stop using “window.onload” in JavaScript

2012 update: this entry is four years old – time for a modern revisit of my old, ugly code – try this instead:

window.addEventListener ? 
window.addEventListener("load",yourFunction,false) : 
window.attachEvent && window.attachEvent("onload",yourFunction);

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I don’t write full blown AJAX but I do code some JavaScript from time to time. One big tip I can offer other “casual” javascript coders, since many times a script must be executed after a page is loaded, is to STOP using window.onload=your_function;

The reason for this is simple – you should be APPENDING your script to the page loaded hook, not CLOBBERING any existing scripts that may be already attached. The “window.onload” method can only hook one function at a time and these days it’s common to use several third party scripts on a site. If you are not careful, you may be left scratching your head why something stopped working – because you accidentally stopped it from loading.

There used to be some clever workarounds for this, like making a function that checked onload and appending it to a list of functions to call and replacing it with your own onload. But with modern browsers today, all that mess is not needed. Just replace your code with this (albeit longer) code:
if (window.attachEvent) {window.attachEvent('onload', your_function);}
else if (window.addEventListener) {window.addEventListener('load', your_function, false);}
else {document.addEventListener('load', your_function, false);}

Tested working in all major modern browser (IE, Firefox, Safari, Opera) and it appends instead of replaces the onload process. Hope that helps!

July 25, 2008 | Categories: ajax, javascript | Tags: ajax, javascript | 31 Comments