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Whic spec define the ready event?

First at all, I want know the difference between the $(document).ready and the $(window).load, then I know this.
But, I have some new questions.what is the ready event, what is the detail. what is the load event.
I have find the description of load event in the HTML spec.
But I can't find the description of the ready event.

ready() is an abstraction implemented by jQuery based on DOMContentLoaded.
load() (Note this jQuery method is depreciated) is based on window.onload.
The MDN articles link to the specification.

In JavaScript window is one of core object and defines several useful events e.g. onload, before jQuery comes, if want to execute any code, once DOM is loaded completely, we use window.onload event
There is a problem with window.onload , it not exactly executed when DOM is loaded but it executes after all content including big images are loaded completely. Browser normally delay executing onload code, until all page content is loaded, because of this user can see significant delay between they first see the page and the time that code inside onload get executed, this delay is particularly notable, if your page content heavy images, flash videos or other heavy content with low bandwidth internet connection.
jQuery solves this problem by introducing ready event, you might have seen code like below in several JavaScript files or HTML pages :
$(document).ready(function(){
alert("Inside jQuery ready method");
});
here $() is a shortcut for jQuery() function, and we wrap document object into jQuery object to use ready() method. We are passing an anonymous function to ready() method, which will be executed once DOM is loaded. It doesn't wait till all DOM content available e.g. images. By the way, instead of using $(document).ready() function, you can also use following short-cut, which has same effect :
$(function() {
alert("shortcut for document.ready method in jQuery");
});
Apart from faster execution, one of the key advantage of jQuery ready method over JavaScript window onload event is that, you can use them multiple times in your page, unlike onload event, which can only be bind to a single function. Browser will ensure to execute all document.ready code to execute in the order, they are specified in the HTML page.
Hope this will be useful for you.
Thanks

jQuery onLoad x jQuery onDomready

Every time I use jsFiddle I see two options to initialize the contents via jQuery: onLoad or onDomReady.
I Tested with most of the scripts I wrote and there was no functional difference. Searching on Google I saw that one of the main differences is that via onLoad, scripts will only start running after all the elements are loaded and that includes CSS external files, JS external files, images and etc., which can be useful if you need to load JS files in a certain order, but at any given moment one of these files makes reference to another who has not been loaded yet, while via onDomReady once the HTML page content is loaded, scripts begin to be loaded already without necessarily others have been.
Got this difference right? Is there any other differences to be studied and perceived?

Making comment as answer:
One major difference, jquery ready 'pseudo' event will be fired even handler
is set after DOM is effectively 'ready', jquery using internally a
promise. Window onload event won't be fired if handler is set after
window is loaded. e.g: http://jsfiddle.net/c58a6/
It should be then noted that there is no in-build equivalent of jQuery document ready event. For example, DOMContentLoaded in-build event won't be fired if settled after the DOM is ready.

Is placing script tag before </body> tag equivalent to jQuery's document.ready method

If we call a javascript method myMethod() in a script tag which is before closing body, is it equivalent to calling myMethod() inside jQuery's document.ready function ? If not, Why ?

From here:
Under the hood: $(document).ready() As you would expect from John
Resig, jQuery’s method for determining when the DOM is ready uses an
assortment of optimizations. For example, if a browser supports the
DOMContentLoaded event (as many non-IE browsers do), then it will fire
on that event. However, IE can’t safely fire until the document’s
readyState reaches “complete”, which is typically later. If none of
those optimizations are available, window.onload will trigger the
event.
These events are independent of a location within the HTML tag, as other event still are going on even at the time of rendering </body>.

No it's not the same, you place the <script> tags before the closing </body> tag to avoid blocking the rendering of html on older browsers, AFAIK, but you have no guarantee that the DOM is "ready"

Not exactly. $(document).ready(); reacts on the so called DOMContentLoaded event which is fired right after the DOM has been loaded and the browser is aware of all the elements on the page (not the content itself).
The main reason why code is usually put inside these blocks is not that much related to preventing blocking of parallel loading but to ensure that the elements which are to be manipulated during page load are actually loaded and present in the DOM tree. Not much sense in manipulating elements which the browser is not aware of right?
Putting JavaScript content (or any other content for that matter) at the bottom of the page is actually more closely related to the onload event which is fired after the page has finished loading, including the content itself. Either way its almost certain that content inside $(document).ready() blocks will be executed before the one at the bottom of the page however if you load external libraries on which the code inside the ready() block relies you can't put those at the bottom of the page.
In general if you have code that isn't dependent on external libraries and a successful loading of DOM you can safely put it at the bottom of the page. If you however have stuff that needs to be executed right after the DOM has been loaded you most definitely want that code in the $(document).ready() block, but do have in mind that you can place that block wherever you want, even in the middle of the page (which can be a nice trick sometimes).

jQuery load external JavaScript with $(document).ready fails

I would like to load an external HTML. In addition, the external HTML contains some JavaScript code. Since the load function does not load this script, I have to use getScript:
<div id="external-content"></div>
<script type="text/javascript">
$("#external-content").load("external.html #myid", function() {
// do something
});
$.getScript("external.js");
</script>
That works like a charm unless the external.js has a $(document).ready command, which tries to access elements from external.html. It seems to me, that the event is fired too early. I also tried to put the command $.getScript("external.js"); into the .load callback, but did not succeed. Note that sing external.html separately and including the external.js directly works like expected.

