Isomorphic Webpack: both on client and server
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Helps setting up isomorphic (universal) Webpack build: the one that's working both on client and server.

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Webpack 2

This library now only supports Webpack 2.

npm install webpack@2.2.0 --save
npm install extract-text-webpack-plugin@2.0.0-rc.2 --save

I've been supporting Webpack 1 in this library for a long time but now the time has come to move to Webpack 2 since it's almost ready to be released.

See Webpack 1 to 2 migration notes

For Webpack 1 use the 0.1.x version of this library.


In summer 2015 I wrote webpack-isomorphic-tools as an experiment to make isomorphic (universal) React rendering work on server-side when the project was built with Webpack. At that time I barely knew how that "Webpack" thing worked so I ended up actually reimplementing some of the Webpack functionality on the server-side.

Still, the goal was met, the whole thing worked, and many people started using webpack-isomorphic-tools in their apps to implement isomorphic (universal) rendering.

But while webpack-isomorphic-tools supports the core Webpack functionality (resolve.aliases and such) it still lacks some Webpack features like various plugins and other advanced stuff.

So I did some research on Webpack builds for Node.js and came up with this proof-of-concept solution which seems to work good enough. It supports all Webpack features (all plugins, etc).

universal-webpack vs webpack-isomorphic-tools

webpack-isomorphic-tools runs on the server-side and hooks into Node.js require() function with the help of require-hacker and does what needs to be done.

universal-webpack doesn't hook into require() function - it's just a helper for transforming client-side Webpack configuration to a server-side Webpack configuration. It doesn't run on the server-side or something. It's just a Webpack configuration generator - turned out that Webpack has a target: "node" parameter which makes it output code that runs on Node.js without any issues.

I wrote webpack-isomorphic-tools before universal-webpack, so universal-webpack is the recommended tool. However many people still use webpack-isomorphic-tools (including me) and find it somewhat less complicated for beginners.


npm install universal-webpack --save
npm install extract-text-webpack-plugin --save

Example project

You may refer to this sample project as an example of using this library (see webpack directory, package.json and code/page-server/web server.js).

Another sample project (from another author) can be found here.


Suppose you have a typical webpack.config.js file. Create two new files called webpack.config.client.babel.js and webpack.config.server.babel.js with the following contents:


import { clientConfiguration } from 'universal-webpack'
import settings from './universal-webpack-settings'
import configuration from './webpack.config'

export default clientConfiguration(configuration, settings)


import { serverConfiguration } from 'universal-webpack'
import settings from './universal-webpack-settings'
import configuration from './webpack.config'

export default serverConfiguration(configuration, settings)


		"input": "./source/server.js",
		"output": "./build/server/server.js"

Use webpack.config.client.babel.js instead of the old webpack.config.js for client side Webpack builds.

The serverConfiguration() function takes the client-side Webpack configuration and tunes it a bit for server-side usage (target: "node").

The server-side bundle (settings.server.output file) is generated from settings.server.input file by Webpack when it's run with the webpack.config.server.babel.js configuration. An example of settings.server.input file may look like this (it must export a function):


// express.js
import path from 'path'
import http from 'http'
import express from 'express'
import http_proxy from 'http-proxy'

// react-router
import routes from '../client/routes.js'

// Redux
import store from '../client/store.js'

// The server code must export a function
// (`parameters` may contain some miscellaneous library-specific stuff)
export default function(parameters)
	// Create HTTP server
	const app = new express()
	const server = new http.Server(app)

	// Serve static files
	app.use(express.static(path.join(__dirname, '..', 'build/assets')))

	// Proxy API calls to API server
	const proxy = http_proxy.createProxyServer({ target: 'http://localhost:xxxx' })
	app.use('/api', (req, res) => proxy.web(req, res))

	// React application rendering
	app.use((req, res) =>
		// Match current URL to the corresponding React page
		// (can use `react-router`, `redux-router`, `react-router-redux`, etc)
		react_router_match_url(routes, req.originalUrl).then((error, result) =>
			if (error)
				return res.send('Server error')

