Detecting the onload event of a window opened with window.open

var myPopup = window.open(...);
myPopup.addEventListener('load', myFunction, false);

If you care about IE, use the following as the second line instead:

myPopup[myPopup.addEventListener ? 'addEventListener' : 'attachEvent'](
  (myPopup.attachEvent ? 'on' : '') + 'load', myFunction, false
);

As you can see, supporting IE is quite cumbersome and should be avoided if possible. I mean, if you need to support IE because of your audience, by all means, do so.

If the pop-up's document is from a different domain, this is simply not possible.

Update April 2015: I was wrong about this: if you own both domains, you can use window.postMessage and the message event in pretty much all browsers that are relevant today.

If not, there's still no way you'll be able to make this work cross-browser without some help from the document being loaded into the pop-up. You need to be able to detect a change in the pop-up that occurs once it has loaded, which could be a variable that JavaScript in the pop-up page sets when it handles its own load event, or if you have some control of it you could add a call to a function in the opener.

As noted at Detecting the onload event of a window opened with window.open, the following solution is ideal:

/* Internet Explorer will throw an error on one of the two statements, Firefox on the other one of the two. */
(function(ow) {
    ow.addEventListener("load", function() { alert("loaded"); }, false);
    ow.attachEvent("onload", function() { alert("loaded"); }, false);
})(window.open(prompt("Where are you going today?", location.href), "snapDown"));

Other comments and answers perpetrate several erroneous misconceptions as explained below.

The following script demonstrates the fickleness of defining onload. Apply the script to a "fast loading" location for the window being opened, such as one with the file: scheme and compare this to a "slow" location to see the problem: it is possible to see either onload message or none at all (by reloading a loaded page all 3 variations can be seen). It is also assumed that the page being loaded itself does not define an onload event which would compound the problem.

The onload definitions are evidently not "inside pop-up document markup":

var popup = window.open(location.href, "snapDown");
popup.onload = function() { alert("message one"); };
alert("message 1 maybe too soon\n" + popup.onload);
popup.onload = function() { alert("message two"); };
alert("message 2 maybe too late\n" + popup.onload);

What you can do:

• open a window with a "foreign" URL
• on that window's address bar enter a javascript: URI -- the code will run with the same privileges as the domain of the "foreign" URL
  The javascript: URI may need to be bookmarked if typing it in the address bar has no effect (may be the case with some browsers released around 2012)

Thus any page, well almost, irregardless of origin, can be modified like:

if(confirm("wipe out links & anchors?\n" + document.body.innerHTML))
    void(document.body.innerHTML=document.body.innerHTML.replace(/<a /g,"< a "))

Well, almost:

jar:file:///usr/lib/firefox/omni.ja!/chrome/toolkit/content/global/aboutSupport.xhtml, Mozilla Firefox's troubleshooting page and other Jar archives are exceptions.

As another example, to routinely disable Google's usurping of target hits, change its rwt function with the following URI:

javascript: void(rwt = function(unusurpURL) { return unusurpURL; })

(Optionally Bookmark the above as e.g. "Spay Google" ("neutralize Google"?)

This bookmark is then clicked before any Google hits are clicked, so bookmarks of any of those hits are clean and not the mongrelized perverted aberrations that Google made of them.

Tests done with Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:11.0) Gecko/20100101 Firefox/11.0 UA string.

It should be noted that addEventListener in Firefox only has a non-standard fourth, boolean parameter, which if true allows untrusted content triggers to be instantiated for foreign pages.

Reference:
element.addEventListener | Document Object Model (DOM) | MDN:
Interaction between privileged and non-privileged pages | Code snippets | MDN: