Onloads and Alternatives

Let's just say that you have a script that you want to distribute or that could be loaded with other scripts and you really don't want them to conflict with each other. If you need an onload event, things can get a bit weird. You'll maybe wonder why your pull-down menus don't work and your background changing does work. This snippet of code will help you out.

In order to use this, you will need to slightly edit your JavaScript. You'll likely add these lines at the bottom of your code. Make sure to change the XXXX to something unique for your program.

function XXXX_onload()
    // Insert your code here to initialize your program
    // Then, this little bit calls the old onload function
    if (XXXX_old_onload != null)
XXXX_old_onload = window.onload;
window.onload = XXXX_onload;

With the above code, you can ensure that your JavaScript will not interfere with another person's JavaScript, especially if yours is loaded after the other person's code. If the other person's code is loaded after yours, and they don't use code like what is above, you can obviously alter their program to do your bidding and make sure it won't interfere with other things you have going for yourself.

As a general rule, you should never code JavaScript that sets window.onload unless you handle the scenario where window.onload was already set.


If you want to avoid this problem completely, you can use the window.setTimeout() function.

function XXXXXXX_startup()
   // This is your startup function.
   // You'd do stuff in here.

// Set the startup function to run.
window.setTimeout('XXXXXXX_startup()', 100);

If you have external javascript files that you need to load before your function runs, you just add a couple lines. If you interact with a named div or span tag, you can also make sure that it is loaded. I just use document.getElementById(), but you can use whatever method you prefer.

// In your other .js files, make sure to add lines similar to this:
document.JS_File_Loaded = 1;
// Just change the JS_File_Loaded to be a unique variable for each
// of the different external files you need to load.

function XXXXXXX_startup()
   // Check to make sure that everything is loaded
   // Make sure to add one check per external javascript file and
   // one check per div/span tag you require.
   if (! document.JS_File_Loaded ||
       ! document.getElementById('required_div_id'))
      // Something is not yet loaded, so try again in 250 ms
      window.setTimeout('XXXXXXX_startup()', 250);
   // This is your startup function.
   // You'd do stuff in here.

// Set the startup function to run.
if (document.getElementById())
   window.setTimeout('XXXXXXX_startup()', 100);

How about a library?

This little bit of code will take over the window.onload, and then all you need to do is call onloadAdd() with your function to add more and more behaviors that should be executed when the page loads.

var onloadQueue = [];

// Pass your function or function name
function onloadAdd(func) {
    if (typeof(func) == 'function') {
    } else {
        onloadQueue.push(function() {

// Take over window.onload
if (window.onload) {

window.onload = function() {
	while (onloadQueue.length) {
		var loadFunc = onloadQueue.shift();

With this, all you need to do is keep calling onloadAdd() and hundreds of things can run without them stepping on each other's toes.

Yet Another Way

Sometimes the above code will not work for you or you prefer to use newer techniques to register your onload behavior. The below code is what I came up with. It uses the browser-specific extensions to automatically work with Firefox 2+, IE 7+, or fall back and work with the window.onload that most of the older versions and other browsers support.

Personally, I like the easier window.onload methods as detailed above. They work splendidly for me, and I only put this up here because people asked nicely.

// We use these variables to see if the onload code ran,
// to possibly store the old window.onload function, and to see
// if we have been set up.
// Make sure to change the names in your script to something unique!
var onload_ran = 0;
var onload_old = null;
var onload_init = 0;

// Here is our onload function.  Note:  We can call it multiple times,
// and that's why there is that 'if' statement right away
function my_onload() {
    if (onload_ran) return;
    onload_ran = 1;
    // Do your other onload stuff here
    // ...
    // Now we may need to handle other onloads
    if (onload_old && onload_old != null) {

// This line is required for the code to work with IE
document.write('<script id="__init_script" defer="true" src="//[]"><\/script>');

if (document.addEventListener) {
    // Mozilla
    document.addEventListener("DOMContentLoaded", my_onload, false);
    onload_init = 1;
if (! onload_init && document.getElementById) {
    // IE
    var deferScript = document.getElementById('__init_script');
    if (deferScript) {
        deferScript.onreadystatechange = function() {
	    if (this.readyState == 'complete') {
	        // Needed to add tiny delay for IE
		window.setTimeout('my_onload()', 100);
		this.onreadystatechange = '';
	// Check if the script has already been completed
	// Clear reference to prevent IE memory leaks
	deferScript = null;
	onload_init = 1;
if (! onload_init) {
    // Older and different browsers
    onload_old = window.onload;
    window.onload = my_onload;
    onload_init = 1;

Feel free to expand these methods as you see fit. It is used on my site for several of the cipher tools and the password strength checker.

Information travels in the nerves at speeds up to 439 kph (268 mph). Tyler Akins <>
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