			// Render React page

			const page = redux.provide(result, store)

			res.send('<!doctype html>' + '\n' + ReactDOM.renderToString(<Html>{page}</Html>))

	// Start the HTTP server

The last thing to do is to create a startup file for the server side. This is the file you're gonna run with Node.js, not the file provided above.


import { server } from 'universal-webpack'
import settings from '../universal-webpack-settings'
// `configuration.context` and `configuration.output.path` are used
import configuration from '../webpack.config'

server(configuration, settings)


// Enable ES6
// (ignoring all `build` and `node_modules` folders for speed-up)
require('babel-register')({ ignore: /\/(build|node_modules)\// })

// Run `source/start-server.js`

Calling source/start-server.js will basically call the function exported from source/server.js built with Webpack.

In the end you run all the above things like this (in parallel):

webpack-dev-server --hot --inline --config "./webpack.config.client.babel.js" --port XXXX --colors --display-error-details
webpack --watch --config "./webpack.config.server.babel.js" --colors --display-error-details
nodemon "./source/start-server.babel" --watch "./build/server"

The above three commands are for development mode. For production mode the same command sequence would be:

webpack --config "./webpack.config.client.babel.js" --colors --display-error-details
webpack --config "./webpack.config.server.babel.js" --colors --display-error-details
node "./source/start-server.babel"


This library will pass the chunks() function parameter (inside the parameters argument of the server-side function) which returns webpack-compiled chunks filename info:


		main: `/assets/main.785f110e7775ec8322cf.js`

		main: `/assets/main.785f110e7775ec8322cf.css`

These filenames are required for <script src=.../> and <link rel="style" href=.../> tags in case of isomorphic (universal) rendering on the server-side.


It emits no assets from the server-side code so make sure you include all assets at least on the client side (e.g. "favicon").


The third argument – options object – may be passed to clientConfiguration function. If options.development === false, then it will apply extract-text-webpack-plugin to CSS styles automatically, i.e. it will extract all CSS styles into one big bundle file. This is considered the "best practice" for production deployment.

Advanced configuration

	// By default, all `require()`d packages
	// (e.g. everything from `node_modules`, `resolve.modules`),
	// except for `resolve.alias`ed ones,
	// are marked as `external` for server-side Webpack build
	// which means they won't be processed and bundled by Webpack,
	// instead being processed and `require()`d at runtime by Node.js.
	// With this setting one can explicitly define which modules
	// aren't gonna be marked as `external` dependencies.
	// (and therefore are gonna be compiled and bundled by Webpack)
	// Can be used, for example, for ES6-only `node_modules`.
	// ( a more intelligent solution would be accepted
	// )
	// Another use case is including assets from `node_modules`:
	// in order to do so one must add those assets to `exclude_from_externals`.
	// Otherwise, for example, when `require()`ing CSS files from `node_modules`
	// Node.js will just throw `SyntaxError: Unexpected token .`
	// because these CSS files need to be compiled by Webpack's `css-loader` first.

	// Enable `silent` flag to prevent client side webpack build
	// from outputting chunk stats to the console.
	silent: true,

	// By default, chunk_info_filename is `webpack-chunks.json`
	chunk_info_filename: 'submodule-webpack-chunks.json'

Source maps

I managed to get source maps working in my Node.js server-side code using source-map-support module.


// Enables proper source map support in Node.js
import 'source-map-support/register'

// The rest is the same as in the above example

import { server } from 'universal-webpack'
import settings from '../universal-webpack-settings'
import configuration from '../webpack.config'

server(configuration, settings)

Without source-map-support enabled it would give me No element indexed by XXX error (which means that by default Node.js thinks there are references to other source maps and tries to load them but there are no such source maps).

devtool is set to source-map for server-side builds.


I recommend using nodemon for running server-side Webpack bundle. Nodemon has a --watch <directory> command line parameter which restarts Node.js process each time the <directory> is updated (e.g. each time any file in that directory is modified).

In other words, Nodemon will relaunch the server every time the code is rebuilt with Webpack.

There's one little gotcha though: for the --watch feature to work the watched folder needs to exist by the time Nodemon is launched. That means that the server must be started only after the settings.server.output path folder has been created.

