Is the 'onload' necessary when the code is

I was wondering if the window.onload = function(){} (or any other kind of onload, like the jQuery $(document).ready(); is necessary if the code is placed at the bottom of my <body>?

Or there could be highly unexpected side-effects?

回答1:

Yes, there could be unexpected consequences. But, no, it's not absolutely necessary. The timing could be off for things still loading, like complicated layouts, deep DOM structures, dynamic HTML from other scripts, or images. To avoid these situations, it's always safest to wrap your script in an onload event.

Here are some examples that demonstrate this. All examples tested on Chrome 17.0.963.12 dev on OS X. Browser results may vary when not using onload, which demonstrates its unpredictable behavior. The examples return fail if the result is different than what you'd expect (i.e. what your design specifies) and return success when the result matches what you would expect. With onload they always return success.

Example 1

In this example, the code is expecting the image to be a certain width. If the code is wrapped in an onload event the width is correct, otherwise, it's not.

Demo: http://jsfiddle.net/ThinkingStiff/qUWxX/

HTML:

<div id="result"></div>
<img id='image' src="http://thinkingstiff.com/images/matt.jpg" />

Script:

document.getElementById( 'result' ).innerHTML 
    = document.getElementById( 'image' ).offsetWidth == 346 ? 'success': 'fail';

You'll see the jsFiddle is set to "onLoad" in the upper left corner of the page and the result above the image is success.

Change that to "onDomReady" or "no wrap (body)":

Now press "Run" at the top left of the page:

The result above the image will now be fail.

Example 2

Here is another example that doesn't use images. In this one, an inline script has been added to the HTML. The code is expecting the width to be what it was set to by the inline script. With onload it's corrent, without, it's not. Use the same instructions as before for this demo.

Demo: http://jsfiddle.net/ThinkingStiff/n7GWt/

HTML:

<div id="result"></div>
<div id="style"></div>

<script>
    window.setTimeout( function() { 
        document.getElementById( 'style' ).style.width = '100px'; 
    }, 1 );
</script>

Script:

document.getElementById( 'result' ).innerHTML 
    = document.getElementById( 'style' ).style.width ? 'success' : 'fail';

Example 3

Here's an example that uses no images or Javascript in the body, just CSS. Again, the results are different between onload and not.

Demo: http://jsfiddle.net/ThinkingStiff/HN2bH/

CSS:

#style {
    animation:             style 5s infinite;
        -moz-animation:    style 5s infinite;
        -ms-animation:     style 5s infinite;
        -o-animation:      style 5s infinite;
        -webkit-animation: style 5s infinite;
    border: 1px solid black;
    height: 20px;
    width: 100px;    
}

@keyframes             style { 0% { width: 100px; } 100% { width: 500px; } }
    @-moz-keyframes    style { 0% { width: 100px; } 100% { width: 500px; } }
    @-ms-keyframes     style { 0% { width: 100px; } 100% { width: 500px; } }
    @-o-keyframes      style { 0% { width: 100px; } 100% { width: 500px; } }
    @-webkit-keyframes style { 0% { width: 100px; } 100% { width: 500px; } }

HTML:

<div id="result"></div>
<div id="style"></div>

Script:

document.getElementById( 'result' ).innerHTML 
    = document.getElementById( 'style' ).clientWidth > 100 ? 'success' : 'fail';

There are just too many scenarios where not wrapping your code can cause issues that you won't be able to anticipate. To avoid these situations, it's always safest to wrap your script in an onload event.

回答2:

Couple of different things going on.

onload is called only after embedded content such as images is loaded. This means you can put code in onload that depends on that content being there.
ready handlers are fired before that, when the DOM (ie internal structure) of your page is fully loaded. This isn't that different from putting it at the bottom, but one difference is that if someone navigates away from your page and then back, these handlers will fire again.

Technically scripts that run at the end of the document can use methods like getElementById to pull in elements that are already rendered. You may still want to put those in a ready or load handler for the above reasons. This isn't to say the scripts themselves shouldn't be at the bottom - there's still a benefit to perceived performance from having them there.

回答3:

A script tag at the bottom of an HTML page is equivalent to DOMContentLoaded. All the html code has been downloaded, and Javascript is now capable of accessing DOM elements.

load is called when all other resources, such as images, have completely downloaded.