How to make JavaScript execute after page load?

javascripthtmldomdom-eventspageload
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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?

Solution 1

These solutions will work:

As mentioned in comments use defer:

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

or

<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.

Solution 2

Triggering scripts in the right moment

A quick overview on how to load / run the script at the moment in which they intend to be loaded / executed.

Using "defer"

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

Using defer will trigger after domInteractive (document.readyState = "interactive") and just before "DOMContentLoaded" Event is triggered. If you need to execute the script after all resources (images, scripts) are loaded use "load" event or target one of the document.readyState states. Read further down for more information about those events / states, as well as async and defer attributes corresponding to script fetching and execution timing.

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.

Scripts with the defer attribute will prevent the DOMContentLoaded event from firing until the script has loaded and finished evaluating.

Resource: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#attributes

* See the images at the bottom for feather explanation.

Event Listeners - Keep in mind that loading of the page has more, than one event:

"DOMContentLoaded"

This event is fired when the initial HTML document has been completely loaded and parsed, without waiting for style sheets, 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 images 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 style sheets

"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.

Executes after everything is loaded and parsed:

document.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

Event Listeners with radyStates - Alternative solution (readystatechange):

You can also track document.readystatechange states to trigger script execution. 
// Place in header (do not use async or defer)
document.addEventListener('readystatechange', event => {
  switch (document.readyState) {
    case "loading":
      console.log("document.readyState: ", document.readyState,
       `- The document is still loading.`
       );
      break;
    case "interactive":
      console.log("document.readyState: ", document.readyState, 
        `- The document has finished loading DOM. `,
        `- "DOMContentLoaded" event`
        );
      break;
    case "complete":
      console.log("document.readyState: ", document.readyState, 
        `- The page DOM with Sub-resources are now fully loaded. `,
        `- "load" event`
        );
      break;
  }
});

MDN Resources: https://developer.mozilla.org/en-US/docs/Web/API/Document/readyState

Where to place your script (with & without async/defer)?

This is also very important to know where to place your script and how it positions in HTML as well as parameters like defer and async will affects script fetching, execution and HTML blocking.

* On the image below the yellow label Ready indicates the moment of ending loading HTML DOM. Then it fires: document.readyState = "interactive" >>> defered scripts >>> DOMContentLoaded event (it's sequential);

If your script uses async or defer read this: https://flaviocopes.com/javascript-async-defer/

Solution 3

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;
    }
}

Solution 4

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.

Solution 5

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.

Solution 6

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

Solution 7

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

Solution 8

JavaScript

document.addEventListener('readystatechange', event => { 

    // When HTML/DOM elements are ready:
    if (event.target.readyState === "interactive") {   //does same as:  ..addEventListener("DOMContentLoaded"..
        alert("hi 1");
    }

    // When window loaded ( external resources are loaded too- `css`,`src`, etc...) 
    if (event.target.readyState === "complete") {
        alert("hi 2");
    }
});

same for jQuery:

$(document).ready(function() {   //same as: $(function() { 
     alert("hi 1");
});

$(window).load(function() {
     alert("hi 2");
});




NOTE: - Don't use the below markup ( because it overwrites other same-kind declarations ) :

document.onreadystatechange = ...

Solution 9

<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

Solution 10

Working Fiddle on <body onload="myFunction()">

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

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

Solution 11

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); };

Solution 12

If you are using jQuery,

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

is equivalent to

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

or another short hand:

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

See What event does JQuery $function() fire on? and https://api.jquery.com/ready/

Solution 13

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

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

Solution 14

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() { }.

Solution 15

The DOMContentLoaded event fires when the initial HTML document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading.

document.addEventListener('DOMContentLoaded', (event) => {
    console.log('DOM fully loaded and parsed');
});
Expand snippet

The load event is fired when the whole page has loaded, including all dependent resources such as stylesheets and images. This is in contrast to DOMContentLoaded, which is fired as soon as the page DOM has been loaded, without waiting for resources to finish loading.

window.addEventListener('load', (event) => {
  console.log('page is fully loaded');
});
Expand snippet

Solution 16

<body onload="myFunction()">

This code works well.

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

Solution 17

Comparison

In below snippet I collect choosen methods and show their sequence. Remarks

  • the document.onload (X) is not supported by any modern browser (event is never fired)
  • if you use <body onload="bodyOnLoad()"> (F) and at the same time window.onload (E) then only first one will be executed (because it override second one)
  • event handler given in <body onload="..."> (F) is wrapped by additional onload function
  • document.onreadystatechange (D) not override document .addEventListener('readystatechange'...) (C) probably cecasue onXYZevent-like methods are independent than addEventListener queues (which allows add multiple listeners). Probably nothing happens between execution this two handlers.
  • all scripts write their timestamp in console - but scripts which also have access to div write their timestamps also in body (click "Full Page" link after script execution to see it).
  • solutions readystatechange (C,D) are executed multiple times by browser but for different document states:
    • loading - the document is loading (no fired in snippet)
    • interactive - the document is parsed, fired before DOMContentLoaded
    • complete - the document and resources are loaded, fired before body/window onload

Show code snippet

Solution 18

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" );
    });

Solution 19

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

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

  // your javascript event

});

Solution 20

$(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>

Solution 21

You can write a function on a specific script file and call it in to your body element using onload attribute.

Exemple:

<script>
  afterPageLoad() {
   //your code here
}
</script>

Now call your script into your html page using script tag:

<script src="afterload.js"></script>

into your body element; add onload attribute like this:

<body onload="afterPageLoad();">

Solution 22

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.

Solution 23

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>

Solution 24

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>

Solution 25

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

// your javascript event here

});
</script>
Expand snippet

Solution 26

i can catch page load by this code

<script>
console.log("logger saber");

window.onload = (event) => {
  console.log('page is fully loaded');
document.getElementById("tafahomNameId_78ec7c44-beab-40de-9326-095f474519f4_$LookupField").value = 1;;
};

</script>

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