Sass support in jest test files appear to be broken

[Tasemu]

Is this a bug report?

Yes

Did you try recovering your dependencies?

Yes

(Write your answer here.)

Which terms did you search for in User Guide?

Sass support

(Write your answer here if relevant.)

Environment

1. node -v:
2. npm -v:
3. yarn --version (if you use Yarn):
4. npm ls react-scripts (if you haven’t ejected):

Then, specify:

1. Operating system:
2. Browser and version (if relevant):

Steps to Reproduce

(Write your steps here:)

1: Update react-scripts to latest release
2: Import styles from 'component.module.scss' in component file
3: Import styles from 'component.module.scss' in component.test.js

Expected Behavior

(Write what you thought would happen.)

Classnames are imported for component file and test file correctly for comparison

see #3815

Actual Behavior

(Write what happened. Please add screenshots!)

Component imports classNames correctly, test file classnames are undefined

Reproducible Demo

(Paste the link to an example project and exact instructions to reproduce the issue.)

[Aftabnack]

This is because we don't have scss preprocessor configured for jest. And I am guessing the same problem will be there when we use .module.css files.

Can I work on this?

[Tasemu]

I'm going to attempt a PR too :)

[Tasemu]

Would it be a good idea to follow the 'mocking css modules' section of the jest-webpack documentation found here?

https://facebook.github.io/jest/docs/en/webpack.html#mocking-css-modules

This would allow tests to check that correct classNames were being applied without having to set up preprocessor transforms.

[Timer]

This is odd, because we already configured that for CSS Modules (and I believe we even have tests); I wonder why Sass isn't working (or is just not what's expected)?

[Timer]

Ah, it appears we need to add {sass,scss} to these locations:

create-react-app/packages/react-scripts/scripts/utils/createJestConfig.js

Lines 46 to 49 in 789b856

	transformIgnorePatterns: [
	'[/\\\\]node_modules[/\\\\].+\\.(js|jsx|mjs)$',
	'^.+\\.module\\.css$',
	],

create-react-app/packages/react-scripts/scripts/utils/createJestConfig.js

Line 52 in 789b856

'^.+\\.module\\.css$': 'identity-obj-proxy',

[Tasemu]

I'd be happy to make a PR on this, one question however (I cannot seem to find an answer in the documentation I have searched), How am I supposed to test my change with the cloned repo? I cannot see any way to test the changes with a generated create-react-app. Also if I were able to generate an app using the changes, shall I just have the extra generated files not committed then make a PR? Sorry in advance if these are dumb questions. Fairly new to the contributing thing.

[iansu]

You can test your changes by running yarn create-react-app my-app from the base of your cloned repo. That will create a new app called my-app using your local changes.

You should not commit my-app when submitting a PR. You can create my-app outside of your cloned repo by doing something like this instead: yarn create-react-app ../my-app.

[Tasemu]

It appears that after modifying createJestConfig.js and generating an app, doing the following:
import styles from test.module.scss
results in:
console.log(styles) // => 'test.module.scss'

So it appears to be importing the sass module as a string of the file name. I'm going to continue looking into this, if this pops out to you as something obvious then feel free to let me know. :)

[Tasemu]

I think I have an idea, going to test it tomorrow as generating the node_modules folder every time for CRA is murdering my mobile internet.

[Tasemu]

it also appears that importing css modules is not working as expected either.

import styles from './test.module.css';
import sass from './sass.module.scss';

it('should handle css modules', () => {
  console.log(styles) // => {}
})

it('should handle sass modules', () => {
  console.log(styles) // => 'sass.module.scss'
})

Using this configuration:

transformIgnorePatterns: [
      '[/\\\\]node_modules[/\\\\].+\\.(js|jsx|mjs)$',
      '^.+\\.module\\.{css,sass,scss}$',
    ],
    moduleNameMapper: {
      '^react-native$': 'react-native-web',
      '^.+\\.module\\.{css,sass,scss}$': 'identity-obj-proxy',
    },