I've been building a site. At some stage I noticed that IE display was a little broken and Chrome had all but rendered nothing but the body tag (empty), and FF all looked good.
After throwing my keyboard around the room and bashing my head against my mouse, I discovered the problem. I had left (don't ask how or why, must have been some lightning speed cut and paste error) an HTML comment unclosed in an inline script block.
<script type="text/javascript">
<!--
...
</script>
I'm guessing (not tested) the problem would have either not come up, or manifested itself in a far more noticeable way if the script was external. So anyways, I got to thinking, is there ever a time when you have a really good reason to write inline script??
No. Write Unobtrusive Javascript.
If you want your Javascript to run as early as possible, it might make sense to include inline Javascript, since it will run before any other HTTP requests have necessarily completed.
And in some cases, you're including Javascript from a 3rd party provider and you don't really have a choice. Certain ad systems, as well as Google Analytics, spring to mind.
If the script must be dynamically generated (say by a PHP or ASP.NET MVC page) would be one reason to have it inline :-)
Depends on how much JS do you plan to write. If you're writing many support routines (lots of validation checks, text processing, animation and effects) then it makes sense to have the code in a separate file. This allows code reuse and removes a lot of junk from your HTML page.
On the other hand, there is no need to put 10 lines of code, or a single function (a refresh JS comes to mind) in a separate file. It will also load slightly faster, since the browser does not need to make an additional HTTP request to download the separate JS file.
Most XSS vulnerabilities can only be exploited using inline javascript.
It's not necessarily enough of a reason, but the pages will load faster. To this end, sometimes, even when you write the script in another file, you want it to show up as inline on the client side.
I sometimes place javascript inline in pages that get partially reloaded (to bind some events to newly added form-fields for example) and / or pages that use some unique javascript that I will not use on any other page.
Having many external scripts can ultimately slow down the page as the browser must call each file separately. Combining the JavaScript into one file or into the page itself can sometimes alleviate this problem.
On the other hand, I believe the browser may cache a script file once it's been called for the first time so if you have a lot of the same code across your site, external is the way to go.
I work a good deal in something called Flex, which combines XML and ActionScript to create the final bytecode. It is ALWAYS best practice to separate the two as much as possible. That way, you can very clearly and easily separate the View (the HTML or MXML in my case) from the Controller (the script)
It also means that you do not have to worry about looking through five files for one line of code -- all of your code is in one place.
来源：https://stackoverflow.com/questions/1064166/is-there-any-good-reason-for-javascript-to-be-inline