What are the differences between JavaScript's window.onload and jQuery's $(document).ready() method?

15 Answers 15




  • 147
    @baptx: You're wrong. The ready event is specific to jQuery. It's using the DOMContentLoaded event if it's available in the browser, otherwise it emulates it. There is no exact equivalent in the DOM because the DOMContentLoaded event is not supported in all browsers. – Guffa May 19 '12 at 21:45
  • 17
    Ok but the same result exists with DOMContentLoaded, maybe you should have specified this – baptx May 19 '12 at 21:51
  • 55
    @baptx: I didn't think that it was releveant to the question, and I still don't. – Guffa May 19 '12 at 22:07
  • 1
    @baptx not same result, document ready handler will be fired even setted after DOM is effectively ready, using a promise. DOMContentLoaded won't. So document jquery ready handler can really be useful when setted in external script load asynchronously – A. Wolff Mar 7 '14 at 12:35
  • 17
    Why the downvote? If you don't explain what it is that you think is wrong, it can't improve the answer. – Guffa Mar 7 '14 at 12:53

window.onload is the built-in JavaScript event, but as its implementation had subtle quirks across browsers (Firefox, Internet Explorer 6, Internet Explorer 8, and Opera), jQuery provides document.ready, which abstracts those away, and fires as soon as the page's DOM is ready (doesn't wait for images, etc.).

$(document).ready (note that it's not document.ready, which is undefined) is a jQuery function, wrapping and providing consistency to the following events:

  • document.ondomcontentready / document.ondomcontentloaded - a newish event which fires when the document's DOM is loaded (which may be some time before the images, etc. are loaded); again, slightly different in Internet Explorer and in rest of the world
  • and window.onload (which is implemented even in old browsers), which fires when the entire page loads (images, styles, etc.)
  • 11
    There's a slight misconception here: the load event of window is implemented reasonably consistently across browsers. The problem that jQuery and other libraries are trying to solve is the one you mentioned, which is that the load event is not fired until all dependent resources such as images and stylesheets have loaded, which could be a long time after the DOM is completely loaded, rendered and ready for interaction. The event that fires in most browsers when the DOM is ready is called DOMContentLoaded, not DOMContentReady. – Tim Down Sep 13 '10 at 8:31
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    @Tim Down: reasonably is the key word here; at least some object sniffing used to be necessary, even with onload (there are differences wrt FF/IE/Opera). As for the DOMContentLoaded, you are entirely correct. Editing to clarify. – Piskvor Sep 13 '10 at 8:44
  • What kind of object sniffing do you mean? – Tim Down Sep 13 '10 at 8:55
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    @Tim Down: I know Opera had it, but the event trigger on it was slightly quirky (to trigger reliably, document.onload was usable). As far as the window.onload goes: window.onload = fn1;window.onload=fn2; - only fn2 would get invoked onload. Some free webhosts insert their own code into documents, and sometimes this clobbered the in-page code. – Piskvor Sep 13 '10 at 11:06
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    Isn't writing "document.ready" incorrect? the document object doesn't have a ready method, the jQuery object does that is returned from the $(document) call. Please edit this answer if I'm right because this is very confusing. – A. Sallai Mar 6 '14 at 9:10

$(document).ready() is a jQuery event. JQuery’s $(document).ready() method gets called as soon as the DOM is ready (which means that the browser has parsed the HTML and built the DOM tree). This allows you to run code as soon as the document is ready to be manipulated.

For example, if a browser supports the DOMContentLoaded event (as many non-IE browsers do), then it will fire on that event. (Note that the DOMContentLoaded event was only added to IE in IE9+.)

Two syntaxes can be used for this:

$( document ).ready(function() {
   console.log( "ready!" );

Or the shorthand version:

$(function() {
   console.log( "ready!" );

Main points for $(document).ready():

  • It will not wait for the images to be loaded.
  • Used to execute JavaScript when the DOM is completely loaded. Put event handlers here.
  • Can be used multiple times.
  • Replace $ with jQuery when you receive "$ is not defined."
  • Not used if you want to manipulate images. Use $(window).load() instead.

window.onload() is a native JavaScript function. The window.onload() event fires when all the content on your page has loaded, including the DOM (document object model), banner ads and images. Another difference between the two is that, while we can have more than one $(document).ready() function, we can only have one onload function.

