Bootstrap 4 in Phoenix Framework using Webpack

app.js

// We need to import the CSS so that webpack will load it.
// The MiniCssExtractPlugin is used to separate it out into
// its own CSS file.

import css from "../css/app.scss"

// webpack automatically bundles all modules in your
// entry points. Those entry points can be configured
// in "webpack.config.js".
//
// Import dependencies
//
import "phoenix_html"
import "bootstrap"

// Import local files
//
// Local files can be imported directly using relative paths, for example:
// import socket from "./socket"

app.scss

@import "~bootstrap/scss/bootstrap";

packages.json

{
  "repository": {},
  "license": "MIT",
  "scripts": {
    "deploy": "webpack --mode production",
    "watch": "webpack --mode development --watch"
  },
  "dependencies": {
    "autoprefixer": "^9.5.1",
    "bootstrap": "^4.3.1",
    "jquery": "^3.4.1",
    "node-sass": "^4.12.0",
    "phoenix": "file:../deps/phoenix",
    "phoenix_html": "file:../deps/phoenix_html",
    "popper.js": "^1.15.0",
    "postcss-loader": "^3.0.0",
    "precss": "^4.0.0",
    "sass-loader": "^7.1.0",
    "style-loader": "^0.23.1"
  },
  "devDependencies": {
    "@babel/core": "^7.0.0",
    "@babel/preset-env": "^7.0.0",
    "babel-loader": "^8.0.0",
    "copy-webpack-plugin": "^4.5.0",
    "css-loader": "^2.1.1",
    "mini-css-extract-plugin": "^0.4.0",
    "optimize-css-assets-webpack-plugin": "^4.0.0",
    "uglifyjs-webpack-plugin": "^1.2.4",
    "webpack": "4.4.0",
    "webpack-cli": "^2.0.10"
  }
}

webpack.config.js

const path = require('path');
const glob = require('glob');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
const CopyWebpackPlugin = require('copy-webpack-plugin');

module.exports = (env, options) => ({
  optimization: {
    minimizer: [
      new UglifyJsPlugin({ cache: true, parallel: true, sourceMap: false }),
      new OptimizeCSSAssetsPlugin({})
    ]
  },
  entry: {
    './js/app.js': ['./js/app.js'].concat(glob.sync('./vendor/**/*.js'))
  },
  output: {
    filename: 'app.js',
    path: path.resolve(__dirname, '../priv/static/js')
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: { loader: 'babel-loader' }
      },
      {
        test: /\.scss$/,
        use: [
          MiniCssExtractPlugin.loader,
          'css-loader',
          {
            loader: 'postcss-loader',
            options: {
              plugins: function () {
                return [
                  require('precss'),
                  require('autoprefixer')
                ];
              }
            }
          },
          'sass-loader'
        ]
      }
    ]
  },
  plugins: [
    new MiniCssExtractPlugin({ filename: '../css/app.css' }),
    new CopyWebpackPlugin([{ from: 'static/', to: '../' }])
  ]
});