Unfortunately, the timing of this makes sense (ie. this is correct behaviour). The script inside the tag fires up a retrieval of external.html and does not wait for it to return, it keeps moving on to the retrieval of external.js. In the meantime, the 'base' document is still loading and may (in fact probably) will complete before externa.html is retrieved.
So, you can run into a timing situation whereby:
Document is indeed ready (external.html being loaded is not a condition of the DOM being scriptable), external.js is loaded and available, but external.html hasn't been fully loaded yet. The document ready function will fire (because the document IS ready!) before the elements of external.html are present.
If you want to continue this approach, there are probably ways to rejig it such that you are using on() inside external.js to delegate to a listener to the base document (#myid) for events on elements inside external.html.

difference between pageLoad , onload & $(document).ready()

i need to know in more detail of what is the differences between pageLoad , onload & $(document).ready()
I found answer but that is not ver clear to me.
the answer is like
The ready event occurs after the HTML document has been loaded, while the onload event occurs later, when all content (e.g. images) also has been loaded.
The onload event is a standard event in the DOM, while the ready event is specific to jQuery. The purpose of the ready event is that it should occur as early as possible after the document has loaded, so that code that adds funcionality to the elements in the page doesn't have to wait for all content to load.
the person trying to say ready event occurs after the HTML document has been loaded
and onload event occur after all page element like image etc being loaded.
So what is HTML document load? I know HTML document load means all page element load complete.
What does mean like dom is ready or loaded? What is the difference between HTML document load & dom load?
Please make me understand with example.
Thanks

I don't know what you mean by pageLoad, but here's some info on onload and $(document).ready().
window.onload fires when everything in the page has finished loading. That means that not only the entire DOM is loaded, but any linked resources such as images are fully loaded. Because this waits for images to finish loading, it can sometimes take a long time to fire window.onload. Unless you really need to wait until images are finished loading, you do not generally want to wait this long to start running your javascript that modifies the page or hooks up event handlers, etc...
$(document).ready() is a jQuery-specific event that fires as soon as the DOM is ready for manipulation, but potentially long before images have finished loading. This occurs after all objects present in the page HTML have been parsed and initialized by the browser and after all scripts in the page have been loaded. At the moment of this event, it is safe to modify the DOM in all browsers. This even may occur a little earlier or later in some browsers as the mechanism for detecting when the DOM is safely loaded varies between older and newer browsers.
The jQuery 1.6.x implementation for $(document).ready() uses a number of different detection mechanisms for when the DOM is ready. The preferred method is when the DOMContentLoaded event triggers on the document object. But, this event is only supported by some browsers so it has fallback mechanisms for other browsers.
Both of these events will fire only once per page.

Let's draw an analogy to compare the process of loading a web page to a chef with a recipe:
First, the chef (browser) reads the entire recipe (downloads the HTML document), to make sure that he understands the steps (HTML code) and that he knows what ingredients (images), utensils (style sheets), and appliances (external scripts) he will need to have in his kitchen (browser cache).
As the chef continues reading, he sends his assistant to the pantry to get the ingredients, utensils, and appliances (download the other files from the server). When he is finished reading the recipe ($(document).ready()), he starts to follow the steps (display the page), but sometimes he gets to a step before his assistant returns with the necessary materials to complete that step. He's pretty skilled, though, so he's still able to complete the later steps while he waits. (The analogy breaks down just a bit here, but basically: the browser lays out the page as well as it can based on the HTML; when you see a page load and then the fonts or layout change after a couple of seconds because it finally got the style sheet... just imagine that this chef is somehow able to add eggs to a cake that's already in the oven.)
It's only after the chef's assistant has brought everything identified in the recipe back to the kitchen (the browser has loaded all of the content) that the chef can put the completed meal onto the plate and garnish it (run the onload event code).
The onload event is a standard event in the DOM, while the ready event is specific to jQuery.
The DOM is the Document Object Model, a basic component of ordinary JavaScript. This means that all modern web browsers already know what onload means.
jQuery is a widely-used JavaScript library. In order for your script to properly use $(document).ready(), you will have to link to a copy of the jQuery library. Browsers don't know what $(document).ready() means without jQuery.