Asked 25/2, 2009 at 21:45 Answered 11/5, 2020 at 8:49

Solved javascript event-handling dom-events

896

Which is more widely supported: window.onload or document.onload?

Scrape answered 25/2, 2009 at 21:45 Comment(2)

MDN docs explains these window events: onload and DOMContentLoaded. Usage example:, window.addEventListener('DOMContentLoaded', callback). As of mid 2019, compatible with all major browsers. ----- developer.mozilla.org/en-US/docs/Web/API/Window/… ------ developer.mozilla.org/en-US/docs/Web/API/Window/load_event – Detergent 8/7, 2019 at 9:32

For me even still, in Firefox 75.0 today, window.onload and document.onload are different from each other! window.onload seems to take place afterwards and has a bit more loaded than document.onload! (Some things are working with window that aren't working with document! That goes for document.onreadstatechange 'complete' as well!) – Girlhood 24/4, 2020 at 16:33

893

When do they fire?

window.onload

By default, it is fired when the entire page loads, including its content (images, CSS, scripts, etc.).

In some browsers it now takes over the role of document.onload and fires when the DOM is ready as well.

document.onload

It is called when the DOM is ready which can be prior to images and other external content is loaded.

How well are they supported?

window.onload appears to be the most widely supported. In fact, some of the most modern browsers have in a sense replaced document.onload with window.onload.

Browser support issues are most likely the reason why many people are starting to use libraries such as jQuery to handle the checking for the document being ready, like so:

$(document).ready(function() { /* code here */ });
$(function() { /* code here */ });

For the purpose of history. window.onload vs body.onload:

A similar question was asked on codingforums a while back regarding the usage of window.onload over body.onload. The result seemed to be that you should use window.onload because it is good to separate your structure from the action.

Derrickderriey answered 25/2, 2009 at 21:46 Comment(11)

Actually that statement seems to be directed at a choice between window.onload and <body onload=""> which is completely different (and the "separate structure from action" makes a lot more sense in this context). Not that the answer is wrong, but the basis of it is. – Schleicher 17/9, 2012 at 10:39

That quote is grammatically horrible... shouldn't some (marked) editing help? – Kutchins 16/1, 2013 at 15:43

@Thor84no I finally found the time to look take another look at this. I have made a few improvements. – Derrickderriey 23/9, 2013 at 14:4

@Kutchins I decided to paraphrase the quote since it was rather rough. – Derrickderriey 23/9, 2013 at 14:4

@JoshMein Does it mean that document.onload is JS-equivalent of jQuery document.ready? – Spark 14/2, 2017 at 15:22

Does the onload event waits for xhr reqests ? – Daimyo 18/9, 2017 at 13:33

@JeyenthaaranNanjappan It does not wait for xhr requests. – Derrickderriey 18/9, 2017 at 14:21

@JeyenthaaranNanjappan XHR components have a readystatechange event that fires at 5 different stages in the operation (codes of 0-4). 4 represents that the response was received back from the server. – Cairn 21/9, 2017 at 18:31

I think this answer needs some updates. On MDN now it's classified merely into DOMContentLoaded and load events. I don't see the differences between document and window prefix as this answer describes. Check the links of the first comment under the question. – Paltry 28/7, 2020 at 2:29

Wow, this answer really has gotten a lot of (editing) love over the years! Thank you for keeping it pretty. – Kettledrummer 12/5, 2021 at 19:18

I think we couldn't call content the stuff like images, CSS, scripts (those are actually the external assets of the page, invoked by the page itself - from the html code) – Arrowwood 8/4, 2022 at 4:25

303

The general idea is that window.onload fires when the document's window is ready for presentation and document.onload fires when the DOM tree (built from the markup code within the document) is completed.

Ideally, subscribing to DOM-tree events, allows offscreen-manipulations through Javascript, incurring almost no CPU load. Contrarily, window.onload can take a while to fire, when multiple external resources have yet to be requested, parsed and loaded.

