The difference between jquery $(document).ready() and window.onload

The function of $(

document).ready() in Jquery is similar to the window.onload method in traditional JavaScript, but it is different from the window.onload method There is still a difference.

1. Execution time

Window.onload must wait until all the elements included in the page are loaded before it can be executed.
          $(document).ready() is executed after the DOM structure is drawn, without waiting until it is loaded.

2. The difference in writing is different. (document) .Ready () can be written at the same time, and all can be executed

3. Simplified writing

## Window.Onload. (document).ready(function(){}) can be abbreviated as $(function(){});
In my previous development, I generally used javascript, and I always used jquery mode. That is to say, most of the time, the first line is:
The code is as follows:

$(document).ready(function(){

...

}) ;

At this time, you don’t have to wait for all js and images to be loaded before you can execute some methods. However, sometimes, you must wait for all

elements to be loaded. , only when some methods can be executed, for example, some pictures or other aspects have not been loaded. At this time, clicking some buttons will cause unexpected situations. At this time, you need to use Go to:

The code is as follows:

$(window).load(function() {

$("#btn-upload").click(function(){ //For example Say:

uploadPhotos();

});

});

The following is the reproduced content,

use $(window).load(function() {...}) instead of body.onload() for several reasons
First of all, they are all images in all elements of the page (including
html tags
and
references
, Flash and other media) are executed after loading, this is what they have in common.

Reason 1 for not using body.Onload():

If we want to load multiple functions at the same time, we must Writing like thised5cb0397b63c7a89b89c225a7d5c20b36cc49f0c466276486e50c850b7e4956 looks extremely ugly. If we use $(window).load() we can load multiple ones like this FunctionThe code is as follows:

$(window).load(function() {

          alert("hello, I am jQuery!");

});

$(window).load(function() {

alert("hello, I am also jQuery");

});

Written like this it will be executed from top to bottom These two functions look much more beautiful.

Reason 2 for not using body.Onload():

Using body.Onload() cannot completely separate js and html. This It is a very serious problem.

In addition, using $(window).load(function(){...}) and body.onload() has the same problem, because as mentioned at the beginning, they You need to wait until all the content of the page

is loaded before executing. However, if the network speed is relatively slow, it often takes a long time to load a page (ranging from a few seconds to more than ten seconds, or even longer). ...), so we often

encounter the situation that the page has not been completely loaded but the user is already operating the page, so the effect displayed by the page is different from what we expected.

So here I recommend using $(document).ready(function(){}), or abbreviated as $(function(){}), because it will be loaded after the dom element of the page is loaded. Execute,

without waiting for pictures or other media to be downloaded.

But sometimes we do need to wait until everything on the page is loaded before executing the function we want to execute, so it is Whether to use $(window).load(function(){...}) or

Whether to use $(function(){}) often requires different choices based on specific needs.

Finally, attach a piece of jQuery code that is executed before all DOM elements are loaded.

The code is as follows:

4ec11beb6c39d0703d1751d203c17053

(function() {

          alert("DOM has not been loaded yet!");

        })(jQuery)  

2cacc6d41bbb37262a98f745aa00fbf0

Haha, sometimes we also have this need!

Take the browser loading a document as an example. After the page is loaded, the browser will add event to the DOM element through Javascript. In regular Javascript code, the window.onload method is usually used, while in Jquery, the $(document).ready() method is used. The $(document).ready() method is the most important function in the event module, which can greatly improve the speed of web applications.

window.load $(document).ready()
Execution timing You must wait for all content in the webpage to be loaded (including pictures) before executing it. Execute after all DOM structures in the webpage are drawn. You can The content associated with the DOM element has not been loaded.
The number of writes cannot be written multiple times at the same time.
The following code cannot be executed correctly:

window.onload = function(){ 
 alert(“text1”); 
}; 
window.onload = function(){ 
 alert(“text2”); 
};

The result is only the second one. Multiple writing can be done at the same time.
The following code is executed correctly:

$(document).ready(function(){ 
 alert(“Hello World”); 
}); 
$(document).ready(function(){ 
 alert(“Hello again”); 
});

The result is output twice
Simplified writing method None

 $(function(){ 
 // do something 
});

In addition, you need to pay attention to one thing, because it is registered in the $(document).ready() method The event will be executed as soon as the DOM is ready, so the associated file of the element may not be downloaded at this time. For example, the html related to the image has been downloaded and parsed into a DOM tree, but it is very likely that the image has not been loaded yet, so attributes such as the height and width of the image may not be valid at this time. To solve this problem, you can use another page loading method in Jquery --- the load() method. The Load() method binds a handler function to the element's onload event. If the handler function is bound to the window object, it will be triggered after all content (including windows, frames, objects, images, etc.) is loaded. If the handler function is bound to an element, it will be triggered after the content of the element is loaded.
Jquery code is as follows:
$(window).load(function (){
// Write code
}); Equivalent to the following code in JavaScript
Window.onload = function (){
// Write code
}

------------------------------

When I recently changed a page embedded in a frame, I used jquery for effect, and the page itself was also bound to the onload event. After the modification, the test runs normally and smoothly under Firefox, but it takes more than ten seconds for the jquery effect to appear under IE, and the day lily is cold.

At first I thought it was conflicting with my own onload loading method. A common saying on the Internet is that $(document).ready() is executed after the page DOM parsing is completed, and the onload event is executed after all resources are prepared. In other words, $(document).ready() is executed after the DOM parsing of the page is completed. Executed before onload, especially when the page pictures are larger and more, the time difference may be larger. But on my page, the picture has been displayed for more than ten seconds, but the jquery effect has not yet appeared.

Try deleting the onload loading method, but the result is still the same. It seems that there is no need to use $(document).ready() to write the original onload event binding. So what is the reason why Firefox works but IE does? Then debugging, I found that the originally bound onload method under IE was executed before the content of $(document).ready(), while Firefox executed the content of $(document).ready() first, and then executed the original onload method. . This doesn’t seem to be completely consistent with what’s said online. Haha, it’s interesting. It seems to be getting closer to the truth.

Look through the source code of jquery to see how $(document).ready() is implemented

if ( jQuery.browser.msie && window == top ) (function(){ 
if (jQuery.isReady) return; 
try { 
document.documentElement.doScroll("left"); 
} catch( error ) { 
　　　　　　setTimeout( arguments.callee, 0 ); 
　　　　　　 return; 
　　　　} 
　　 // and execute any waiting functions 
　　　jQuery.ready(); 
})(); 
jQuery.event.add( window, "load", jQuery.ready );

The result is very clear. IE only works when the page is not embedded in a frame. Like Firefox, etc., the content of $(document).ready() is executed first, and then the original onload method is executed. For the page embedded in the frame, it is only bound to the load event for execution, so naturally it is the turn after the original onload binding method is executed. And this page happens to have a resource that is inaccessible in the test environment, and the delay of more than ten seconds is exactly the time difference it amplifies.

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