iframe behaviour of onload vs addEventListener('load')
I've been playing around with adding hidden iframe elements to a page, and I want to manipulate the DOM of the these once loaded. I've noticed that I can't start manipulating the DOM immediately after adding the iframe to a page since it hasn't loaded yet. This can't be done with the DOMContentLoaded event since that fires against the document which doesn't exist in the iframe until it is added to the page, so we have to use the load event.
Here is some test code:
var iframe = document.createElement('iframe');
iframe.onload = function() { console.log('loaded!'); };
document.getElementsByTagName('body')[0].appendChild(iframe);
This works as expected, however when I change it to addEventListener it doesn't even get added to the DOM:
var iframe = document.createElement('iframe');
iframe.addEventListener('load', function() { console.log('loaded!'); });
document.getElementsByTagName('body')[0].appendChild(iframe);
I haven't tested attachEvent in IE.
Anyone shed any light on this?
Answer
addEventListener() function needs 3 arguments! Take a look at https://developer.mozilla.org/en/DOM/element.addEventListener
The 3rd argument is marked as optional, but then they write:
Note that this parameter is not optional in all browser versions.
I'm not sure when and where it is required, but my tests on FF4 threw an exception when calling the addEventListener with 2 arguments:
uncaught exception: [Exception... "Not enough arguments" nsresult: "0x80570001 (NS_ERROR_XPC_NOT_ENOUGH_ARGS)" location: "JS frame :: http://localhost/index.php :: :: line 10" data: no]
By the way, your code works well in Chrome [the string loaded! is logged in console].
Like FF, IE9 needs the 3rd argument in the standards mode (with <!DOCTYPE html>). IE9 is the first IE that supports W3C's event model. So in the earlier versions we need to try attachEvent. I don't have earlier IEs, but it worked in IE7/8 Standards Mode and even in Quirks Mode in IE9. Here is the code I used:
<!DOCTYPE html>
<html>
<head><title></title></head>
<body>
<script>
window.onload=function(){
var iframe = document.createElement('iframe');
var func = function() { console.log('loaded!');};
if(iframe.addEventListener)
iframe.addEventListener('load', func, true);
else if(iframe.attachEvent)
iframe.attachEvent('onload',func);
document.body.appendChild(iframe);
}
</script>
</body>
</html>