在HTML页面中何处加载JavaScript脚本

Question

I am developing a jquery page and I think it is a little bit slow when you load it because I load all js files at the begining.

Would it be faster if I load just which I need at the begining and in each section, loads which I need?.

Now my webpage has all script calling in and after that, in all sections, home, page2, page3...

I structurated the webpage in one html file and I navigate to each section. Each section is like this:

<div data-role="page" id="secion2" data-theme="e">
<!--<script type='text/javascript' src='javascript/createFbAlbums.js'></script> -->
**LOAD SCRIPTS HERE**
<div data-theme = "f" data-role="header" data-id="fixedNav" data-position="fixed">
    <h1>Page 2</h1>
    <a href="#home" data-icon="home" data-iconpos="notext"></a>

</div> <!-- /header -->
<div data-role="content">
     <ul data-role="listview" data-inset="true" class="managementEvents">   
        <li data-theme = "f"><a href="#expositivo">Page2</a></li>
        <li data-theme = "f"><a href="#page3">Modify Exposiciones</a></li>
    </ul>        
</div> <!-- /content -->

Loading them in sections like the commented in this piece of code. Is this equal to load at the begining or not?

Or doing something like this inside javascript:

$(document).on("pageinit", '#section2', function(){
**LOAD HERE SCRIPTS**
Javascript code here doing everything in that page
});

I am a little bit lost because I don't know if this is ok or will break the page or will not speedup the loading.

Answer 1

The usual recommendation (for instance, in YUI's Best Practices for Speeding Up Your Web Site ) is that unless you have a good reason to do something else, put your scripts at the very end of the page, just before the closing </body> tag:

<p>Content</p>
<div>More content</div>
<script src="/some/script.js"></script>
</body>
</html>

Side note: This also makes it essentially unnecessary to use jQuery's ready feature. If the script is at the very bottom of the page, all of the elements defined by the markup above it are available for scripting.

There's lots more useful advice in the page linked above. For example: To the extent possible, combine all of your scripts into a single file. (And make the file cachable, etc., etc.)

The downside : If you have content on your page that requires JavaScript to work correctly, the above means that the events related to that won't be hooked up right away. There can be a brief window of time between the content being available to the user, and the event handlers being registered. To deal with that, the concept of "progressive enhancement" comes into play: Make things work well without JavaScript, and then make them work better with JavaScript. Things that just can't work without JavaScript should be added by the JavaScript, or loaded initially-hidden and then shown by the JavaScript.

Answer 2

Whenever possible, JS files should be loaded at the bottom of the page. Prior to the closing body tag.

http://developer.yahoo.com/performance/rules.html#js_bottom

The reason is that JS files loading can block parallel downloads of other page assets. So they can cause a bottleneck.

In addition to that advice, since you're using jQuery, load jQuery from a CDN such as Google. There are a couple of benefits to doing that:

• a popular CDN such as Google increases the odds that the end-user has already cached the JS library
• it further reduces the parallel downloading bottleneck as it's another domain (the bottleneck is per-domain)

Answer 3

The recommendation is to put JavaScripts at the bottom of the page. This does not take into account dynamically loading your scripts, however, because there are differences in how browsers actually handle downloading external resources such as JS files between using tags and dynamically loading via JavaScript DOM manipulation.

Further Explanation

Browsers load HTML and any embedded resources synchronously as they parse the HTML. This means that as your page loads, when you have scripts up front, the loading of the page stops, the browser makes a request to download, then parse and evaluate/execute the JavaScript file, and then continues to load the rest of the HTML. It does this with each script.

Therefore, unless there are crucial elements that need to use some JS functions or variables during page load, and in some cases this is needed, it's better to load the JS at the bottom of the page because your page will render quicker. And in those crucial cases, only load enough JS to get the page load done as quick as possible.

As an interesting aside, loading JS files dynamically using JavaScript itself (ie, dynamically inject script objects via javascript after page load), those scripts will load asynchronously and further improve page load times.

So to answer your question ,

If you use the jQuery.ready() method of loading JS, it conforms to a best practice since the scripts will load after the page has already been loaded by the browser. This is in effect even better than placing tags at the bottom of the page.

$(document).ready(function () {
    $.getScript( "yourScript.js" )
});

(Full example for jQuery getScript can be found here for more in-depth examples: http://api.jquery.com/jquery.getscript/

Answer 4

You should always put your scripts at the bottom of your page before closing </body> tag

Refer here for more informations why you should do it.

Answer 5

是的，正如每个人都提到的那样，将脚本放在</body>标记之前的页面底部不会有任何问题，而将脚本放在head标记中的某个时间可能会造成问题。

Answer 6

如果您使用$(document).ready(function(){然后添加您的代码，并使用})关闭它，则实际上不必将脚本放在HTML末尾，尽管这两种方法都具有相同的效果被认为是良好的编程习惯。