Extract CSS from SCSS and deferred lazy load in React app

Question

I have a few SCSS theme files I want to extract to CSS files and later load them into the page. I want to be able to use contenthash for long term caching.

Since I'm using Webpack 4, I am also using mini-css-extract-plugin. I started down the path of creating a splitChunks in my webpack config.

// webpack.config.js
module.exports = {
  plugins: [
      new MiniCssExtractPlugin({
      // Options similar to the same options in webpackOptions.output
      // both options are optional
      filename: "[name].[contenthash].css",
      chunkFilename: "[id].[contenthash].css"
    })
  ],
  optimization: {
    splitChunks: {
      cacheGroups: {
        'vendor': {
            // custom commons chunk for js
        },
        'theme-a': {
            test: /theme-a.\scss/,
        },
        'theme-b': {
            test: /theme-b.\scss/,
        },
        // more themes
      }
    }
  }
  module: {
    rules: [
      {
        test: /\.scss$/,
        use: [
          MiniCssExtractPlugin.loader,
          "css-loader",
          "sass-loader"
        ]
      }
    ]
  }
}

I've then tried dynamically importing the css in my app:

// app.js
class App extends React.Component {
  // constructor

  login(themeName) {
    import(/* webpackChunkName: "`${themeName}`" */ `./path/to/${themeName}.scss`).then(theme => {
      // do something with `theme`
    }
  }

  // other stuff
}

I need to be able to load that css file dynamically in login() and I'm just not sure how to reference it when it has a generated [contenthash].

TLDR: Is there a good way to both extract css and import the referenced CSS bundle to lazy load later? I'm not tied to mini-css-extract-plugin.

Edit: Created a mini-css-extract-plugin issue here.

Answer 1

My solution ended up using extract-text-webpack-plugin. My config now looks like this:

// webpack.config.js
const ExtractTextPlugin = require('extract-text-webpack-plugin');
const ExtractThemeA = new ExtractTextPlugin({ filename: 'themeA.[hash].css', allChunks: true});

module.exports = {
  plugins: [
      ExtractThemeA,
      // more plugins for other css files
  ],
  optimization: {
    splitChunks: {
      cacheGroups: {
        // Note: No changes to splitChunks
        'vendor': {
            // custom commons chunk for js
        }
    }
  }
  module: {
    rules: [
      {
        test: /theme-a\.scss$/,
        use: ExtractThemeA.extract([ 'css-loader', 'sass-loader' ])
      },
      // more module rules for each css file needed
    ]
  }
}

Then, these chunks are available by file name in my HtmlWebpackPlugin:

<!-- HtmlWebpackPlugin Template -->
<script>
// provides me with an array of file name strings
var themesManifest = <%= htmlWebpackPlugin.files.css %>
</script>

Answer 2

Sorry for my miss understanding,

You could probably just make two different scss-files and import them as needed. theme.scss admin.scss or like so

This is how I am doing scss in React right now

In App.js

import styles from '../../stylesheet/main.scss'
// could be as well
import styles1 from '../../stylesheet/theme.scss' // some file
import styles2 from '../../stylesheet/admin.scss' // some other file

const App = () => {
  <div className={styles.action_feed} >{ content }</div>
} 

In main.scss

.action_feed {
  position: fixed;
  width: 350px;
  height: auto;
  max-height: 100%;
  max-width: 100%;
  top: 0;
  left: 0;
  z-index: 9999;
}

I think you could just as well do it like so

const themeName = 'main'
import(`./stylesheet/${themeName}.scss`, (css) => {
  // meaby set this to state? not quite sure how should properly handle
  // dynamically imported css
  this.setState({ css: css.action_feed })

  // or possible this way. I have done some non-React dom manipulation
  // this way before
  document.querySelector('body').classList.add(css.action_feed)
})

<div className={this.state.css}>{ content }</div>

You should probably check out React's new Refs API as well. It might give you some nice flexibility for giving className-attr to required element.

Having set to splitChunks.chunks to all works though i think in this case anyway