Javascript Deferred Functions - javascript

I've been looking around at JavaScript to try and get some code to load asynchronously
I found this code sample
<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>
from http://www.feedthebot.com/pagespeed/defer-loading-javascript.html
The thing is it doesn't quite do what I'm trying to achieve. I would like to be able to do something similar but setup a deferred or some type of function that is called after this has all loaded. Is that possible? If so can anyone help explain how?

You can try to use the onload event of the script tags you are loading. See this question, for example: Trying to fire the onload event on script tag. However, this mechanism seems pretty sketchy and may not be cross-browser.
Another possible approach is to have the script that is being loaded trigger an event that can be handled by the existing javascript on the page. This may or may not make sense for your particular case, and it requires you to have control over the loaded script.
Finally, these days it's rare for javascript loading to be a performance bottleneck for your website. So why are you trying to dynamically load javascript? Could you solve the same problem by loading some other resource, e.g. by doing an AJAX request?

You've tagged jQuery on your question. It has $.getScript() which does exactly what you're asking for in a purely cross browser fashion. It will load the script asynchronously and then call the specified callback when the script has finished loading and initializing:
$.getScript("defer.js", function() {
// script loaded here
// you can run code here that uses that script
});
If you really want to wait until the DOM is loaded before loading this script (generally not necessary), you could do this:
$(document).ready(function() {
$.getScript("defer.js", function() {
// script loaded here
// you can run code here that uses that script
});
});
Or, to wait until ALL other resources are loaded including images:
$(window).load(function() {
$.getScript("defer.js", function() {
// script loaded here
// you can run code here that uses that script
});
});
If you're interested in some references on when scripts are loaded (particularly with defer and async attributes, you can read this detailed post.

Related

How to wait to execute JS until readyState == Complete for document and window? [duplicate]

I need to execute some JavaScript code when the page has fully loaded. This includes things like images.
I know you can check if the DOM is ready, but I don’t know if this is the same as when the page is fully loaded.
That's called load. It came waaaaay before DOM ready was around, and DOM ready was actually created for the exact reason that load waited on images.
window.addEventListener('load', function () {
alert("It's loaded!")
})
For completeness sake, you might also want to bind it to DOMContentLoaded, which is now widely supported
document.addEventListener("DOMContentLoaded", function(event){
// your code here
});
More info: https://developer.mozilla.org/en-US/docs/Web/Events/DOMContentLoaded
Usually you can use window.onload, but you may notice that recent browsers don't fire window.onload when you use the back/forward history buttons.
Some people suggest weird contortions to work around this problem, but really if you just make a window.onunload handler (even one that doesn't do anything), this caching behavior will be disabled in all browsers. The MDN documents this "feature" pretty well, but for some reason there are still people using setInterval and other weird hacks.
Some versions of Opera have a bug that can be worked around by adding the following somewhere in your page:
<script>history.navigationMode = 'compatible';</script>
If you're just trying to get a javascript function called once per-view (and not necessarily after the DOM is finished loading), you can do something like this:
<img src="javascript:location.href='javascript:yourFunction();';">
For example, I use this trick to preload a very large file into the cache on a loading screen:
<img src="bigfile"
onload="this.location.href='javascript:location.href=\'javascript:doredir();\';';doredir();">
Try this it Only Run After Entire Page Has Loaded
By Javascript
window.onload = function(){
// code goes here
};
By Jquery
$(window).bind("load", function() {
// code goes here
});
Try this code
document.onreadystatechange = function () {
if (document.readyState == "complete") {
initApplication();
}
}
visit https://developer.mozilla.org/en-US/docs/DOM/document.readyState for more details
Javascript using the onLoad() event, will wait for the page to be loaded before executing.
<body onload="somecode();" >
If you're using the jQuery framework's document ready function the code will load as soon as the DOM is loaded and before the page contents are loaded:
$(document).ready(function() {
// jQuery code goes here
});
the window.onload event will fire when everything is loaded, including images etc.
You would want to check the DOM ready status if you wanted your js code to execute as early as possible, but you still need to access DOM elements.
You may want to use window.onload, as the docs indicate that it's not fired until both the DOM is ready and ALL of the other assets in the page (images, etc.) are loaded.
In modern browsers with modern javascript (>= 2015) you can add type="module" to your script tag, and everything inside that script will execute after whole page loads. e.g:
<script type="module">
alert("runs after") // Whole page loads before this line execute
</script>
<script>
alert("runs before")
</script>
also older browsers will understand nomodule attribute. Something like this:
<script nomodule>
alert("tuns after")
</script>
For more information you can visit javascript.info.
And here's a way to do it with PrototypeJS:
Event.observe(window, 'load', function(event) {
// Do stuff
});
The onload property of the GlobalEventHandlers mixin is an event
handler for the load event of a Window, XMLHttpRequest, element,
etc., which fires when the resource has loaded.
So basically javascript already has onload method on window which get executed which page fully loaded including images...
You can do something:
var spinner = true;
window.onload = function() {
//whatever you like to do now, for example hide the spinner in this case
spinner = false;
};
Completing the answers from #Matchu and #abSiddique.
This:
window.addEventListener('load', (event) => {
console.log('page is fully loaded');
});
Is the same as this but using the onload event handler property:
window.onload = (event) => {
console.log('page is fully loaded');
};
Source:
https://developer.mozilla.org/en-US/docs/Web/API/Window/load_event
Live example here:
https://developer.mozilla.org/en-US/docs/Web/API/Window/load_event#live_example
If you need to use many onload use $(window).load instead (jQuery):
$(window).load(function() {
//code
});
2019 update: This is was the answer that worked for me. As I needed multiple ajax requests to fire and return data first to count the list items.
$(document).ajaxComplete(function(){
alert("Everything is ready now!");
});

