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?
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.
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 readyStates - 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/
And if all of the above points are still to early...
What if you need your script to run after other scripts are run, including those scheduled to run at the very end (e.g. those scheduled for the "load" event)? See Run JavaScript after all window.onload scripts have completed?
What if you need to make sure your script runs after some other script, regardless of when it is run? This answer to the above question has that covered too.
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;
}
}
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(...).
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 = ...
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); };
<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 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>
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/
document.onreadystatechange = function(){
if(document.readyState === 'complete'){
/*code here*/
}
}
look here: http://msdn.microsoft.com/en-us/library/ie/ms536957(v=vs.85).aspx
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() { }.
<body onload="myFunction()">
This code works well.
But window.onload method has various dependencies. So it may not work all the time.
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
<html>
<head>
<script>
// solution A
console.log(`[timestamp: ${Date.now()}] A: Head script`);
// solution B
document.addEventListener("DOMContentLoaded", () => {
print(`[timestamp: ${Date.now()}] B: DOMContentLoaded`);
});
// solution C
document.addEventListener('readystatechange', () => {
print(`[timestamp: ${Date.now()}] C: ReadyState: ${document.readyState}`);
});
// solution D
document.onreadystatechange = s=> {print(`[timestamp: ${Date.now()}] D: document.onreadystatechange ReadyState: ${document.readyState}`)};
// solution E (never executed)
window.onload = () => {
print(`E: <body onload="..."> override this handler`);
};
// solution F
function bodyOnLoad() {
print(`[timestamp: ${Date.now()}] F: <body onload='...'>`);
infoAboutOnLoad(); // additional info
}
// solution X
document.onload = () => {print(`document.onload is never fired`)};
// HELPERS
function print(txt) {
console.log(txt);
if(mydiv) mydiv.innerHTML += txt.replace('<','<').replace('>','>') + '<br>';
}
function infoAboutOnLoad() {
console.log("window.onload (after override):", (''+document.body.onload).replace(/\s+/g,' '));
console.log(`body.onload==window.onload --> ${document.body.onload==window.onload}`);
}
console.log("window.onload (before override):", (''+document.body.onload).replace(/\s+/g,' '));
</script>
</head>
<body onload="bodyOnLoad()">
<div id="mydiv"></div>
<!-- this script must te at the bottom of <body> -->
<script>
// solution G
print(`[timestamp: ${Date.now()}] G: <body> bottom script`);
</script>
</body>
</html>
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
});
$(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(){
$('.loading-page').delay(3000).fadeOut(250);
});
</script>
You can write a function on a specific script file and call it in to your body element using onload attribute.
Exemple:
<script>
afterPageLoad() {
//your code here
}
</script>
Now call your script into your html page using script tag:
<script src="afterload.js"></script>
into your body element; add onload attribute like this:
<body onload="afterPageLoad();">
As Daniel says, you could use document.onload.
The various javascript frameworks hwoever (jQuery, Mootools, etc.) use a custom event 'domready', which I guess must be more effective. If you're developing with javascript, I'd highly recommend exploiting a framework, they massively increase your productivity.
Using the YUI library (I love it):
YAHOO.util.Event.onDOMReady(function(){
//your code
});
Portable and beautiful! However, if you don't use YUI for other stuff (see its doc) I would say that it's not worth to use it.
N.B. : to use this code you need to import 2 scripts
<script type="text/javascript" src="http://yui.yahooapis.com/2.7.0/build/yahoo/yahoo-min.js" ></script>
<script type="text/javascript" src="http://yui.yahooapis.com/2.7.0/build/event/event-min.js" ></script>
i can catch page load by this code
<script>
console.log("logger saber");
window.onload = (event) => {
console.log('page is fully loaded');
document.getElementById("tafahomNameId_78ec7c44-beab-40de-9326-095f474519f4_$LookupField").value = 1;;
};
</script>
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>
I have a small mobile page with a few big js files (including jQuery)
The page speed is pretty slow and the google page speed analysis suggests to move the 'render-blocking javascript below the fold'. My page is pretty small to begin with, so everything on it is already above the fold.
I would normally async the javascript, but the javascript needs to run in order
(First jQuery, then the plugin, and finally my own code which is inline)
So async loading on the code would cause it to run out of order, usually jquery loading last and everything else breaking before hand.
