JavaScript - How do I call a JavaScript function AFTER everything has been rendered on the page?

Posted on 2008-06-25
Last Modified: 2010-04-21
Does anybody know how to call a JavaScript function after the page has been completely rendered. I am trying to hide certain parts of the page using display:none but when I use window.onload I get an error saying the element cant be found, I know it is definately there.
Question by:R2C
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Expert Comment

ID: 21866041
I normally do this by downloading the jQuery library and plugging it into the page

eg. <script type="text/javascript" src="jquery.js"></script>

and doing the following:


Expert Comment

ID: 21866256
Another way that relies on a library would be to use prototype.js (

First include the script:

<script src="prototype.js" type="text/javascript" language="javascript"></script>

And then you can call functions after the page is finished loading like so:

Event.observe(window, 'load', function() {
  // call a function or run some code.

The benefit to this approach is that you can have as many of those blocks of code as you would like and none of them will override the other. When you write something into the window.onload property, you overwrite any previous value. Thus,

window.onload = function() { alert("Hello."); };
window.onload = function() { alert("World."); };

Would result in an alert dialog saying "World" and nothing more. If you wanted them both to fire, you could use Event.observe for both.

Event.observe(window, 'load', function() { alert("Hello."); });
Event.observe(window, 'load', function() { alert("World."); });

Then you would get two dialogs in succession. You can use Event.observe() to observe any event from any object, as well. Such as clicks, window resizes, etc. Documentation is at
LVL 41

Expert Comment

ID: 21866335
What does your


look like?  And what does the specified code look like?

The function specified by window.onload will be called once the page is
"completely" loaded.  However, if the function that you invoke changes
(or tries to change) the DOM, or the page, you could get the error that
you are seeing.

I have used this construct numerous times, with great success:

<body onload='init()'>
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LVL 16

Expert Comment

ID: 21880104
The issue here is not with window.onload. This is a simple event and should work every time.

The issue is with your code in getting the element. Make sure you are using the correct id name. Another common mistake is trying to get an element before the page has loaded. You must get the element after the page has loaded (or at least until after the element has been created), and then you can do anything you like with it.

For example:

<!-- This script will result in an error in function load -->
   var element = document.getElementById("divExample");
   var load()
   { = "none";
   window.onload = load;
<!-- This script will succeed -->
   var element;
   var load()
      element = document.getElementById("divExample"); = "none";
   window.onload = load;

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Author Comment

ID: 21882533
I think I have figured out what is happening, I could still do with a hand figuring out how to fix it though!
The page is using AJAX to do partial page postbacks, on this page there are several panels that are made invisible using server-side code.
One of these hidden panels has the elements I am trying to hide and show using JavaScript.
This panel becomes visible when I click on a certain object (an image button I have put in a template field of a grid).
I am assuming the JavaScript cant find the object when the page first loads because it is within a panel hidden with server-side code.
I need this JavaScript to run at some point so put the code in the onClientClick of the image button (in the grid). It had no effect, the JavaScript ran before the page was re-rendered.
To slightly alter my original question, how do I call a JavaScript function after a partial page postback and all the objects have been redrawn?
Sorry if im being a pain.
LVL 41

Expert Comment

ID: 21882660
You could use a timer to check periodically for the presence of the document element.

function Something( id ) {
  var ele = document.getElementById( id );
  if ( ele ) {
    // do whatever you want with the element
  } else {
    SetTimeout( 'Something("' + id + '")', 1000 );  // Try again in 1 second

Open in new window


Author Comment

ID: 21883808
I think you might be on to something, that would work perfectly - except it doesnt want to!!
I have put the name of the object straight into the JavaScript as I am only going to use this code once. The problem seems to be with SetTimeout, everytime it gets to it I get 'SetTimeout is not defined' in Firebug. I have tried a whole load double quote, single quote and bracket combinations incase I have the syntax wrong but nothing seems to work!!

function Timer() {
  var ele = document.getElementById("Tab1");
  if ( ele ) {
else {
    SetTimeout(Timer, 1000)   }
LVL 41

Accepted Solution

HonorGod earned 500 total points
ID: 21887298
Drat...  Please forgive my typographical error.

It should have a leading lowercase s


Author Closing Comment

ID: 31470575
Excellent work, I probably should have noticed or checked that myself. Thank you very much
LVL 41

Expert Comment

ID: 21917814
You are very welcome.  Thank you for the grade & points.

Good luck & have a great day

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