Drupal 8 webpack

mytheme.libraries.yml

global-styling:
  js:
    build/js/vendor.js: {}
    build/js/main.js: {}
  dependencies:
    - core/drupal
  css:
    theme:
      build/css/main.css: {}

package.json

{
  "name": "d8-webpack",
  "version": "1.0.0",
  "description": "[![Build Status](https://travis-ci.org/drupal-composer/drupal-project.svg?branch=8.x)](https://travis-ci.org/drupal-composer/drupal-project)",
  "main": "app.js",
  "scripts": {
    "start": "webpack --mode development --watch --config webpack.dev.js",
    "build": "webpack --mode production --config webpack.prod.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@babel/core": "^7.2.2",
    "@babel/plugin-syntax-dynamic-import": "^7.2.0",
    "@babel/preset-env": "^7.2.2",
    "autoprefixer": "^9.4.4",
    "babel-loader": "^8.0.5",
    "clean-webpack-plugin": "^1.0.0",
    "css-loader": "^2.1.0",
    "cssnano": "^4.1.8",
    "eslint": "^5.3.0",
    "eslint-config-airbnb-base": "^13.1.0",
    "eslint-loader": "^2.1.1",
    "eslint-plugin-import": "^2.14.0",
    "html-webpack-plugin": "^3.2.0",
    "mini-css-extract-plugin": "^0.5.0",
    "node-sass": "^4.11.0",
    "postcss-loader": "^3.0.0",
    "postcss-preset-env": "^6.5.0",
    "sass-loader": "^7.1.0",
    "style-loader": "^0.23.1",
    "webpack": "^4.28.1",
    "webpack-cli": "^3.2.1",
    "webpack-dev-server": "^3.1.14",
    "webpack-merge": "^4.2.1"
  }
}

webpack.common.js

// webpack v4

const path = require('path');

const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const CleanWebpackPlugin = require('clean-webpack-plugin');

const themePath = path.resolve(__dirname, 'web/themes/custom/mytheme'); // Replace theme folder with your own
const themeJsPath = path.resolve(__dirname, `${themePath}/js`);
const themeBuildFolder = path.resolve(__dirname, `${themePath}/build`);

module.exports = {
    target: 'web',
    entry: {
        main: path.resolve(__dirname, `${themeJsPath}/app.js`)
    },
    output: {
        path: path.resolve(__dirname, themeBuildFolder),
        filename: 'js/[name].js',
        publicPath: '/'
    },
    optimization: {
        splitChunks: {
            cacheGroups: {
                vendor: {
                    test: /node_modules/,
                    chunks: 'all',
                    name: 'vendor',
                    enforce: true
                }
            }
        }
    },
    module: {
        rules: [
            {
                test: /\.js$/,
                exclude: /node_modules/,
                use: {
                   loader: 'babel-loader'
                }
            },
            {
                test: /\.js$/,
                exclude: /node_modules/,
                use: {
                    loader: 'eslint-loader',
                    options: {
                        failOnError: true
                    }
                }
            },
            {
                test: /\.scss$/,
                use: [
                    'style-loader',
                    MiniCssExtractPlugin.loader,
                    'css-loader',
                    'postcss-loader',
                    'sass-loader'
                ]
            }
        ]
    },
    plugins: [
        new CleanWebpackPlugin(themeBuildFolder, {} ),
        new MiniCssExtractPlugin({
            filename: 'css/main.css'
        })
    ]
};

webpack.dev.js

const merge = require('webpack-merge');
const baseConfig = require('./webpack.common.js');

module.exports = merge(baseConfig, {
    devtool: 'inline-source-map'
});

webpack.prod.js

// Empty for now

const merge = require('webpack-merge');
const baseConfig = require('./webpack.common.js');

module.exports = merge(baseConfig, {

});

When developing a D8 site, ensure that ' Aggregate CSS files' and 'Aggregate JavaScript files' checkboxes are unticked in admin/config/development/performance