Is there an easy way to push everything below the fold? or at least make the browser treat the javascript like it is below the fold all while loading everything in order?
If all of your content is above the fold you should be able to simply put all of your javascript files towards the bottom of your document and above your closing </body> tag.
I ended up fixing the problem by wrapping the code in a callback after the page loads. Here is the code that I used:
function appendScript(src, callback) {
callback = callback || function(){};
var element = document.createElement("script");
element.src = src;
element.async = false;
element.onload = callback;
document.body.appendChild(element);
}
function downloadJSAtOnload() {
appendScript("http://code.jquery.com/jquery-1.11.1.min.js", function(){
appendScript("http://code.jquery.com/mobile/1.4.5/jquery.mobile-1.4.5.min.js", doStuff);
});
}
if (window.addEventListener)
window.addEventListener("load", downloadJSAtOnload, false);
else if (window.attachEvent)
window.attachEvent("onload", downloadJSAtOnload);
else window.onload = downloadJSAtOnload;
doStuff(){
// MY CODE HERE
}
I forgot to mention in my question that the code embedded on the page isn't much at all. It is just a few snippets but would be much harder without jquery.
I hope this helps out anyone else who is trying what I tried to do
I want to defer the following script. I need it to launch only after Keyup (aftet the mouse was clicked then I start typing). this is delaying page load I specially want to delay jquery.js.. see script below..
<script type="text/javascript" src="scripts/jQuery.js"></script>
<script defer type="text/javascript">
$(document).ready(function(){
$("#searchquery").keyup(function(){
$.get("suggest.php", {searchquery: $(this).val()}, function(data){
$("datalist").empty();
$("datalist").html(data);
});
});
});
$(document).ready(function(){
$("#searchquery2").keyup(function(){
$.get("suggest1.php", {searchquery2: $(this).val()}, function(data){
$("datalist").empty();
$("datalist").html(data);
});
});
});
</script>
The most reliable method for deferring scripts seems to be to insert a <script> element into the page at runtime. If you're deferring things like jQuery, then you'll probably want to load them once the DOM is loaded rather than when the user performs an action, which will typically get your script loaded sooner and probably give better performance to the user (e.g. when they first click something, they won't have to wait for jQuery to load before something awesome happens).
Below is an example of loading a script in about the same way as Google Analytics. If your browser supports the JS API I'm using, and the events I'm listening to, and console.log(), then you should see that window.jQuery is logged as undefined when the page loads, and then a function five seconds later (as long as jQuery loads by then!).
Disclaimer: Absolutely nothing has been done here to ensure cross-browser compatibility. This is just a simple demonstration of deferring a script.
<!doctype html>
<html>
<head></head>
<body>
<script>
(function() {
function init() {
loadDeferredScript('http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js');
console.log(window.jQuery);
setTimeout(function() { console.log(window.jQuery); }, 5000);
}
function loadDeferredScript(src) {
var script = document.createElement('script');
script.src = src;
var firstScript = document.getElementsByTagName('script')[0];
firstScript.parentNode.insertBefore(script, firstScript);
}
document.addEventListener('DOMContentLoaded', init);
})();
</script>
</body>
</html>
Google tells us we should defer loading of jquery until after the page has loaded.
I took the sample code that Google provides and modified it slightly so that the jquery library is loaded, a callback is invoked and my own custom .js file is then loaded.
It's pretty straightforward stuff.
Seems to work on FireFox, does not work on IE8.
My question:
Is there anything in the code that IE8 would not support.
(i.e. why is this code not running on IE8)
Thanks for your ideas.
David
<script type="text/javascript">
function downloadJSAtOnload()
{
getScript("//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js",function(){
getScript("/course/java/linguashop2013.js", function() {});
});
}
function getScript(url,callback)
{
var element = document.createElement("script");
element.src = url;
document.body.appendChild(element);
element.onload=callback;
}
if (window.addEventListener)
window.addEventListener("load", downloadJSAtOnload, false);
else if (window.attachEvent)
window.attachEvent("onload", downloadJSAtOnload);
else window.onload = downloadJSAtOnload;
</script>
In older IE there's no load event for script tags.
This article explains how to load scripts properly:
http://www.nczonline.net/blog/2009/07/28/the-best-way-to-load-external-javascript/