►Test scenario:

To observe the difference and how your browser of choice implements the aforementioned event handlers, simply insert the following code within your document's - <body>- tag.

<script language="javascript">
window.tdiff = []; fred = function(a,b){return a-b;};
window.document.onload = function(e){ 
    console.log("document.onload", e, Date.now() ,window.tdiff,  
    (window.tdiff[0] = Date.now()) && window.tdiff.reduce(fred) ); 
}
window.onload = function(e){ 
    console.log("window.onload", e, Date.now() ,window.tdiff, 
    (window.tdiff[1] = Date.now()) && window.tdiff.reduce(fred) ); 
}
</script>

►Result:

Here is the resulting behavior, observable for Chrome v20 (and probably most current browsers).

No document.onload event.
onload fires twice when declared inside the <body>, once when declared inside the <head> (where the event then acts as document.onload ).
counting and acting dependent on the state of the counter allows to emulate both event behaviors.
Alternatively declare the window.onload event handler within the confines of the HTML-<head> element.

►Example Project:

The code above is taken from this project's codebase (index.html and keyboarder.js).

For a list of event handlers of the window object, please refer to the MDN documentation.

Worship answered 10/9, 2011 at 12:5 Comment(0)

243

Add Event Listener

<script type="text/javascript">
  document.addEventListener("DOMContentLoaded", function(event) {
      /* 
        - Code to execute when only the HTML document is loaded.
        - This doesn't wait for stylesheets, 
          images, and subframes to finish loading. 
      */
  });
</script>

`Update March 2017`

1 Vanilla JavaScript

window.addEventListener('load', function() {
    console.log('All assets are loaded')
})

2 jQuery

$(window).on('load', function() {
    console.log('All assets are loaded')
})

Good Luck.

Wholesale answered 28/6, 2016 at 11:57 Comment(6)

"The DOMContentLoaded event is fired when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading." - developer.mozilla.org/en/docs/Web/Events/DOMContentLoaded So you seem to be incorrect about everything being loaded at this event. – Tritheism 29/3, 2017 at 8:11

@ProfK, thank you for your feedback. Can you try window.addEventListener('load', function() {...}). I've also updated my answer. – Wholesale 30/3, 2017 at 0:3

What I like about this answer is that it gives a plain-old-javascript solution. You'd think that most people think jQuery is built into all browsers, given how often it is given as the only answer. – Castellated 24/8, 2018 at 21:27

Double problem when the damn jquery takes too long to load and you get $ not found. I'm of the opinion that the $(window).ready type solutions should NEVER be trusted to work. – Arum 13/9, 2018 at 14:10

I've tried both in todays chrome. It's not waiting for css and fonts. – Feint 10/10, 2018 at 15:10

This doesn't answer the question – Measures 23/11, 2023 at 15:32

113

According to Parsing HTML documents - The end,

The browser parses the HTML source and runs deferred scripts.
A DOMContentLoaded is dispatched at the document when all the HTML has been parsed and have run. The event bubbles to the window.
The browser loads resources (like images) that delay the load event.
A load event is dispatched at the window.

Therefore, the order of execution will be

DOMContentLoaded event listeners of window in the capture phase
DOMContentLoaded event listeners of document
DOMContentLoaded event listeners of window in the bubble phase
load event listeners (including onload event handler) of window

A bubble load event listener (including onload event handler) in document should never be invoked. Only capture load listeners might be invoked, but due to the load of a sub-resource like a stylesheet, not due to the load of the document itself.

