Webpack loaders are great. With them, you can
require() just about any file and the loaders will take care of transpiling
CSS Modules are great because you can write CSS for each of your components
without worrying about rules from one stepping on the rules of another. The
aforementioned webpack loaders (the css-loader
in particular) will let you
require() your CSS and return a nice map of
original class names to generated CSS Module class names so you can do
There are several solutions to this problem. The most common solutions either
attempt to parse the CSS faithfully or attempt to ignore the CSS
In the first case, you're just complicating things and wasting time. In my
automated tests, I don't need to know that
myClass is going to become
_23_aKvs-b8bW2Vg3fwHozO, so why should I waste the time to parse the CSS to
find that out? Further, if there's an error in my CSS, the parsing will fail
and cause my component test to fail... is that where the failure belongs? I
dunno... maybe, maybe not...
In the second case, all of my class names become empty strings in my automated
tests. While it's true that I don't need to know exactly what my class names
will become after transpiling, I might want to be able to test that there is
some class and I can't do that if just make my CSS
require()s return null.
mock-css-modules' solution is somewhere between the former and the latter.
mock-css-modules registers a handler for requiring CSS files. When node comes
require() for a CSS file, it will run mock-css-modules' handler which
will return a Proxy
object. This Proxy object will trap getters and return the name of the
requested property as a string. So, for example:
This gives all of our classes names without the overhead of parsing the actual CSS files. And since code that is using CSS Modules shouldn't be making any assumptions about the names of the generated classes, these values are just as valid as the real ones so they shouldn't cause any issues.
This package makes use of the
--harmony-proxies node option. I'm not sure in
what version that was added, but you may need a newish version. Install with
npm install --save-dev mock-css-modules
As noted above, this package makes use of the
--harmony-proxies option. This
means that however you start node, you'll need to pass that option. Then,
require("mock-css-modules") before any CSS files and you'll be rockin'.
By default, mock-css-modules will handle
require()d .css files. If your
project has some other extensions (such as .sass, .scss, etc), you'll need to
register handlers for those, too:
var mockCssModules = ;mockCssModules;
Unfortunately, this means that if you are taking advantage of webpack's
require() files without extensions, it won't work. You should
use extensions for your CSS files.
If you are using mocha to run your tests, you can use mock-css-modules from the command line:
mocha --require mock-css-modules --harmony-proxies ...
If you need to handle additional extensions, copy the two lines above into a
test-setup.js, for example, and require the file instead of
mocha --require test-setup.js --harmony-proxies ...