition to the Window
interface, the event handler property onunhandledrejection
is also available on the following targets:
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.
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', function (event) {
event.preventDefault();
});