I'm using a JavaScript upload script that says to run the initialize function as soon as the DOM is ready. I currently have it working just fine with either a call to the function with body.onload or directly after the function is defined. The function builds some HTML in a placeholder div that acts as the file uploader tool.
My question is what is the best practice here? Since it works for now, why would the instructions say to run the init function as soon as the DOM is ready? Should I be adding a <script> tag directly after the placeholder DIV for example?
<script>
window.addEventListener("DOMContentLoaded", function() {
// do stuff
}, false);
</script>
You do that so you know all the parsed elements are available in the DOM etc.
The easiest solution is using jQuery and its $(document).ready(function() { .... }); function. Instead of .... you put your own code.
Note that it basically does the same thing #Shadow2531 suggested, but also works in old browsers not supporting that event.
The DOM is usually ready before onLoad runs. onLoad only runs after everything loads - external scripts, images, stylesheets, etc.
But the DOM, i.e. the HTML structure is ready before that. If you run the code at the bottom of the page (or after the parts of the page the script works with) that will work fine as well.
In 2015 you have two options with modern browsers:
document.onload
this fires when the document is loaded, but other resources (most notably images) have not necessarily finished loading.
window.onload
this fires when the document is loaded, AND all other resources (again, most notably images) are loaded.
Both of the above events would be better utilized with window.addEventListener() of course, as multiple listeners would be allowed.
You could also just move the <script> to the bottom of your page like this:
<html>
<body>
<main></main>
<script>
// code
</script>
</body>
</html>
As you probably know you should not run init functions before the DOM is fully loaded.
The reason you must run the init function as soon as the DOM is ready, is that once the page has loaded the user starts hitting buttons etc. You have to minimize the small inavoidable gap where the page is loaded and the init-functions haven't run yet. If this gap gets too big (ie. too long time) your user might experience inappropiate behaviour... (ie. your upload will not work).
Other users have provided fine examples of how to call the init function, so I will not repeat it here... ;)
Get jQuery and use the following code.
$(document).ready(function(){
// Do stuff
});
var Tette =
{
init: function()
{
/* Your HTML code */
}
};
Core.start(Tette);
You can try in this code, registering "Core.start(your object)" on the last line of the script. This is a way to load in safety your functions after the DOM loading.
Related
I was using $(window).load(function(){}); for my projects until somewhere I saw that somebody said we could just use $(function(){}); and they would perform identically.
But now that I have more experience I have noticed that they are not identical. I noticed that the first piece kicks in a little bit after the second piece of code.
I just want to know what's the difference?
$(document).ready(function(){})
will wait till the document is loaded(DOM tree is loaded) and not till the entire window is loaded. for example It will not wait for the images,css or javascript to be fully loaded . Once the DOM is loaded with all the HTML components and event handlers the document is ready to be processed and then the $(document).ready() will complete
$(window).load(function(){});
This waits for the entire window to be loaded. When the entire page is loaded then only the $(window).load() is completed. Hence obviously $(document).ready(function(){}) finishes before $(window).load() because populating the components(like images,css) takes more time then just loading the DOM tree.
So $(function(){}); cannot be used as a replacement for $(window).load(function(){});
From the jQuery docs itself.
The first thing that most Javascript programmers end up doing is adding some code to their program, similar to this:
window.onload = function(){ alert("welcome"); }
Inside of which is the code that you want to run right when the page is loaded. Problematically, however, the Javascript code isn't run until all images are finished downloading (this includes banner ads). The reason for using window.onload in the first place is that the HTML 'document' isn't finished loading yet, when you first try to run your code.
To circumvent both problems, jQuery has a simple statement that checks the document and waits until it's ready to be manipulated, known as the ready event:
$(document).ready(function(){
// Your code here
});
Now,
$(window).load(function(){}); is equal to window.onload = function(){ alert("welcome"); }
And, $(function(){}); is a shortcut to $(document).ready(function(){ });
I think , this clears everything :)
$(window).load from my experience waits until everything including images is loaded before running where as $(function() {}); has the same behaviour as $(document).ready(function() {});
Please someone correct me if I am wrong.
The second is/was a shortcut for $(document).ready(), which should run before window's load event.
