JavaScript Onerror

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JavaScript Onerror

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By using the onerror event is the old standard solution to catch errors in a webpage.

Whenever a Java Script error occurs in a page. Using it, we can create custom Java Script error dialog boxes, or better yet, suppress all Java Script errors in the document.

Understanding the onerror event

As mentioned above, the onerror event fires whenever an Java Script error occurs (you know when that happens through those annoying error boxes that pop up). Look at the title of this document again... it reads "The onerror event of the window object". The onerror event is of the window object, a rather unusual object to be attached to. It is attached this way so it can monitor all Java Script errors on a page, even those in the <head> section of the page. Recall that event handlers are usually inserted inside tags, like the below example with the onClick event handler:

<a href="http://www.academictutorial.com" onClick="alert('Vyom')">academictutorial</a>

Since onClick events react to a link being clicked on, they are naturally inserted inside the <a> tag. Now get ready for something that breaks all the rules. The onerror event, unlike most event handlers, is not inserted inside any tag. It can't, since there doesn't exist a "window" tag. Since no such tag exists in HTML, the creators of JavaScript had to devise an alternate way of declaring this event, and the syntax is as follows:

<script> window.onerror=//function to run when an error occurs </script>

The onerror event is directly attached to the window object, and is declared inside the script tags. The code on the right has to be a function call to the function that will be executed when the event fires. A picture (or in this case, an actual code) is worth a thousand words, so before anything else, lets first see an example that alerts "an error has occurred!" when a Java Script error occurs in a webpage:

<head>
<script>
function tellerror()
{
alert('An error has occurred!')
}
window.onerror=tellerror
</script>
<script type="text/javascript">
document.write('hi there'
</script>
</head>

We were defined a function (tellerror) that simply alerts a message by itself. Now we look at how it is hooked up to the onerror event- through the function's name, without the normal parentheses. You should follow this rule whenever hooking up a function to onerror events (or any event defined in this manner). In the above example, the careless Java Script programmers had forgotten to close the document.write() method with a closing parentheses.

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Keywords:JavaScript onerror, Event onerror, event of the window object

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