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:
promise
- The actual
Promise
which was rejected with no handler available to deal with the rejection. reason
- The reason that would have been passed into the rejection handler if one had existed. See
catch()
for details.
Basic error logging
This example simply 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 | Status | Comment |
---|---|---|
HTML Living Standard The definition of 'unhandledrejection' in that specification. |
Living Standard | Initial definition. |
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
unhandledrejection event | Chrome Full support 49 | Edge Full support ≤79 | Firefox
Full support
69
| IE No support No | Opera Full support 36 | Safari Full support 11 | WebView Android Full support 49 | Chrome Android Full support 49 | Firefox Android
Full support
68
| Opera Android Full support 36 | Safari iOS Full support 11.3 | Samsung Internet Android Full support 5.0 |
Legend
- Full support
- Full support
- No support
- No support
- User must explicitly enable this feature.
- User must explicitly enable this feature.
See also
- Promise rejection events in Using Promises
onunhandledrejection
event handler property1rejectionhandled
eventPromise
[1] The corresponding event handler property is defined on the WindowEventHandlers
mixin, which is available on both the Window
interface and all types of Worker
interfaces.