Window: unhandledrejection event
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.
Bubbles | No |
---|---|
Cancelable | Yes |
Interface | PromiseRejectionEvent |
Event handler property | onunhandledrejection |
Usage notes
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.
Examples
Here we have a few examples showing ways you can make use of the unhandledrejection
event. The event includes two useful pieces of information:
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 the onunhandledrejection
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 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) {
// ...your code here to handle the unhandled rejection...
// Prevent the default handling (such as outputting the
// error to the console)
event.preventDefault();
});
Specifications
Specification |
---|
HTML Standard # event-unhandledrejection |
Browser compatibility
BCD tables only load in the browser
See also
- Promise rejection events in Using Promises
onunhandledrejection
event handler propertyrejectionhandled
eventPromise