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?

Answers:

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>
Answer

This is working for me:

html:

iframe source code: <br />
<textarea id="output" rows="20" cols="60">loading ...</textarea>

javascript (on documentReady):

var iframe = document.createElement('iframe');
iframe.id = iframe.name = "testframe";
iframe.src = "http://fiddle.jshell.net";
iframe.width = 400;
iframe.height = 100;
iframe.style.display = "none";

if (iframe.addEventListener)
    iframe.addEventListener("load", loaded, false);
else
    iframe.attachEvent("onload", loaded);

function loaded() {
    var html;
    if (iframe.contentDocument)
        html = iframe.contentDocument.getElementsByTagName("HTML")[0].innerHTML;
    else
        html = window.frames[iframe.name].document.getElementsByTagName("html")[0].innerHTML;

    document.getElementById("output").value = html;
}

document.getElementsByTagName("body")[0].appendChild(iframe);

See the Demo at: http://jsfiddle.net/WcKEz/

Works with addEventListener, but includes the fallback to attachEvent. Access to the DOM of the IFRAME of course only on the same domain.

Answer

Does the first example work? Not sure exactly what you're looking for, but this should illuminate when events work: jQuery document.ready source

$(document).ready equivalent without jQuery

addEventListener does not work in IE, so if that's where you're testing, then the 2nd would fail before the iframe gets appended.

You could also add a callback from the page itself, though (for example, using jQuery so you don't reinvent the wheel) I suspect $(iframe).ready() {..} would give you consistent behavior.

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