Execute jQuery after Other JS file Dynamically loads content

I'm working to modify some content which is dynamically loaded via another script(let's call is script #1) onto my site. Script #1 loads some markup and content and I've been using the setTimeout() function to call my script (Script #2) using a delay of a few seconds, in order to wait to be sure that Script #1 has executed and the content is present in the DOM.
My issue is that Script#1 has different loading times, based on the server load and can be slow or fast depending on these factors, and right now, playing it safe with setTimeout() I'm often left with a second or two where my scripts are still waiting to be fired and Script #1 has already loaded the content.
How can I execute my script as soon as Script#1 successfully loads it's dynamic content?
I've found this post which does seem to address the same issue but using the setInterval function as #Matt Ball has laid out there doesn't work at all for some reason. I'm using the code below where 'div.enrollment' is meant to find in the DOM which is dynamically loaded and execute..
jQuery(window).load(function ($)
{
var i = setInterval(function ()
{
if ($('div.enrollment').length)
{
clearInterval(i);
// safe to execute your code here
console.log("It's Loaded");
}
}, 100);
});
Any help on guidance on this would be greatly appreciated! Thanks for your time.
It seems that the healcode.js is doing a lot of stuff. There is a whole lot of markup added to the <healcode-widget> tag.
I would try to add another tag with an id inside and test for its existence:
<healcode-widget ....><div id="healCodeLoading"></div></healcode-widget>
Test in an interval for the existence of healCodeLoading inside <healcode-widget>: (Assuming jQuery)
var healCodeLoadingInterval = setInterval(function(){
var healCodeLoading = jQuery('healcode-widget #healCodeLoading');
if (healCodeLoading.length == 0) {
clearInterval(healCodeLoadingInterval);
// Everything should be loaded now, so you can do something here
}
}, 100);
healcode.js should replace everything inside <healcode-widget></healcode-widget> during init. So, if your <div>-element is no longer inside, the widget has loaded and initialized.
Hope that helps.
If you just want to load some markup and content and then run some script afterwards, you can use jQuery. You should use something like the following in script#1 to run a function in script#2
$.get( "ajax/test.html", function( data ) {
// Now you can do something with your data and run other script.
console.log("It's Loaded");
});
The function is called, after ajax/test.html is loaded.
Hope that helps

How to execute code before window.load and after DOM has been loaded?