To accomplish that this library provides a command line tool: universal-webpack. No need to install in globally: it is supposed to work locally through npm scripts. Usage example:


  "scripts": {
    "start": "npm-run-all prepare-server-build start-development-workflow",
    "start-development-workflow": "npm-run-all --parallel development-webpack-build-for-client development-webpack-build-for-server development-start-server",
    "prepare-server-build": "universal-webpack --settings ./universal-webpack-settings.json prepare",

The prepare command creates settings.server.output path folder, or clears it if it already exists.

In a moderately sized React project server restart times can reach ~10 seconds. Therefore the right way to go is to extract React rendering server-side code into a separate Node.js process (service), and then in the main Node.js web server just proxy all unmatched URLs to this React rendering service, as explained in the following section. This way only React rendering service will be restarted, and the main Node.js web server will be left untouched.

Separate React rendering service from the main code

If the sole purpose of using universal-webpack in your project is to enable server-side rendering of web pages, then the best practice is to extract the web page rendering code from the main Node.js application into a separate Node.js application (reactRenderingService in the example below), and then in the main Node.js application just proxy all unmatched URLs to this React rendering service.

import express from 'express'
import httpProxy from 'http-proxy'

const app = express()

const reactRenderingService = `http://localhost:3000`
const proxy = httpProxy.createProxyServer({})

// The usual web application middleware
// (serving statics, REST API, etc)

// Proxy all unmatched HTTP requests to webpage rendering service
app.use(function(request, response)
  proxy.web(request, response, { target: reactRenderingService }, (error) =>
  	response.status(500).send('Proxying failed for page rendering service')

This way only the rendering service will have to be restarted (by nodemon) and rebuilt (by Webpack) on code changes.

Flash of unstyled content

(this is an "advanced" section which can be safely skipped)

A "flash of unstyled content" is a well-known thing. One can observe it when refreshing the page in development mode: because Webpack's style-loader adds styles to the page dynamically there's a short period (a second maybe) when there are no CSS styles applied to the webpage (in production mode extract-text-webpack-plugin is used instead of style-loader so there's no "flash of unstyled content").

It's not really a bug, many projects live with it and it doesn't really affect the development process that much, so one can safely skip reading this section. It's just if you're a perfectionist then it can get a little itchy.

I came up with a solution which seems to be working good enough. To enable the anti-unstyled-flash feature one needs to pass the third parameter to the client-side webpack configuration creation function - an options object with:

  • development key set to true indicating that it's a development build configuration
  • css_bundle key set to true

If both development and css_bundle options are set to true, then universal-webpack will enhance the client side Webpack configuration to also output all styles into a single CSS bundle (while retaining style-loader) which is later added to the webpage's <head/> as a <link rel="stylesheet"/> tag on the server side, therefore making that "flash of unstyled content" disappear.

There's a gotcha though. Because the whole CSS bundle gets inserted as a <link rel="stylesheet"/> tag in the <head/> it also means that the styles defined in that CSS bundle are static, not dynamic, and they aren't gonna "hot reload" themselves or something. So, my proposed solution is to have that <link rel="stylesheet"/> tag sit in the <head/> for a while (say, a couple of seconds) and then remove it from there. The styling of the webpage isn't gonna disappear at that moment because by that time the dynamic styles of style-loader have already kicked in. See an example of how this can be done.

import { clientConfiguration } from 'universal-webpack'
import settings from './universal-webpack-settings'
import configuration from './webpack.config'

export default clientConfiguration(configuration, settings, { development: true, css_bundle: true })


If you were using resolve.moduleDirectories for global paths instead of relative paths in your code then consider using resolve.alias instead

    components: path.resolve(__dirname, '../src/components'),

Server-side startup time optimization

In case babel-register is used to run the server-side bundle, there have been reports that leveraging the ignore option of babel-register might speed things up significantly by telling Babel explicitly not to parse the server-side bundle.

// Prevents Babel from transpiling server-side bundle
// resulting in faster server-side hot-reload (startup) times.