window.addEventListener('DOMContentLoaded', function() {
  console.log('window - DOMContentLoaded - capture'); // 1st
}, true);
document.addEventListener('DOMContentLoaded', function() {
  console.log('document - DOMContentLoaded - capture'); // 2nd
}, true);
document.addEventListener('DOMContentLoaded', function() {
  console.log('document - DOMContentLoaded - bubble'); // 2nd
});
window.addEventListener('DOMContentLoaded', function() {
  console.log('window - DOMContentLoaded - bubble'); // 3rd
});

window.addEventListener('load', function() {
  console.log('window - load - capture'); // 4th
}, true);
document.addEventListener('load', function(e) {
  /* Filter out load events not related to the document */
  if(['style','script'].indexOf(e.target.tagName.toLowerCase()) < 0)
    console.log('document - load - capture'); // DOES NOT HAPPEN
}, true);
document.addEventListener('load', function() {
  console.log('document - load - bubble'); // DOES NOT HAPPEN
});
window.addEventListener('load', function() {
  console.log('window - load - bubble'); // 4th
});

window.onload = function() {
  console.log('window - onload'); // 4th
};
document.onload = function() {
  console.log('document - onload'); // DOES NOT HAPPEN
};

Astto answered 22/7, 2016 at 2:49 Comment(4)

I ran your snippet and document - load - capture does indeed happen, which is counter to what I was expecting in my search as to why document load isn't happening for me. Bizarrely, it is inconsistent. Sometimes it appears, sometimes it doesn't, sometimes it appears twice - but never does a document - load - bubble happen. I would suggest not using document load. – Tombstone 31/8, 2016 at 20:36

@Tombstone Good point. I didn't consider that a load event is dispatched on external resources. That event does not bubble so it's not usually detected on the document, but it should in the capture phase. These entries refer to the load of the <style> and <script> elements. I think Edge is right for showing them, and Firefox and Chrome are wrong. – Astto 31/8, 2016 at 21:19

Thanks Oriol, the useCapture option taught me something new. – Reactor 19/2, 2020 at 16:9

Thanks for summarizing the parsing and rendering flow from w3. I just wonder after step 4 once ‘load’ event is fired, what else can happen? I noticed on my browser that sometimes there are still some objects being fetched after load event is fired although I haven’t touched or interacted with the page at all. Do you know what are those objects called? ‘Non-blocking rendering objects? – Wirer 27/4, 2020 at 4:43

In Chrome, window.onload is different from <body onload="">, whereas they are the same in both Firefox(version 35.0) and IE (version 11).

You could explore that by the following snippet:

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
        <!--import css here-->
        <!--import js scripts here-->

        <script language="javascript">

            function bodyOnloadHandler() {
                console.log("body onload");
            }

            window.onload = function(e) {
                console.log("window loaded");
            };
        </script>
    </head>

    <body onload="bodyOnloadHandler()">

        Page contents go here.

    </body>
</html>

And you will see both "window loaded"(which comes firstly) and "body onload" in Chrome console. However, you will see just "body onload" in Firefox and IE. If you run "window.onload.toString()" in the consoles of IE & FF, you will see:

"function onload(event) { bodyOnloadHandler() }"

which means that the assignment "window.onload = function(e)..." is overwritten.

Deeply answered 26/1, 2015 at 4:8 Comment(0)

window.onload and onunload are shortcuts to document.body.onload and document.body.onunload

document.onload and onload handler on all html tag seems to be reserved however never triggered

'onload' in document -> true

Teddi answered 12/6, 2014 at 0:21 Comment(0)

In short

window.onload is not supported by IE 6-8
document.onload is not supported by any modern browser (event is never fired)

window.onload   = () => console.log('window.onload works');   // fired
document.onload = () => console.log('document.onload works'); // not fired

Subclass answered 11/5, 2020 at 8:49 Comment(0)

window.onload however they are often the same thing. Similarly body.onload becomes window.onload in IE.

Chalcedony answered 25/2, 2009 at 21:47 Comment(0)

Window.onload is the standard, however - the web browser in the PS3 (based on Netfront) doesn't support the window object, so you can't use it there.

Frasco answered 19/5, 2010 at 11:31 Comment(0)

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