Type safe CSS Modules with Flow

June 21, 20173 minute read

CSS Modules + Flow = type safety and editor autocompletion
CSS Modules + Flow = type safety and editor autocompletion

I’ve been dreaming about type safety and editor autocompletion when using CSS Modules. There are a few TypeScript tools for this (see this and this), but I didn’t find any solid tools for Flow.

TL;DR I’ve made some new tools, and when trying them on a codebase I’m working on, Flow found a lot of dead code (and potential bugs) 😬

The problems

It is quite easy to misspell a class name or forget to update consumers after removing a class in a .css file. As an example, the class foo might not be defined in Button.css:

/* @flow */
import styles from "./Button.css";
const Button = () => <button className={styles.foo} />;


To teach Flow about CSS Modules file, we can create a definition file Button.css.flow containing the class names exposed by Button.css. By doing so we can get:

  • static type checks showing usage of non existing classes
  • editor autocompletion of CSS classes (for editors supporting Flow)

To generate these .flow files I was thinking of two use cases. One using a simple CLI and another using webpack.

Solution: CLI

css-modules-flow-types-cli is a CLI that quickly generate .flow files.

Let us install it:

$ npm install --save-dev css-modules-flow-types-cli
# or
$ yarn install -D css-modules-flow-types-cli

And then just run the CLI on your source directory:

$ css-modules-flow-types src

I recommend using the CLI on your CI system (like Travis or Circle) to ensure that all .flow files are up-to-date before running Flow. This will catch potential styling errors before deploying.

Another use case is quick feedback loop when developing and changing CSS Modules files. The CLI includes a watch mode for this, but I myself would like to avoid having yet another tool that needs to be running while developing. As a lot of people already have webpack running, I did a small loader consuming the tokens from style-loader.

Solution: webpack loader

css-modules-flow-types-loader is a webpack loader keeping .flow files updated by consuming the tokens from style-loader. I recommend using this when developing as part of a webpack-dev-server setup. It will give a small slowdown, as the loader potentially needs write a lot of files.

To get started:

$ npm install --save-dev css-modules-flow-types-loader

Then update your webpack config:

  test: /\.css$/,  // or the file format you are using
  use: [
    'css-modules-flow-types-loader',  // right after style-loader
    // Other loaders like css-loader after this:

And then sit back and enjoy CSS Modules being type checked by Flow. 🍺

Please let me know what you think… And give a little ⭐️ over at GitHub.

This blog post was originally posted on hackernoon.

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