Here is the circumstance:
I have 2 pages:
1 x html page
1 x external Javascript
Now in the html page, there will be internal Javascript coding to allow the placement of the window.onload, and other page specific methods/functions.
But, in the external Javascript I want certain things to be done before the window.onload event is triggered. This is to allow customized components to be initialized first.
Is there a way to ensure initialization to occur in the external Javascript before the window.onload event is triggered?
The reason I have asked this, is to attempt to make reusable code (build once - use all over), to which the external script must check that it is in 'order/check' before the Javascript in the main html/jsp/asp/PHP page takes over. And also I am not looking for a solution in jQuery #_#
Here are some of the links on Stack Overflow I have browsed through for a solution:
Javascript - How to detect if document has loaded (IE 7/Firefox 3)
How to check if page has FULLY loaded(scripts and all)?
Execute Javascript When Page Has Fully Loaded
Can someone help or direct me to a solution, your help will be muchness of greatness appreciated.
[updated response - 19 November 2012]
Hi all, thanks for you advice and suggested solutions, they have all been useful in the search and testing for a viable solution.
Though I feel that I am not 100% satisfied with my own results, I know your advice and help has moved me closer to a solution, and may indeed aid others in a similar situation.
Here is what I have come up with:
test_page.html
<html>
<head>
<title></title>
<script type="text/javascript" src="loader.js"></script>
<script type="text/javascript" src="test_script_1.js"></script>
<script type="text/javascript" src="test_script_2.js"></script>
<script type="text/javascript">
window.onload = function() {
document.getElementById("div_1").innerHTML = "window.onload complete!";
}
</script>
<style type="text/css">
div {
border:thin solid #000000;
width:500px;
}
</head>
<body>
<div id="div_1"></div>
<br/><br/>
<div id="div_2"></div>
<br/><br/>
<div id="div_3"></div>
</body>
</html>
loader.js
var Loader = {
methods_arr : [],
init_Loader : new function() {
document.onreadystatechange = function(e) {
if (document.readyState == "complete") {
for (var i = 0; i < Loader.methods_arr.length; i++) {
Loader.method_arr[i]();
}
}
}
},
load : function(method) {
Loader.methods_arr.push(method);
}
}
test_script_1.js
Loader.load(function(){initTestScript1();});
function initTestScript1() {
document.getElementById("div_1").innerHTML = "Test Script 1 Initialized!";
}
test_script_2.js
Loader.load(function(){initTestScript2();});
function initTestScript2() {
document.getElementById("div_2").innerHTML = "Test Script 2 Initialized!";
}
This will ensure that scripts are invoked before invocation of the window.onload event handler, but also ensuring that the document is rendered first.
What do you think of this possible solution?
Thanking you all again for the aid and help :D
Basically, you're looking for this:
document.onreadystatechange = function(e)
{
if (document.readyState === 'complete')
{
//dom is ready, window.onload fires later
}
};
window.onload = function(e)
{
//document.readyState will be complete, it's one of the requirements for the window.onload event to be fired
//do stuff for when everything is loaded
};
see MDN for more details.
Do keep in mind that the DOM might be loaded here, but that doesn't mean that the external js file has been loaded, so you might not have access to all the functions/objects that are defined in that script. If you want to check for that, you'll have to use window.onload, to ensure that all external resources have been loaded, too.
So, basically, in your external script, you'll be needing 2 event handlers: one for the readystatechange, which does what you need to be done on DOMready, and a window.onload, which will, by definition, be fired after the document is ready. (this checks if the page is fully loaded).
Just so you know, in IE<9 window.onload causes a memory leak (because the DOM and the JScript engine are two separate entities, the window object never gets unloaded fully, and the listener isn't GC'ed). There is a way to fix this, which I've posted here, it's quite verbose, though, but just so you know...
If you want something to be done right away without waiting for any event then you can just do it in the JavaScript - you don't have to do anything for your code to run right away, just don't do anything that would make your code wait. So it's actually easier than waiting for events.
For example if you have this HTML:
<div id=one></div>
<script src="your-script.js"></script>
<div id=two></div>
then whatever code is in your-script.js will be run after the div with id=one but before the div with id=two is parsed. Just don't register event callbacks but do what you need right away in your JavaScript.
javascript runs from top to bottom. this means.. if you include your external javascript before your internal javascript it would simply run before the internal javascript runs.
It is also possible to use the DOMContentLoaded event of the Window interface.
addEventListener("DOMContentLoaded", function() {
// Your code goes here
});
The above code is actually adding the event listener to the window object, though it's not qualified as window.addEventListener because the window object is also the global scope of JavaScript code in webpages.
DOMContentLoaded happens before load, when images and other parts of the webpage aren't still fully loaded. However, all the elements added to the DOM within the initial call stack are guaranteed to be already added to their parents prior to this event.
You can find the official documentation here.

