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How to get Webpack Stats

Below is an example of the recommended approach to getting stats. You can also do the below in your npm scripts, but it's far more robust to do it in code and keep your npm scripts cleaner.

If you haven't seen it before, it uses a multi-compiler. That means Webpack can compile 2 bundles at once: both the client and the server bundles. In fact, it's faster this way since shared modules are cached.

More importantly though, this approach makes "universal HMR" for both the client and server more frictionless than ever, thanks to @richardscarrot's amazing webpack-hot-server-middleware package.

const express = require('express')
const webpack = require('webpack')

const webpackDevMiddleware = require('webpack-dev-middleware')
const webpackHotMiddleware = require('webpack-hot-middleware')
const webpackHotServerMiddleware = require('webpack-hot-server-middleware')

const clientConfig = require('../webpack/client.dev')
const serverConfig = require('../webpack/server.dev')

const clientConfigProd = require('../webpack/client.prod')
const serverConfigProd = require('../webpack/server.prod')

const publicPath = clientConfig.output.publicPath
const outputPath = clientConfig.output.path
const DEV = process.env.NODE_ENV === 'development'
const app = express()

if (DEV) {
  const compiler = webpack([clientConfig, serverConfig]) // multi-compiler
  const clientCompiler = compiler.compilers[0]

  app.use(webpackDevMiddleware(compiler, { publicPath }))
  app.use(webpackHotMiddleware(clientCompiler))
  app.use(webpackHotServerMiddleware(compiler)) // passes stats to `serverRender`

  compiler.plugin('done', done)
}
else {
  webpack([clientConfigProd, serverConfigProd]).run((err, stats) => {
    const clientStats = stats.toJson().children[0]
    const serverRender = require('../buildServer/main.js').default

    app.use(publicPath, express.static(outputPath))
    app.use(serverRender({ clientStats }))      // production version of `serverRender`

    done()
  })
}

let isBuilt = false

const done = () =>
  !isBuilt &&
  app.listen(3000, () => {
    isBuilt = true
    console.log('BUILD COMPLETE -- Listening @ http://localhost:3000/')
  })

So the idea here is that your serverRender method is the default export of the entry module for your server webpack bundle:

webpack/server.prod.js + webpack/server.dev.js:

entry: path.resolve(__dirname, '../server/render.js'),

serverRender()

Lastly server/render.js which you should be familiar with by now looks like this in its entirety:

import React from 'react'
import ReactDOM from 'react-dom/server'
import { flushChunkNames } from 'react-universal-component/server'
import flushChunks from 'webpack-flush-chunks'
import App from '../src/components/App'

export default ({ clientStats }) => (req, res) => {
  const app = ReactDOM.renderToString(<App />)
  const chunkNames = flushChunkNames()
  const { js, styles, cssHash } = flushChunks(clientStats, { chunkNames })

  res.send(
    `<!doctype html>
      <html>
        <head>
          ${styles}
        </head>
        <body>
          <div id="root">${app}</div>
          ${cssHash}
          ${js}
        </body>
      </html>`
  )
}

The key takeaway is that the export IS A FUNCTION THAT RETURNS A TYPICAL EXPRESS MIDDLEWARE FUNCTION. And that first function takes an object with a clientStats key/val thanks to webpack-hot-server-middleware.

And finally since that's how our super awesome universal HMR middleware works, we simply replicate the same in production, passing clientStats in the same place. Again, that looks like this:

production call to serverRender:

app.use(publicPath, express.static(outputPath))
app.use(serverRender({ clientStats })) // function returned which becomes the middleware

Stats from a file

If you really need to get stats from a file, here's how you do it:

server/index.js:

const clientStats = require('../buildClient/stats.json')
const serverRender = require('../buildServer/main.js').default

app.use(publicPath, express.static(outputPath))
app.use(serverRender({ clientStats, outputPath }))

webpack/client.prod.js:

const StatsPlugin = require('stats-webpack-plugin')

// ...

plugins: [
  new StatsPlugin('stats.json')

Getting Stats using a Babel server

To get stats when using a Babel server (especially in development where HMR is important), you use the following:

server/index.js:

import serverRender from './render'

let serverRenderWithHmr = (req, res) => res.send('not ready yet')
const appMiddleware = (req, res) => serverRenderWithHmr

compiler.plugin('done', stats => {
  const clientStats = stats.toJson()
  serverRenderWithHmr = serverRender({ clientStats })
})

app.use(appMiddleware)

Do not do this:

compiler.run((error, stats) => {
  const clientStats = stats.toJson()
  // etc
})

The stats in the run callback are not complete as they are in the compiler plugin's done callback.

serverRenderWithHmr will continue to be replaced on every compilation that HMR triggers. If you're using a webpack server, webpack-hot-server-middleware does the same.

Additional Tricks

If you want to embed CSS in your served responses (rather than use stylesheets), pass outputPath to serverRender like this:

app.use(serverRender({ clientStats, outputPath }))

Then you can pass it to flushChunks like this:

const { js, css } = flushChunks(clientStats, {
  chunkNames,
  outputPath
})