  • 4
    Minor point: The discussion of IE is poorly worded. It isn't that IE (8 and before) "can't safely fire" until the document's readyState reaches complete, it is that IE lacked a DOMContentLoaded event. Not a matter of "safety", but a missing feature in IE, added in IE 9. – ToolmakerSteve Aug 12 '14 at 0:36
  • You are correct, so I edited the answer to reflect your comment and thanks! – James Drinkard May 13 '15 at 22:42
  • You say "It will not wait for the images to get loaded." what about other files, js most importantly? Often before calling function from another file - I need to know if it is loaded. – Darius.V Aug 12 '15 at 9:35
  • This is another topic altogether, but if I understand what you are asking, it's based on how you structure your page, including what order you place the js files. Here is a link that goes into more detail: ablogaboutcode.com/2011/06/14/… HTH, James – James Drinkard Aug 12 '15 at 14:34
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    Also, its $(document).ready(), not document.ready(). – Undefined Jan 12 '16 at 19:34

A Windows load event fires when all the content on your page is fully loaded including the DOM (document object model) content and asynchronous JavaScript, frames and images. You can also use body onload=. Both are the same; window.onload = function(){} and <body onload="func();"> are different ways of using the same event.

jQuery $document.ready function event executes a bit earlier than window.onload and is called once the DOM(Document object model) is loaded on your page. It will not wait for the images, frames to get fully load.

Taken from the following article: how $document.ready() is different from window.onload()

  • I think this is the best answer! – Haoyu Chen Nov 8 '15 at 20:56
  • 3
    I might agree, if it weren't copied word-for-word. – Kevin B Jun 30 '16 at 18:28

A word of caution on using $(document).ready() with Internet Explorer. If an HTTP request is interrupted before the entire document is loaded (for example, while a page is streaming to the browser, another link is clicked) IE will trigger the $(document).ready event.

If any code within the $(document).ready() event references DOM objects, the potential exists for those objects to be not found, and Javascript errors can occur. Either guard your references to those objects, or defer code which references those objects to the window.load event.

I have not been able to reproduce this problem in other browsers (specifically, Chrome and Firefox)


$(document).ready(function() {

    // Executes when the HTML document is loaded and the DOM is ready
    alert("Document is ready");

// .load() method deprecated from jQuery 1.8 onward

     // Executes when complete page is fully loaded, including
     // all frames, objects and images
     alert("Window is loaded");



$(document).on('ready', handler) binds to the ready event from jQuery. The handler is called when the DOM is loaded. Assets like images maybe still are missing. It will never be called if the document is ready at the time of binding. jQuery uses the DOMContentLoaded-Event for that, emulating it if not available.

$(document).on('load', handler) is an event that will be fired once all resources are loaded from the server. Images are loaded now. While onload is a raw HTML event, ready is built by jQuery.


$(document).ready(handler) actually is a promise. The handler will be called immediately if document is ready at the time of calling. Otherwise it binds to the ready-Event.

Before jQuery 1.8, $(document).load(handler) existed as an alias to $(document).on('load',handler).

Further Reading

  • can you please explain $(document).load(handler) behaves same as binding to the load-event. Unlike $.fn.ready, it will not call through immediately ? – Nabeel Khan Jan 27 '16 at 2:21
  • I think you got confused by $.fn.load that doesn't behave as event binder anymore. In fact, it's obsoleted since jquery 1.8. I updated it accordingly – abstraktor Jan 27 '16 at 11:05
  • yap! exactly, thanks :) – Nabeel Khan Jan 28 '16 at 8:30

window.onload: A normal JavaScript event.

document.ready: A specific jQuery event when the entire HTML have been loaded.


The $(document).ready() is a jQuery event which occurs when the HTML document has been fully loaded, while the window.onload event occurs later, when everything including images on the page loaded.

Also window.onload is a pure javascript event in the DOM, while the $(document).ready() event is a method in jQuery.

$(document).ready() is usually the wrapper for jQuery to make sure the elements all loaded in to be used in jQuery...

Look at to jQuery source code to understand how it's working:

jQuery.ready.promise = function( obj ) {
    if ( !readyList ) {

        readyList = jQuery.Deferred();

        // Catch cases where $(document).ready() is called after the browser event has already occurred.
        // we once tried to use readyState "interactive" here, but it caused issues like the one
        // discovered by ChrisS here: http://bugs.jquery.com/ticket/12282#comment:15
        if ( document.readyState === "complete" ) {
            setTimeout( jQuery.ready );

        // Standards-based browsers support DOMContentLoaded
        } else if ( document.addEventListener ) {
            // Use the handy event callback
            document.addEventListener( "DOMContentLoaded", completed, false );

            // A fallback to window.onload, that will always work
            window.addEventListener( "load", completed, false );

        // If IE event model is used
        } else {
            // Ensure firing before onload, maybe late but safe also for iframes
            document.attachEvent( "onreadystatechange", completed );

            // A fallback to window.onload, that will always work
            window.attachEvent( "onload", completed );

            // If IE and not a frame
            // continually check to see if the document is ready
            var top = false;

            try {
                top = window.frameElement == null && document.documentElement;
            } catch(e) {}

            if ( top && top.doScroll ) {
                (function doScrollCheck() {
                    if ( !jQuery.isReady ) {

                        try {
                            // Use the trick by Diego Perini
                            // http://javascript.nwbox.com/IEContentLoaded/
                        } catch(e) {
                            return setTimeout( doScrollCheck, 50 );

                        // detach all dom ready events

                        // and execute any waiting functions
    return readyList.promise( obj );
jQuery.fn.ready = function( fn ) {
    // Add the callback
    jQuery.ready.promise().done( fn );

    return this;

Also I have created the image below as a quick references for both:

enter image description here


Document.ready (a jQuery event) will fire when all the elements are in place, and they can be referenced in the JavaScript code, but the content is not necessarily loaded. Document.ready executes when the HTML document is loaded.

