OnError

Object.Onerror = Action

I have created a script that will deliberately create an error message, click Link One.

You will recieve an error message similar to the following.

Line: 18
Error: 'document.fiction' is null or not an object
URL: http:\\www.huntingground.freeserve.co.uk\scripts\error.htm

Now, by including window.onerror = Do_This in the script, instead of showing the error message function Do_This will be run..
Do_This can be any function name of your choosing.

I am using function Do_This to show the difference between having a Return value of True and a Return value of False.

Click Link Two.

Notice that the error box did not show itself, this is because the statement had a ReturnValue of true

Below is the script I used for this example.

<script type="text/javascript">
<!--
function test1(){ // link one
which_test="test1"
document.fiction.value=""
}

function test2(){ // link two
which_test="test2"
document.fiction.value=""
}

window.onerror = Do_This

function Do_This() { // error function
if(which_test=="test2"){
alert("This could be a secondary function")
return true // cancels the error message
}
else{
return false // shows the error message
}
}

// -->
</script>

Here's another example using images for an onmouseover effect.

Passing your mouse over image 1 will swap the image.
Passing your mouse over image 2 will invoke the onerror event because I have deliberately left out the image source so no image will load.

Notice the only difference between the two codes is the onmouseover src and whilst both include the onerror method only the second image invokes it..

<IMG src="red1.gif" alt="A Simple Mouseover Image Change" onmouseover="this.src = 'red.gif'" onmouseout="this.src = 'red1.gif'" onerror="alert('Oops I did not put in the image source.')">

<IMG src="red1.gif" alt="What! .... No Image?" onmouseover="this.src = ' ';" onmouseout="this.src = 'red1.gif'" onerror="alert('Oops I did not put in the image source.')">

Onerror creates an array of arguments
alert(arguments.length+'\n'+arguments[0]+'\n'+arguments[1]+'\n'+arguments[2])