The onerror
property of the GlobalEventHandlers
mixin is an EventHandler
that processes error
events.
Error events are fired at various targets for different kinds of errors:
- When a JavaScript runtime error (including syntax errors and exceptions thrown within handlers) occurs, an
error
event using interfaceErrorEvent
is fired atwindow
andwindow.onerror()
is invoked (as well as handlers attached bywindow.addEventListener
(not only capturing)). - When a resource (such as an
<img>
or<script>
) fails to load, anerror
event using interfaceEvent
is fired at the element that initiated the load, and theonerror()
handler on the element is invoked. These error events do not bubble up to window, but can be handled with awindow.addEventListener
configured withuseCapture
set totrue
.
Installing a global error
event handler is useful for automated collection of error reports.
Syntax
For historical reasons, different arguments are passed to window.onerror
and element.onerror
handlers (as well as on error-type window.addEventListener
handlers).
window.onerror
window.onerror = function(message, source, lineno, colno, error) { ... };
Function parameters:
message
: error message (string). Available asevent
(sic!) in HTMLonerror=""
handler.source
: URL of the script where the error was raised (string)lineno
: Line number where error was raised (number)colno
: Column number for the line where the error occurred (number)error
: Error Object (object)
When the function returns true
, this prevents the firing of the default event handler.
window.addEventListener('error')
window.addEventListener('error', function(event) { ... })
event
of type ErrorEvent
contains all the information about the event and the error.
element.onerror
element.onerror = function(event) { ... }
element.onerror
accepts a function with a single argument of type Event
.
A good example for this is when you are using an image tag, and need to specify a backup image in case the one you need is not available on the server for any reason.
<img src="imagenotfound.gif" onerror="this.onerror=null;this.src='imagefound.gif';" />
The reason we have the this.onerror=null
in the function is that the browser will be stuck in an endless loop if the onerror image itself generates an error.
Notes
When an error occurs in a script, loaded from a different origin, the details of the error are not reported to prevent leaking information (see bug 363897). Instead the error reported is simply "Script error."
This behavior can be overriden in some browsers using the crossorigin
attribute on <script>
and having the server send the appropriate CORS HTTP response headers. A workaround is to isolate "Script error." and handle it knowing that the error detail is only viewable in the browser console and not accessible via JavaScript.
window.onerror = function (msg, url, lineNo, columnNo, error) {
var string = msg.toLowerCase();
var substring = "script error";
if (string.indexOf(substring) > -1){
alert('Script Error: See Browser Console for Detail');
} else {
var message = [
'Message: ' + msg,
'URL: ' + url,
'Line: ' + lineNo,
'Column: ' + columnNo,
'Error object: ' + JSON.stringify(error)
].join(' - ');
alert(message);
}
return false;
};
When using the inline HTML markup (<body onerror="alert('an error occurred')">
), the HTML specification requires arguments passed to onerror
to be named event
, source
, lineno
, colno
, error
. In browsers that have not implemented this requirement, they can still be obtained via arguments[0]
through arguments[2]
.
Specifications
Specification | Status | Comment |
HTML Living StandardThe definition of 'onerror' in that specification. | Living Standard |
Browser compatibility
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Update compatibility data on GitHub
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
onerror
|
Chrome
Full support 10 |
Edge
Full support 12 |
Firefox
Full support 1 |
IE
Full support 9 |
Opera
Full support 11.6 |
Safari
Full support 6 |
WebView Android
Full support ≤37 |
Chrome Android
Full support 18 |
Firefox Android
Full support 4 |
Opera Android
Full support 12 |
Safari iOS
Full support 6 |
Samsung Internet Android
Full support 1.0 |
Legend
- Full support
- Full support
See also
- Capture and report JavaScript errors with window.onerror (blog.sentry.io, 2016)
- How to catch JavaScript Errors with window.onerror (even on Chrome and Firefox) (danlimerick.wordpress.com, 2014)
GlobalEventHandlers.onerror by Mozilla Contributors is licensed under CC-BY-SA 2.5.