$(document).ready(function() {

    // Code to be executed
    alert("Document is ready");

The window.load however will wait for the page to be fully loaded. This includes inner frames, images, etc.

$(window).load(function() {

    //Fires when the page is loaded completely
    alert("window is loaded");

A little tip:

Always use the window.addEventListener to add an event to window. Because that way you can execute the code in different event handlers .

Correct code:

window.addEventListener('load', function () {

window.addEventListener('load', function () {

Invalid code:

window.onload = function () {
  alert('Hello!') // it will not work!!!

window.onload = function () {

This is because onload is just property of the object, which is overwritten.

By analogy with addEventListener, it is better to use $(document).ready() rather than onload.

  • Nice one! Wonder why you only got my thumbs up – gedamial Sep 8 '18 at 20:32
  • 1
    This doesn't answer the question asked. – Tiny Giant Dec 13 '18 at 3:49

One thing to remember (or should I say recall) is that you cannot stack onloads like you can with ready. In other words, jQuery magic allows multiple readys on the same page, but you can't do that with onload.

The last onload will overrule any previous onloads.

A nice way to deal with that is with a function apparently written by one Simon Willison and described in Using Multiple JavaScript Onload Functions.

function addLoadEvent(func) {
    var oldonload = window.onload;
    if (typeof window.onload != 'function') {
        window.onload = func;
    else {
        window.onload = function() {
            if (oldonload) {

// Example use:
addLoadEvent(function() {
  /* More code to run on page load */

The document.ready event occurs when the HTML document has been loaded, and the window.onload event occurs always later, when all content (images, etc) has been loaded.

You can use the document.ready event if you want to intervene "early" in the rendering process, without waiting for the images to load. If you need the images (or any other "content") ready before your script "does something", you need to wait until window.onload.

For instance, if you are implementing a "Slide Show" pattern, and you need to perform calculations based on image sizes, you may want to wait until window.onload. Otherwise, you might experience some random problems, depending on how fast the images will get loaded. Your script would be running concurrently with the thread that loads images. If your script is long enough, or the server is fast enough, you may not notice a problem, if images happen to arrive in time. But the safest practice would be allowing for images to get loaded.

document.ready could be a nice event for you to show some "loading..." sign to users, and upon window.onload, you can complete any scripting that needed resources loaded, and then finally remove the "Loading..." sign.

Examples :-

// document ready events
     alert("document is ready..");

// using JQuery
   alert("document is ready..");

// window on load event
function myFunction(){
  alert("window is loaded..");
window.onload = myFunction;

window.onload is a JavaScript inbuilt function. window.onload trigger when the HTML page loaded. window.onload can be written only once.

document.ready is a function of the jQuery library. document.ready triggers when HTML and all JavaScript code, CSS, and images which are included in the HTML file are completely loaded. document.ready can be written multiple times according to requirements.


When you say $(document).ready(f), you tell script engine to do the following:

  1. get the object document and push it, since it's not in local scope, it must do a hash table lookup to find where document is, fortunately document is globally bound so it is a single lookup.
  2. find the object $ and select it, since it's not in local scope, it must do a hash table lookup, which may or may not have collisions.
  3. find the object f in global scope, which is another hash table lookup, or push function object and initialize it.
  4. call ready of selected object, which involves another hash table lookup into the selected object to find the method and invoke it.
  5. done.

In the best case, this is 2 hash table lookups, but that's ignoring the heavy work done by jQuery, where $ is the kitchen sink of all possible inputs to jQuery, so another map is likely there to dispatch the query to correct handler.

Alternatively, you could do this:

window.onload = function() {...}

which will

  1. find the object window in global scope, if the JavaScript is optimized, it will know that since window isn't changed, it has already the selected object, so nothing needs to be done.
  2. function object is pushed on the operand stack.
  3. check if onload is a property or not by doing a hash table lookup, since it is, it is called like a function.

In the best case, this costs a single hash table lookup, which is necessary because onload must be fetched.

Ideally, jQuery would compile their queries to strings that can be pasted to do what you wanted jQuery to do but without the runtime dispatching of jQuery. This way you have an option of either

  1. do dynamic dispatch of jquery like we do today.
  2. have jQuery compile your query to pure JavaScript string that can be passed to eval to do what you want.
  3. copy the result of 2 directly into your code, and skip the cost of eval.

protected by Rajaprabhu Aravindasamy Jun 13 '14 at 7:59


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