mue/webpack.dev.js

const { merge } = require('webpack-merge');
const common = require('./webpack.common.js');
const path = require('path');

module.exports = merge(common, {
  mode: 'development',
  module: {
    rules: [
      {
        test: /\.js$/,
        enforce: 'pre',
        use: ['source-map-loader']
      }
    ]
  },
  devServer: {
    contentBase: path.resolve(__dirname, './build'),
    open: true,
    port: 3000
  }
});