import css file into es6 returns string instead of object

Issue

TL;DR

I'm importing a css file into a typescript module, but the import resolves to a string instead of an object. Can anyone tell me why I don't get an object??

Example

// preview.ts

import test from './src/assets/test.theme.css';

// also tried this:
// import * as test from './src/assets/test.theme.css';

console.log('typeof test: ', typeof test);
console.log(test);

Console output

Detailed explanation

Actually, I'm trying to set up a Storybook for my Angular12 component library.

In order to provide various themes, I want to use the @etchteam/storybook-addon-css-variables-theme plugin, which in its documentation refers to the inline loader syntax of Webpack.

import myTheme from '!!style-loader?injectType=lazyStyleTag!css-loader!./assets/my-theme.css';

When applying this to my code my browser console started to complain

Error: myTheme.use is not a function

During my research I recognized that the imported stylesheet is not an evaluated javascript object, but instead it is provided as a string containing the sourcecode generated by the style-loader.

I also recognized, that this issue is not specific to the style-loader, but also occurs for all other loaders, e.g. css-loader, raw-loader, etc.

This issue is also not related to inline loader syntax, as it also shows up with loaders being defined in a minimalistic webpack config.

Environment:

• Angular 12
• Webpack 5

Reproduction

I have set up a GIT repo reproducing the issue.

The readme file explains the repro and the issue.

Solution

Finally I got my issue solved!

Analysis

The main issue here is the webpack configuration generated by the @angular-devkit/build-angular package. I was able to analyze it by debugging a fresh angular12 application (you can check it out here).

By setting a break-point at /node_modules/@angular-devkit/build-angular/src/utils/webpack-browser-config.js, function: generateWebpackConfig(...), I could inspect the final webpackConfig object in the debugger.

The relevant rule looks like this:

The important part here is the rule setting the module type to asset/source, instructing webpack not to evaluate the loader's result.

Solution concept 1: inline loader

With the help of alexander-kait and his great hints at this issue, I was able to find an inline-loader syntax that overrides webpack's module declaration:

import Test from 'test.css.webpack[javascript/auto]!=!!!style-loader?injectType=lazyStyleTag!css-loader!./test.css';

console.log(typeof Test); // output: object
console.log(Test); // output: Object { use: () => void, unuse: () => void }
Test.use(); // this should usually be called by a theme switcher...

I'm not really sure about the url pattern here, as it seems to be an undocumented feature, but I assume that it's something like <query-pattern>.webpack[<module-type>]!=!<loaders><query>.

However, since this is an undocumented feature, I was rather reluctant to use it.

Solution concept 2: webpackConfig customization

Since I'm in a storybook context, I decided to customize the webpack configuration according to the storybook documentation.

My solution requires to set up a naming convention (e.g. *.theme.css).

// .storybook/main.js

module.exports = {
  webpackFinal: async (config) => {

    // exclude *.theme.css from the *.css ruleset
    config.module.rules.find(rule => '.css'.match(rule.test)).exclude = [ /\.(?:theme\.css)$/i ];

    // add a rule for *.theme.css
    config.module.rules.push({
      test: /\.(?:theme\.css)$/i,
      use: [
        { loader: 'style-loader', options: { injectType: 'lazyStyleTag' } },
        'css-loader',
      ],
    });
  },
};

With these rules in place, I can now simply do the following:

// preview.js

import LightTheme from './light.theme.css';
import DarkTheme from './dark.theme.css';

setupThemeSwitcher(LightTheme, DarkTheme);

Please note that the setupThemeSwitcher function is just pseudocode merely there for the example. In reality I'm using the @etchteam/storybook-addon-css-variables-theme addon...

Answered By - Markus Windhager