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How to make JavaScript execute after page load?


Question

I'm executing an external script, using a <script> inside <head>.

Now since the script executes before the page has loaded, I can't access the <body>, among other things. I'd like to execute some JavaScript after the document has been "loaded" (HTML fully downloaded and in-RAM). Are there any events that I can hook onto when my script executes, that will get triggered on page load?

2020/06/17
1
768
6/17/2020 6:49:55 PM

Accepted Answer

These solutions will work:

<body onload="script();">

or

document.onload = function ...

or even

window.onload = function ...

Note that the last option is a better way to go since it is unobstrusive and is considered more standard.

2017/05/23
869
5/23/2017 12:26:35 PM

Keep in mind that loading the page has more than one stage. Btw, this is pure JavaScript

"DOMContentLoaded"

This event is fired when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading. At this stage you could programmatically optimize loading of images and css based on user device or bandwidth speed.

Executes after DOM is loaded (before img and css):

document.addEventListener("DOMContentLoaded", function(){
    //....
});

Note: Synchronous JavaScript pauses parsing of the DOM. If you want the DOM to get parsed as fast as possible after the user requested the page, you could turn your JavaScript asynchronous and optimize loading of stylesheets

"load"

A very different event, load, should only be used to detect a fully-loaded page. It is an incredibly popular mistake to use load where DOMContentLoaded would be much more appropriate, so be cautious.

Exectues after everything is loaded and parsed:

window.addEventListener("load", function(){
    // ....
});

MDN Resources:

https://developer.mozilla.org/en-US/docs/Web/Events/DOMContentLoaded https://developer.mozilla.org/en-US/docs/Web/Events/load

MDN list of all events:

https://developer.mozilla.org/en-US/docs/Web/Events

2020/01/13

Reasonably portable, non-framework way of having your script set a function to run at load time:

if(window.attachEvent) {
    window.attachEvent('onload', yourFunctionName);
} else {
    if(window.onload) {
        var curronload = window.onload;
        var newonload = function(evt) {
            curronload(evt);
            yourFunctionName(evt);
        };
        window.onload = newonload;
    } else {
        window.onload = yourFunctionName;
    }
}
2015/10/08

You can put a "onload" attribute inside the body

...<body onload="myFunction()">...

Or if you are using jQuery, you can do

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

or 

$(window).load(function(){ /*code here*/ })

I hope it answer your question.

Note that the $(window).load will execute after the document is rendered on your page.

2009/04/30

If the scripts are loaded within the <head> of the document, then it's possible use the defer attribute in script tag.

Example:

<script src="demo_defer.js" defer></script>

From https://developer.mozilla.org:

defer

This Boolean attribute is set to indicate to a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded.

This attribute must not be used if the src attribute is absent (i.e. for inline scripts), in this case it would have no effect.

To achieve a similar effect for dynamically inserted scripts use async=false instead. Scripts with the defer attribute will execute in the order in which they appear in the document.

2018/02/21

Here's a script based on deferred js loading after the page is loaded,

<script type="text/javascript">
  function downloadJSAtOnload() {
      var element = document.createElement("script");
      element.src = "deferredfunctions.js";
      document.body.appendChild(element);
  }

  if (window.addEventListener)
      window.addEventListener("load", downloadJSAtOnload, false);
  else if (window.attachEvent)
      window.attachEvent("onload", downloadJSAtOnload);
  else window.onload = downloadJSAtOnload;
</script>

Where do I place this?

Paste code in your HTML just before the </body> tag (near the bottom of your HTML file).

What does it do?

This code says wait for the entire document to load, then load the external file deferredfunctions.js.

Here's an example of the above code - Defer Rendering of JS

I wrote this based on defered loading of javascript pagespeed google concept and also sourced from this article Defer loading javascript

2017/03/22

Source: https://stackoverflow.com/questions/807878
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