Note that $(document).ready() is the preferred way of binding something to document load; there are a couple other ways of doing it like the one you showed, but that's preferred.
In JavaScript, when I want to run a script once when the page has loaded, should I use window.onload or just write the script?
For example, if I want to have a pop-up, should I write (directly inside the <script> tag):
alert("hello!");
Or:
window.onload = function() {
alert("hello!");
}
Both appear to run just after the page is loaded. What is the the difference?
The other answers all seem out of date
First off, putting scripts at the top and using window.onload is an anti-pattern. It's left over from IE days at best or mis-understandings of JavaScript and the browser at worst.
You can just move your scripts the the bottom of your html
<html>
<head>
<title>My Page</title>
</head>
<body>
content
<script src="some-external.js"></script>
<script>
some in page code
</script>
</body>
</html>
The only reason people used window.onload is because they mistakenly believed scripts needed to go in the head section. Because things are executed in order if your script was in the head section then the body and your content didn't yet exist by definition of execute in order.
The hacky workaround was to use window.onload to wait for the rest of the page to load. Moving your script to the bottom also solved that issue and now there's no need to use window.onload since your body and content will have already been loaded.
The more modern solution is to use the defer tag on your scripts but to use that your scripts need to all be external.
<head>
<script src="some-external.js" defer></script>
<script src="some-other-external.js" defer></script>
</head>
This has the advantage that the browser will start downloading the scripts immediately and it will execute them in the order specified but it will wait to execute them until after the page has loaded, no need for window.onload or the better but still unneeded window.addEventListener('load', ...
window.onload just runs when the browser gets to it.
window.addEventListener waits for the window to be loaded before running it.
In general you should do the second, but you should attach an event listener to it instead of defining the function. For example:
window.addEventListener('load',
function() {
alert('hello!');
}, false);
Here's the documentation on MDN.
According to it:
The load event fires at the end of the document loading process. At this point, all of the objects in the document are in the DOM, and all the images and sub-frames have finished loading.
Your first snippet of code will run as soon as browser hit this spot in HTML.
The second snippet will trigger popup when the DOM and all images are fully loaded (see the specs).
Considering the alert() function, it doesn't really matter at which point it will run (it doesn't depend on anything besides window object). But if you want to manipulate the DOM - you should definitely wait for it to properly load.
That depends on if you want it to run when the script element is encountered or if you want it to run when the load event fires (which is after the entire document (including such things as images) has loaded).
Neither is always right.
In general, however, I'd avoid assigning functions directly to onload in favour of using addEventListener (with compatibility libraries if I needed to support older browsers).
The reason for waiting for the DOM to be loaded is so that you can target any elements that load after your script. If you're just creating an alert, it doesn't matter. Let's say, however, you were targeting a div that was in your markup after your script, you would get an error if you don't wait until that piece of the DOM tree to load.
document.ready is a great alternative to window.onload if you're using jQuery.
See here: window.onload vs $(document).ready()
You have three alternatives:
Directly inside the script tag runs it as soon as it is parsed.
Inside document.addEventListener( "DOMContentLoaded", function(){}); will run it once the DOM is ready.
Inside window.onload function(){}) will run as soon as all page resources are loaded.
I have a page which has document.body.onload=function(){...} but this is suffering delays because the document also contains external <img> tags etc; onload seems to be only getting fired after these external sites have delivered some kind of response.
I put the code in onload because I thought the DOM tree wasn't fully available until then (e.g. document.getElementById(x) might return null if it is called too soon).
Is there an event which triggers after everything in the DOM is accessible, but before everything has been loaded?
You can just place your <script> tag at the end of the body since the html is parsed in sequence.
Additionally, you could look at jquery's document.ready. Even if you don't want to use jquery, you can have a look at how they handle it. ready does exactly what you're asking for.
Sure you can:
Maybe you have two options:
1)
by using jQuery:
with $(document).ready(), you can get your events to load or fire or whatever you want them to do before the window loads.
