webpack.config.js

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

module.exports = {
  entry: {
    app: path.join(__dirname, 'src/app.js')
  },

  plugins: [
    new MiniCssExtractPlugin({
      filename: '[name].css',
      chunkFilename: '[id].css',
    }),
    new webpack.ProvidePlugin({
      $: "jquery",
      jQuery: "jquery",
      "window.jQuery": "jquery",
      Popper: ["popper.js", "default"]
    })
  ],

  output: {
    filename: '[name].js.liquid',
    path: path.join(__dirname, '/assets'),
  },

  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /(node_modules)/,
        use: {
          loader: 'babel-loader',
          options: {
            presets: [
              '@babel/preset-env',
            ],
          },
        },
      },
      {
        test: /\.(sa|sc|c)ss$/,
        exclude: /node_modules/,
        use: [
          {
            loader: MiniCssExtractPlugin.loader,
            options: {
              publicPath: '../',
              hmr: process.env.NODE_ENV === 'development',
            },
          },
          {
            loader: 'css-loader',
            options: {
              sourceMap: true,
            },
          },
          {
            loader: 'sass-loader',
            options: {
              sourceMap: true,
            },
          },
          {
            loader: 'postcss-loader',
            options: {
              sourceMap: true,
            },
          }
        ],
      },
    ],
  }
};