dynamically created iframe triggers onload event twice

Question

I created an iframe dynamically and found that this iframe trigger onload event twice.

var i=0;
frameOnload=function(){
  console.log(i++);  
};

var ifr=document.createElement("iframe");  
ifr.src="javascript:(function(){document.open();document.write('test');document.close();})();";
ifr.onload=frameOnload;  
document.body.appendChild(ifr);

Why i finally is 1?
How to prevent iframe's onload twice instead of pointing onload function to null inside itself?

Answer 1

I've also encountered the same problem, but get no answer anywhere, so I tested by myself.

The iframe onload event will be triggered twice in webkit browsers ( safari/chrome ), if you attach the onload event BEFORE the iframe is appended to the body.

So you can prevent iframe onload twice by change your codes in the following way.

document.body.appendChild(ifr);
ifr.onload=frameOnload; // attach onload event after the iframe is added to the body

Then, you will only get one onload event, which is the event the document really loaded.

Answer 2

To expand on the top-voted answer: if you can not control when and how things are attached to the DOM -- for instance, when using a framework (we've had this happening in our Angular app) -- you might want to try the solution below.

I did heavy cross-browser testing and I found the following workaround: check the parameter passed to onload callback and inspect evt.target.src in the event listener.

iframe.onload = function(evt) {
    if (evt.target.src != '') {
        // do stuff
    }
}

(if you call global method from HTML, remember to pass event in your HTML markup: <iframe onload="window.globalOnLoad(event)">)

evt.target.src can be be '' (empty string) only in webkit in the first onload call, from my tests.

Study of iframe onload behavior in different situations

iframe.onload = function(evt){
    console.log("frameOnload", ++i);
    console.log("src = '" + evt.target.src + "'");
};

iframe onload behavior with regular URL

// Chrome:  onload 1 src='', onload 2 src=requestedSrc
// IE11:    onload 1 src=requestedSrc
// Firefox: onload 1 src=requestedSrc
document.body.appendChild(iframe);
iframe.src = "http://www.example.org/";

iframe onload behavior with 404 URL

// Chrome:  onload 1 src='', onload 2 src=requestedSrc
// IE11:    onload 1 src=requestedSrc
// Firefox: onload 1 src=requestedSrc
document.body.appendChild(iframe);
iframe.src = "http://www.example.org/404";

iframe onload behavior with non-resolvable (at DNS level) URL

// Chrome:  onload 1 src='', onload 2 src=requestedSrc
// IE11:    onload 1 src=requestedSrc
// Firefox: onload NEVER triggered!
document.body.appendChild(iframe);
iframe.src= 'http://aaaaaaaaaaaaaaaaaaaaaa.example/';

iframe onload behavior with iframe.src explicitly set to about:blank before appending to DOM

// Chrome:  onload 1 src='about:blank', onload 2 src=requestedSrc
// IE11:    onload 1 src=requestedSrc
// Firefox: onload 1 src=requestedSrc
iframe.src = "about:blank";
document.body.appendChild(iframe);
iframe.src = "http://www.example.org/";

iframe onload behavior with iframe.src explicitly set to a javascript expression before appending to DOM

// Chrome:  onload 1 src='javascript:void 0', onload 2 src=requestedSrc
// IE11:    onload 1 src=requestedSrc
// Firefox: onload 1 src=requestedSrc
iframe.src = "javascript:void 0";
document.body.appendChild(iframe);
iframe.src = "http://www.example.org/";

Answer 3

I've had this happen to asynchronously loaded stylesheets, like this one.

<link rel="preload" href="stylesheet.css" as="style" onload="this.rel='stylesheet'">

I find that the simplest solution is to have the event remove itself:

<link rel="preload" href="stylesheet.css" as="style" onload="this.removeAttribute('onload'); this.rel='stylesheet'">