How can I configure different customProperties in postcss.config.js for different Webpack entry points?

I am using Webpack 4.

I have two entry points in my webpack.config.js, for each of which I am generating a separate CSS file using mini-css-extract-plugin.

const webpack = require('webpack')
const path = require('path')
const MiniCssExtractPlugin = require('mini-css-extract-plugin')

module.exports = {
    mode: 'development',
    entry: {
        foo: './foo.js',
        bar: './bar.js'
    },
    output: {
        path: path.resolve(__dirname, 'src/'),
        filename: 'js/[name].js'
    },
    plugins: [
        new MiniCssExtractPlugin({
            filename: "css/[name].css"
        })
    ],
    module: {
        rules: [
            {
                test: /\.js$/,
                exclude: /node_modules/,
                use: {
                    loader: "babel-loader"
                }
            },
            {
                test: /\.css$/,
                use: [
                    {
                        loader: MiniCssExtractPlugin.loader
                    },
                    "css-loader", "postcss-loader"
                ]
            }
        ]
    }
}

I am also using postcss-cssnext.

In my postcss.config.js file in my root folder I am defining a number of CSS variables in customProperties, like this:

module.exports = {
    plugins: {
        'postcss-cssnext': {
            features: {
                customProperties: {
                    variables: {
                        color1: '#333',
                        color2: '#53565A',
                        baseFont: 'Helvetica, sans-serif'
                    }
                }
            }
        }
    }
}

If I wanted my CSS variables to have different values for each of my two entry points, how would I go about doing this?

For instance, let's say in foo.css I want var(--color1) to equal #333, but in bar.css I want it to equal #860000.

I figured this out. Here's how I solved it.

I switched my webpack.config file to have multiple configurations, one for each entry point. I also configured postcss-loader to pass the entry point as context to postcss.config.js.

webpack.config.js

const webpack = require('webpack')
const path = require('path')
const MiniCssExtractPlugin = require('mini-css-extract-plugin')

const entryPoints = ['foo', 'bar']

const config = entry => ({
  mode: 'development',
  entry: `./${entry}.js`,
  output: {
    path: path.resolve(__dirname, 'src/'),
    filename: `js/${entry}.js`
  },
  plugins: [
    new MiniCssExtractPlugin({
      filename: `css/${entry}.css`
    })
  ],
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader'
        }
      },
      {
        test: /\.css$/,
        use: [
          {
            loader: MiniCssExtractPlugin.loader
          },
          'css-loader', 
          {
            loader: 'postcss-loader',
            options: {
              config: {
                ctx: { entry }
              }
            }
          },
        ]
      }
    ]
  }
})

module.exports = entryPoints.map(entry => config(entry))

My postcss.config.js file can then be conditionally configured based on the entry point. In this example I define styles inline, but these could be pulled in from an external module via a require.

postcss.config.js

module.exports = ({ options }) => ({
  plugins: {
    'postcss-cssnext': {
      features: {
        customProperties: {
          variables: 
            options.entry === 'foo' ? 
              {
                color1: '#333',
                color2: '#53565A',
                baseFont: 'Helvetica, sans-serif'
              } :
              {
                color1: '#860000',
                color2: '#555',
                baseFont: 'Arial, sans-serif'
              }
        }
      }
    }
  }
})