The onerror property of the GlobalEventHandlers mixin is an event handler 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 interface ErrorEvent is fired at window and window.onerror() is invoked (as well as handlers attached by EventTarget.addEventListener (not only capturing)). - When a resource (such as an
<img> or <script>) fails to load, an error event using interface Event is fired at the element that initiated the load, and the onerror() handler on the element is invoked. These error events do not bubble up to window, but can be handled with a EventTarget.addEventListener configured with useCapture set to true.
Installing a global error event handler is useful for automated collection of error reports.
For historical reasons, different arguments are passed to window.onerror and element.onerror handlers (as well as on error-type EventTarget.addEventListener handlers).
window.onerror = function(message, source, lineno, colno, error) { };
Function parameters:
-
message: error message (string). Available as event (sic!) in HTML onerror="" 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). May be null if no corresponding Error Object is available.
When the function returns true, this prevents the firing of the default event handler.
window.addEventListener('error', function(event) { })
event of type ErrorEvent contains all the information about the event and the error.
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="imagefound.gif" onerror="this.onerror=null;this.src='imagenotfound.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.
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 "Script error." This behavior can be overridden in some browsers using the crossorigin<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].