When should I use Inline vs. External Javascript?

I would like to know when I should include external scripts or write them inline with the html code, in terms of performance and ease of maintenance.

What is the general practice for this?

Real-world-scenario - I have several html pages that need client-side form validation. For this I use a jQuery plugin that I include on all these pages. But the question is, do I:

• write the bits of code that configure this script inline?
• include all bits in one file that's share among all these html pages?
• include each bit in a separate external file, one for each html page?

Thanks.

At the time this answer was originally posted (2008), the rule was simple: All script should be external. Both for maintenance and performance.

(Why performance? Because if the code is separate, it can easier be cached by browsers.)

JavaScript doesn't belong in the HTML code and if it contains special characters (such as <, >) it even creates problems.

Nowadays, web scalability has changed. Reducing the number of requests has become a valid consideration due to the latency of making multiple HTTP requests. This makes the answer more complex: in most cases, having JavaScript external is still recommended. But for certain cases, especially very small pieces of code, inlining them into the site’s HTML makes sense.

Maintainability is definitely a reason to keep them external, but if the configuration is a one-liner (or in general shorter than the HTTP overhead you would get for making those files external) it's performance-wise better to keep them inline. Always remember, that each HTTP request generates some overhead in terms of execution time and traffic.

Naturally this all becomes irrelevant the moment your code is longer than a couple of lines and is not really specific to one single page. The moment you want to be able to reuse that code, make it external. If you don't, look at its size and decide then.

Externalizing javascript is one of the yahoo performance rules: http://developer.yahoo.com/performance/rules.html#external

While the hard-and-fast rule that you should always externalize scripts will generally be a good bet, in some cases you may want to inline some of the scripts and styles. You should however only inline things that you know will improve performance (because you've measured this).