How can I get file extensions with JavaScript?

return filename.split('.').pop();

Edit:

This is another non-regex solution that I think is more efficient:

return filename.substring(filename.lastIndexOf('.')+1, filename.length) || filename;

There are some corner cases that are better handled by VisioN's answer below, particularly files with no extension (.htaccess etc included).

It's very performant, and handles corner cases in an arguably better way by returning "" instead of the full string when there's no dot or no string before the dot. It's a very well crafted solution, albeit tough to read. Stick it in your helpers lib and just use it.

Old Edit:

A safer implementation if you're going to run into files with no extension, or hidden files with no extension (see VisioN's comment to Tom's answer above) would be something along these lines

var a = filename.split(".");
if( a.length === 1 || ( a[0] === "" && a.length === 2 ) ) {
    return "";
}
return a.pop();    // feel free to tack .toLowerCase() here if you want

If a.length is one, it's a visible file with no extension ie. file

If a[0] === "" and a.length === 2 it's a hidden file with no extension ie. .htaccess

This should clear up issues with the slightly more complex cases. In terms of performance, I think this solution is a little slower than regex in most browsers. However, for most common purposes this code should be perfectly usable.

Newer Edit: Lots of things have changed since this question was initially posted - there's a lot of really good information in wallacer's revised answer as well as VisioN's excellent breakdown

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Edit: Just because this is the accepted answer; wallacer's answer is indeed much better:

return filename.split('.').pop();

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My old answer:

return /[^.]+$/.exec(filename);

Should do it.

Edit: In response to PhiLho's comment, use something like:

return (/[.]/.exec(filename)) ? /[^.]+$/.exec(filename) : undefined;