How to test if jQuery and certain other scripts have loaded?

I know about document.ready() but I don't want to wait for some content on external servers like Google Analytics, some ad serving content or anything else that isn't absolutely required for the website. I see this problem on sites that have user comments, and usually every single external piece of content has to be loaded before comments become available to use and even if one of the many CDNs is late, it blocks everything else.
I don't think you need to test it as much as you need to defer it. Here is a really simple example from Google.
(the following code copied from Google)
// Add a script element as a child of the body
function downloadJSAtOnload() {
var element = document.createElement("script");
element.src = "deferredfunctions.js";
document.body.appendChild(element);
}
// Check for browser support of event handling capability
if (window.addEventListener)
window.addEventListener("load", downloadJSAtOnload, false);
else if (window.attachEvent)
window.attachEvent("onload", downloadJSAtOnload);
else window.onload = downloadJSAtOnload;
Essentially, add a new script tag at the documentReady state when you're executing your code already.

Checking if iframe is ready to be written to

A 3rd party script on my web page creates an iframe. I need to know when this iframe is ready, so I can manipulate its DOM.
I can think of a hacky approach: repeatedly try to modify the iFrame's DOM, and return success when a change we make sticks between two attempts. For this to work, I would prefer a property I can check on the iframe repeatedly.
Is there an alternative, cross-browser evented approach to knowing that the iframe is ready? E.g. can we redefine the onLoad function to call into our code (but I don't know if I can do this, since I didn't create the iframe).
using jquery?
function callIframe(url, callback) {
$(document.body).append('<IFRAME id="myId" ...>');
$('iframe#myId').attr('src', url);
$('iframe#myId').load(function()
{
callback(this);
});
}
Question answered in jQuery .ready in a dynamically inserted iframe
Have a variable in the parent:
var setToLoad = false;
Follow it up with a function to set it:
function SetToLoad() {
setToLoad = true;
}
Then in the child iframe call this function using the window.opener
function CallSetToLoad() {
window.opener.SetToLoad();
}
window.onload = CallSetToLoad;
This won't run until the iframe is finished loading, and if it's in the same domain it'll allow access to the opener. This would require editing a small portion of the 3rd party script.
EDIT: Alternative solution
Given that you can't edit the script, you might try something like:
frames["myiframe"].onload = function()
{
// do your stuff here
}
Can you inject arbitrary scripting code into your iframe? If so, you should be able to make it so that some code executes in the iframe upon the the iframe loading that calls code in the parent page.
First: You probably won't be able to manipulate its dom when it loads html from other domain
And You probably are interested in DOMready.
Look here:
jQuery .ready in a dynamically inserted iframe
Bilal,
There's a document.readyState property that you can check (works in IE).
foo(){
if([your iframe id].document.readyState != "complete")
{
setTimeout("foo()", 500); //wait 500 ms then call foo
}
else
{
//iframe doc is ready
}
}
Arkady

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