2)
DOMContentLoaded
Reference: https://developer.mozilla.org/en/DOM/DOM_event_reference/DOMContentLoaded
function checkDom(yourFunc)
{
window.addEventListener('DOMContentLoaded', yourFunc);
}
You should consider if you really need the whole document loaded. As long as your libraries are already loaded and the DOM nodes you are interested in are on the page, you can act immediately. This sort of code makes a page "come alive" much faster than one that waits for every last resource to load:
<form name="foo">
<!-- ... -->
</form>
<script>
var form = new My.FormWidget(document.forms.foo);
</script>
This has the added benefit of forcing you to encapsulate logic in the FormWidget (you should want to keep an "inline" script like that as brief as possible).
I'm learning Greasemonkey with the hopes of making some improvements to a webpage.
I think I have a good grasp of JavaScript, but I don't understand at what point in the rendering of the document the Greasemonkey user script is executed.
For instance, what if there is JavaScript natively inside the document that inserts some elements to the page. I want my Greasemonkey script to run only after that JS completes.
Let's say this is the document that I'm trying to modify with Greasemonkey
<html>
<script>
//insert a button with id="mybutton"
</script>
</html>
I want the <script> code to complete before my Greasemonkey script is run, because I want to alter its background color.
Greasemonkey runs at the DOMContentLoaded event by default. This means that everything will be in the page except for possibly large images and stuff added by some javascripts (scripts that fire on the load event or that AJAX-in content).
If you want to wait until even large media has loaded and "onload" scripts have run, use:
window.addEventListener ("load", Greasemonkey_main, false);
function Greasemonkey_main () {
//***** PUT YOUR GREASEMONKEY CODE HERE.
}
Do not use unsafeWindow.onload = function(){ ... } or window.onload = function() { /* logic here */ } as others have suggested. These are not only poor practice/won't work in GM, but the unsafeWindow is an unnecessary security risk in this case.
However, dealing with JS-added content:
Since you indicated that the node you care about is added by javascript, waiting for the load event will often not work. JS can add, remove or edit nodes at any time.
The best approach in cases like these is to poll for the element you are interested in ("#mybutton"). See this answer to "Fire Greasemonkey script on AJAX request".
I am trying to run my javascript in my asp.net webform page but I am not sure it runs properly because all the elements are not loaded yet. How can I make sure my script is at the very bottom of the page with using jquery? So it can run when the page is loaded?
Even though everyone says use $(document).ready it's kind of an anti-pattern.
What you really want to do is put any scripts you want to load at the end of the body
<html>
<head> ... </head>
<body>
...
<script src="..." ></script>
</html>
As long as your scripts are at the end of all your other HTML content the javascript will only fire when the content above it has loaded.
With pure JavaScript, you can use
window.onload = function() {
// Page loaded
};
Or you can use jQuery's ready function:
$( document ).ready( function() {
// Dom loaded
} );
Note: jQuery's ready function fires when the DOM has loaded (unless the browser does not support a dom-ready method), not when the whole page has loaded (images, scripts, etc).
Use .ready():
$(document).ready(function($) {
// page is loaded
});
While JavaScript provides the load event for executing code when a
page is rendered, this event does not get triggered until all assets
such as images have been completely received. In most cases, the
script can be run as soon as the DOM hierarchy has been fully
constructed. The handler passed to .ready() is guaranteed to be
executed after the DOM is ready, so this is usually the best place to
attach all other event handlers and run other jQuery code. When using
scripts that rely on the value of CSS style properties, it's important
to reference external stylesheets or embed style elements before
referencing the scripts.
With jQuery, you want to attach to the ready event. This gets fired when all DOM elements have been loaded:
jQuery(document).ready(function(){
// jQuery Code here
});
Shorthand version:
$(function(){
// ...
});
Using this method, I usually put my jQuery in an external file, or in the <head> tag.
You should put your code inside the document ready block as suggested in the JQuery documentation.
This way it will be loaded when all the scripts will be loaded.
$(document).ready(function(){
//Put your code here
});
What is the best way to make sure javascript is running when page is fully loaded?
If you mean "fully loaded" literally, i.e., all images and other resources downloaded, then you have to use an onload handler, e.g.:
window.onload = function() {
// Everything has loaded, so put your code here
};
If you mean "after all of the HTML has been parsed and all elements are accessible from script", at which point images may still be downloading, then you can either put your script at the bottom of the source HTML or use a document.ready handler. Or both. Refer to any of the other answers for details.