Skip to content

Instantly share code, notes, and snippets.

Created September 28, 2015 17:09
Show Gist options
  • Save lambdahands/d19e0da96285b749f0ef to your computer and use it in GitHub Desktop.
Save lambdahands/d19e0da96285b749f0ef to your computer and use it in GitHub Desktop.
FlowType and CSS Modules


So basically FlowType doesn't know about CSS Modules, a really handy way of dealing with the plagues of CSS in codebases (global variables and dependency wackiness mainly).

What WebPack allows us to do is "require" CSS files and use their class names:

import styles from "my_styles.css";
import React from "react";

const MyComponent = React.createClass({
  render() {
    return <h1 className={styles.redHeader}>Hello!</h1>;

Unfortunately, Flow will give us an error Required module not found because, well, let's be honest, importing CSS with JavaScript is pretty out of this world and a little bit crazy (i.e: this).

So here's what I did to fix that. Flow has a nice way of dealing with this in its options, namely one called module.name_mapper. Somebody was kind enough to make an npm module called empty that– you guessed it– returns empty objects and arrays. I'm pretty amazed to have found a use for this.

So as a fix, do this: Run npm install --save empty in your project directory.

Open your .flowconfig, and add the following under [options]:

module.name_mapper='.*\(.css\)' -> 'empty/object'

Ta-da! Another fun day in JavaScript land.

Copy link

Thanks so much for sharing this!! You're a lifesaver.

Copy link

Vages commented Apr 19, 2018

Could not get your solution to work, @Luna2442. According to the documentation (,

module.name_mapper (regex -> string)

Specify a regular expression to match against module names, and a replacement pattern, separated by a ->.

For example:

module.name_mapper='^image![a-zA-Z0-9$_]+$' -> 'ImageStub'

This makes Flow treat require('image!foo.jpg') as if it were require('ImageStub').

These are OCaml regular expressions. Use \( and \) (slashes required!) to create a capturing group, which you can refer to in the replacement pattern as \1 (up to \9).

So (1) how did you get your regex capturing group to work without the required slashes and (2) how did you get Flow to ignore the .css imports by feeding it require('{}')?

Original solution works perfectly, @lambdahands

Copy link

gabrii commented Jun 5, 2018

Slight modification from @jsermeno solution, which no longer works (facebook/flow#1806 (comment)):


  declare var exports: { [key: string]: string };
  declare export default typeof exports;




module.name_mapper='^\(.*\)\.css$' -> 'CSSModule'


I believe @lambdahands solution works, but doesn't offer type coverage.

Copy link

Eugene-Musika commented Oct 24, 2018

As for me, the best solution is css-modules-flow-types.
It's not just a plug. This thing make types for your css modules.

Copy link

KareemSalah commented Oct 27, 2018

Google brought me here, and it's late 2018, and still got that same issue and couldn't really solve it following the comments here, here's what I did eventually:

  1. Create a dump module at PROJECT_ROOT/flow-typed/LESSModule.js.flow
  2. Add this code inside of it
declare module 'LESSModule' {
  declare module.exports: any;
  1. Add this line under [options] in .flowconfig (just this line, nothing more πŸ˜„)
module.name_mapper.extension='less' -> 'LESSModule'

Notes that I had to take are that the file name must begin with exactly the same name as the stub module name, if you're going to change the module name to SCSSModule or CSSModule, then change the file name accordingly, also put the file under that same directory, flow-typed should be already there if you did npm i --save flow-typed and ran flow-typed install, I tried putting that file somewhere else, but it didn't work.

Copy link

πŸ‘ ❀️

Copy link

not working for me,

$ node node_modules/.bin/flow check app.js
Error β”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆβ”ˆ app.js:5:8

Cannot resolve module my.less.

     3β”‚ import "./my.sass"
     4β”‚ import "LESSModule"
     5β”‚ import "my.less"

Found 1 error

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment