hot reloadを自前のexpressサーバーに組み込む ref: https://qiita.com/RyosukeCla/items/d0e02777d0b5cf9358e1

client.js

// any js code. eg vue/react/...

file0.txt

npm i --save-dev express webpack webpack-dev-middleware webpack-hot-middleware

file1.txt

npm i --save-dev pug

file3.js

const webpack = require('webpack')
const webpackConfig = require('./webpack.config.js')
const compiler = webpack(webpackConfig)
app.use(require('webpack-dev-middleware')(compiler, {
  noInfo: true, publicPath: webpackConfig.output.publicPath
}))
app.use(require("webpack-hot-middleware")(compiler))

index.pug

html
  header
    title hot reload
  body
    #app
    script(src="/dist/app.js")

server.js

const express = require('express')
const app = express()
const isProd = process.env.NODE_ENV === 'production'

if (!isProd) {
  const webpack = require('webpack')
  const webpackConfig = require('./webpack.config.js')
  const compiler = webpack(webpackConfig)
  app.use(require('webpack-dev-middleware')(compiler, {
    noInfo: true, publicPath: webpackConfig.output.publicPath
  }))
  app.use(require("webpack-hot-middleware")(compiler))
  }
}

app.set('view engine', 'pug')

app.use((req, res, next) => {
  res.render('index')
})

const server = setup(app)

server.listen(8080)

webpack.config.js

const path = require('path')
const webpack = require('webpack')
const isProd = process.env.NODE_ENV === 'production'

const webpackConfig = {
  // webpack config
  entry: {
    app: [path.resolve(__dirname, 'client.js')]
  },
  output: {
    filename: '[name].js',
    path: path.resolve(__dirname, 'dist'),
    publicPath: '/dist/'
  },
  plugins: []
  // ...
}

if (isProd) {
  // ...
} else {
  // dev
  webpackConfig.entry.app.unshift('webpack-hot-middleware/client')
  webpackConfig.plugins.push(
    new webpack.HotModuleReplacementPlugin()
  )
}

module.exports = webpackConfig