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Universal JavaScript applications are tough to setup. Either you buy into a framework like Next.js or react-server, fork a boilerplate, or set things up yourself. Razzle aims to fill this void by abstracting all the required tooling for your universal JavaScript application into a single dependency, and then leaving the rest of the architectural decisions about frameworks, routing, and data fetching up to you.


Razzle comes with the "battery-pack included" and is part of a complete JavaScript breakfast:

  • ūüĒ• Universal Hot Module Replacement, so both the client and server update whenever you make edits. No annoying restarts necessary
  • Comes with your favorite ES6 JavaScript goodies (through babel-preset-razzle)
  • Comes with the same CSS setup as create-react-app
  • Works with React, Reason-React, Preact, Inferno, and Rax as well as Angular and Vue if that's your thing
  • Escape hatches for customization via .babelrc and razzle.config.js
  • Jest test runner setup with sensible defaults via razzle test

Table of Contents

Quick Start

$ npm i -g create-razzle-app
create-razzle-app my-app
cd my-app
npm start

or with yarn

yarn create razzle-app my-app
cd my-app 
yarn start

Then open http://localhost:3000/ to see your app. Your console should look like this:

Razzle Development Mode

When you’re ready to deploy to production, create a minified bundle with npm run build.

Getting Started


If you have the latest version of Yarn, you can skip this. Otherwise:

Install Razzle globally:

npm i -g create-razzle-app

Creating an app

To create an app, run:

create-razzle-app my-app

or with yarn create (new!):

yarn create razzle-app my-app

You can also bootstrap any one of the examples by adding --example <example-name> to your command.

create-razzle-app --example with-preact my-app

or with yarn create

yarn create razzle-app my-app -- --example with-preact

(The -- is needed for yarn to ignore options meant for create-razzle-app)

It will create a directory called my-app inside the current folder.
Inside that directory, it will generate the initial project structure and install the transitive dependencies.

    client.js            # Client entry point 
    server.js .          # Main server code (an Express application)
    index.js             # Server entry point

Note: The default application is a universal React application with React Router 4 on an Express server. If don't want this setup, have a look at some of the examples. Each one is installable with just a few commands.

Once the installation is done, you can run some commands inside the project folder:

npm start or yarn start

Runs the project in development mode.
You can view your application at http://localhost:3000

The page will reload if you make edits.

npm run build or yarn build

Builds the app for production to the build folder.

The build is minified and the filenames include the hashes. Your app is ready to be deployed!

npm run start:prod or yarn start:prod

Runs the compiled app in production.

You can again view your application at http://localhost:3000

npm test or yarn test

Runs the test watcher (Jest) in an interactive mode. By default, runs tests related to files changed since the last commit.


Extending Babel Config

Razzle comes with most of ES6 stuff you need. However, if you want to add your own babel transformations, just add a .babelrc file to the root of your project.

  "presets": [

Extending Webpack

You can also extend the underlying webpack config. Create a file called razzle.config.js in your project's root.

// razzle.config.js 
module.exports = {
  modify: (config, {target, dev}, webpack) => {
    // do something to config 
    return config

A word of advice: razzle.config.js is an escape hatch. However, since it's just JavaScript, you can and should publish your modify function to npm to make it reusable across your projects. For example, imagine you added some custom webpack loaders and published it as a package to npm as my-razzle-modifictions. You could then write your razzle.config.js like so:

// razzle.config.js
const modify = require('my-razzle-modifictions');

module.exports = {

Last but not least, if you find yourself needing a more customized setup, Razzle is very forkable. There is one webpack configuration factory that is 300 lines of code, and 3 scripts (build, start, and init). The paths setup is shamelessly taken from create-react-app, and the rest of the code related to logging.

razzle API Reference

razzle start

Runs razzle in development mode.
You can view your application at http://localhost:3000

razzle build

Builds a razzle project for production. Final build located in build directory.

razzle test

Runs Jest test runner.


There are just a few settings you should know about.

// razzle.config.js 
module.exports = {
  modify: (config, { target, dev }, webpack) => {
    // do something and return config 
    return config

Environment Variables

The environment variables are embedded during the build time. Since Razzle produces a static HTML/CSS/JS bundle and an equivalent static bundle for your server, it cannot possibly read them at runtime.

  • process.env.RAZZLE_PUBLIC_DIR: Path to the public directory.
  • process.env.RAZZLE_ASSETS_MANIFEST: Path to a file containing compiled asset outputs
  • process.env.VERBOSE: default is false, setting this to true will not clear the console when you make edits in development (useful for debugging).
  • process.env.PORT: default is 3000, unless changed
  • process.env.HOST: default is
  • process.env.NODE_ENV: 'development' or 'production'
  • process.env.BUILD_TARGET: either 'client' or 'server'

You can create your own custom build-time environment variables. They must start with RAZZLE_. Any other variables except the ones listed above will be ignored to avoid accidentally exposing a private key on the machine that could have the same name. Changing any environment variables will require you to restart the development server if it is running.

These environment variables will be defined for you on process.env. For example, having an environment variable named RAZZLE_SECRET_CODE will be exposed in your JS as process.env.RAZZLE_SECRET_CODE.

Adding Temporary Environment Variables In Your Shell

Defining environment variables can vary between OSes. It’s also important to know that this manner is temporary for the life of the shell session.

Windows (cmd.exe)

set RAZZLE_SECRET_CODE=abcdef&&npm start

(Note: the lack of whitespace is intentional.)

Linux, macOS (Bash)

RAZZLE_SECRET_CODE=abcdef npm start

Adding Environment Variables In .env

To define permanent environment variables, create a file called .env in the root of your project:


What other .env files are can be used?

  • .env: Default.
  • .env.local: Local overrides. This file is loaded for all environments except test.
  • .env.development, .env.test, .env.production: Environment-specific settings.
  • .env.development.local, .env.test.local, .env.production.local: Local overrides of environment-specific settings.

Files on the left have more priority than files on the right:

  • npm start: .env.development.local, .env.development, .env.local, .env
  • npm run build: .env.production.local, .env.production, .env.local, .env
  • npm test: .env.test.local, .env.test, .env (note .env.local is missing)

These variables will act as the defaults if the machine does not explicitly set them.
Please refer to the dotenv documentation for more details.

Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely need these defined as well. Consult their documentation how to do this. For example, see the documentation for Travis CI or Heroku.

How Razzle works (the secret sauce)

tl;dr: 2 configs, 2 ports, 2 webpack instances, both watching and hot reloading the same filesystem, in parallel during development and a little webpack.output.publicPath magic.

In development mode (razzle start), Razzle bundles both your client and server code using two different webpack instances running with Hot Module Replacement in parallel. While your server is bundled and run on whatever port your specify in src/index.js (3000 is the default), the client bundle (i.e. entry point at src/client.js) is served via webpack-dev-server on a different port (3001 by default) with its publicPath explicitly set to localhost:3001 (and not / like many other setups do). Then the server's html template just points to the absolute url of the client JS: localhost:3001/static/js/client.js. Since both webpack instances watch the same files, whenever you make edits, they hot reload at exactly the same time. Best of all, because they use the same code, the same webpack loaders, and the same babel transformations, you never run into a React checksum mismatch error.