Webpack + Vue.js hot module replacement config in an existing site

dev-client.js

// build/dev-client.js

/* eslint-disable */
require('eventsource-polyfill')
var hotClient = require('webpack-hot-middleware/client?noInfo=true&reload=true&path=http://localhost:8080/__webpack_hmr')//<-- HMR config

hotClient.subscribe(function (event) {
  if (event.action === 'reload') {
    window.location.reload()
  }
})

index.js

// config/index.js

// see http://vuejs-templates.github.io/webpack for documentation.
var path = require('path')

module.exports = {
  build: {
    env: require('./prod.env'),
    index: path.resolve(__dirname, '../dist/index.html'),
    assetsRoot: path.resolve(__dirname, '../dist'),
    assetsSubDirectory: 'static',
    assetsPublicPath: '/',
    productionSourceMap: true,
    // Gzip off by default as many popular static hosts such as
    // Surge or Netlify already gzip all static assets for you.
    // Before setting to `true`, make sure to:
    // npm install --save-dev compression-webpack-plugin
    productionGzip: false,
    productionGzipExtensions: ['js', 'css']
  },
  dev: {
    env: require('./dev.env'),
    port: 8080,
    assetsSubDirectory: 'static',
    assetsPublicPath: 'http://localhost:8080/',//<-- HMR config
    proxyTable: {},
    // CSS Sourcemaps off by default because relative paths are "buggy"
    // with this option, according to the CSS-Loader README
    // (https://github.com/webpack/css-loader#sourcemaps)
    // In our experience, they generally work as expected,
    // just be aware of this issue when enabling this option.
    cssSourceMap: false
  }
}

your-existing-template.html.twig

<script src="http://localhost:8080/app.js"></script>