JavaScript that executes after page load

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

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?

Was it helpful?

Solution

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.

OTHER TIPS

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;
    }
}

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.

Exectues 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

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.

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.

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

Look at hooking document.onload or in jQuery $(document).load(...).

document.onreadystatechange = function(){
     if(document.readyState === 'complete'){
         /*code here*/
     }
}

look here: http://msdn.microsoft.com/en-us/library/ie/ms536957(v=vs.85).aspx

Best method, recommended by Google also. :)

<script type="text/javascript">
  function downloadJSAtOnload() {
   var element = document.createElement("script");
   element.src = "defer.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>

http://www.feedthebot.com/pagespeed/defer-loading-javascript.html

Working Fiddle

<!DOCTYPE html>
<html>
<head>
<script>
function myFunction()
{
   alert("Page is loaded");
}
</script>
</head>

<body onload="myFunction()">
<h1>Hello World!</h1>
</body>    
</html>

If you are using jQuery,

$(function() {...});

is equivalent to

$(document).ready(function () { })

See What event does JQuery $function() fire on?

<body onload="myFunction()">

This code works well.

But window.onload method has various dependencies. So it may not work all the time.

I find sometimes on more complex pages that not all the elements have loaded by the time window.onload is fired. If that's the case, add setTimeout before your function to delay is a moment. It's not elegant but it's a simple hack that renders well.

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

becomes...

window.onload = function(){ setTimeout( function(){ doSomethingCool(); }, 1000); };

Just define <body onload="aFunction()"> that will be called after the page has been loaded. Your code in the script is than enclosed by aFunction() { }.

$(window).on("load", function(){ ... });

.ready() works best for me.

$(document).ready(function(){ ... });

.load() will work, but it won't wait till the page is loaded.

jQuery(window).load(function () { ... });

Doesn't work for me, breaks the next-to inline script. I am also using jQuery 3.2.1 along with some other jQuery forks.

To hide my websites loading overlay, I use the following:

<script>
$(window).on("load", function(){
$('.loading-page').delay(3000).fadeOut(250);
});
</script>

Using the YUI library (I love it):

YAHOO.util.Event.onDOMReady(function(){
    //your code
});

Portable and beautiful! However, if you don't use YUI for other stuff (see its doc) I would say that it's not worth to use it.

N.B. : to use this code you need to import 2 scripts

<script type="text/javascript" src="http://yui.yahooapis.com/2.7.0/build/yahoo/yahoo-min.js" ></script>
<script type="text/javascript" src="http://yui.yahooapis.com/2.7.0/build/event/event-min.js" ></script>

There is a very good documentation on How to detect if document has loaded using Javascript or Jquery.

Using the native Javascript this can be achieved

if (document.readyState === "complete") {
 init();
 }

This can also be done inside the interval

var interval = setInterval(function() {
    if(document.readyState === 'complete') {
        clearInterval(interval);
        init();
    }    
}, 100);

Eg By Mozilla

switch (document.readyState) {
  case "loading":
    // The document is still loading.
    break;
  case "interactive":
    // The document has finished loading. We can now access the DOM elements.
    var span = document.createElement("span");
    span.textContent = "A <span> element.";
    document.body.appendChild(span);
    break;
  case "complete":
    // The page is fully loaded.
    console.log("Page is loaded completely");
    break;
}

Using Jquery To check only if DOM is ready

// A $( document ).ready() block.
$( document ).ready(function() {
    console.log( "ready!" );
});

To check if all resources are loaded use window.load

 $( window ).load(function() {
        console.log( "window loaded" );
    });

Use this code with jQuery library, this would work perfectly fine.

$(window).bind("load", function() { 

  // your javascript event

});

As Daniel says, you could use document.onload.

The various javascript frameworks hwoever (jQuery, Mootools, etc.) use a custom event 'domready', which I guess must be more effective. If you're developing with javascript, I'd highly recommend exploiting a framework, they massively increase your productivity.

My advise use asnyc attribute for script tag thats help you to load the external scripts after page load 

<script type="text/javascript" src="a.js" async></script>
<script type="text/javascript" src="b.js" async></script>


<script type="text/javascript">
$(window).bind("load", function() { 

// your javascript event here

});
</script>

use self execution onload function

window.onload = function (){
    /* statements */
}();
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