NGINX gzip not compressing JavaScript files

Change this line:

gzip_types    text/plain application/x-javascript text/xml text/css;

To be this:

gzip_types    text/plain application/javascript application/x-javascript text/javascript text/xml text/css;

Note the addition of application/javascript and text/javascript to your list of gzip types.

There are also more details—and a more expansive list of gzip types—in the answer posted here.

This is interesting, because the best-supported, old standard mime type for javascript in the browser is actually text/javascript. And if you configure that, in /etc/nginx/mime.types, it works.

text/javascript (Obsolete): JavaScript; Defined in and made obsolete in RFC 4329 in order to discourage its usage in favor of application/javascript. However, text/javascript is allowed in HTML 4 and 5 and, unlike application/javascript, has cross-browser support. The "type" attribute of the tag in HTML5 is optional and there is no need to use it at all since all browsers have always assumed the correct default (even in HTML 4 where it was required by the specification).

From this thread: text/javascript vs application/javascript

So the gzip module of nginx is simply built against previous standards, and apparently doesn't properly process the application/javascript mime type.