script tag placement

Put Scripts at the Bottom

tag: javascript

The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames.

In some situations it's not easy to move scripts to the bottom. If, for example, the script usesdocument.write to insert part of the page's content, it can't be moved lower in the page. There might also be scoping issues. In many cases, there are ways to workaround these situations.

An alternative suggestion that often comes up is to use deferred scripts. The DEFER attribute indicates that the script does not contain document.write, and is a clue to browsers that they can continue rendering. Unfortunately, Firefox doesn't support the DEFER attribute. In Internet Explorer, the script may be deferred, but not as much as desired. If a script can be deferred, it can also be moved to the bottom of the page. That will make your web pages load faster.

If there is a lot of JS, put it at the end of the document. Although this makes no difference on load time, the user will see the page sooner and can begin to read it while your js loads, as opposed to seeing nothing until your JS is downloaded (which is what happens when you put it in the head). It merely makes the download appear faster. This would also solve the problem mentioned above about the script executing on an unfinished document, although for that, an even better solution is to use window.onload().

Yes, it does affect the performance of the web page.

The problem with JavaScript is that is blocks the execution/loading of the rest of the page. If you have something that takes a long time in your JavaScript then it will prevent the rest of the page from loading:

See these examples:

• Script at top, blocks whole page: http://jsfiddle.net/jonathon/wcQqn/
• Script in the middle, blocks bottom: http://jsfiddle.net/jonathon/wcQqn/1
• Script at bottom, blocks nothing: http://jsfiddle.net/jonathon/wcQqn/3/

You can see the effect the alert has on the rendering of the rest of the page. Any JavaScript that you put into the top of your page will have the same effect. In general, it is better to put anything critical to the layout of your page (i.e. menu plugins or something). Anything that requires a user interaction (popup handlers) or doesn't involve the user at all (Google Analytics) should go to the bottom of the page.

You can get lazy loaders that will inject your script tags into your code. Since the code isn't on the HTML, you can be sure that your whole page has rendered correctly and that the JS you're including will not block anything.

Which is best depends on the relative importance priority (which has to be determined on a case-by-case basis) of having the JS running Vs. having the HTML rendering.