Webpack - React

this post describes how to set up React in a project using Webpack - instructions are geared towards Phoenix project but it must be fairly easy to adapt them to Rails project using Webpacker.

it’s assumed that it’s not React-only project - React is integrated into existing application with traditional HTML templates.

  1. http://whatdidilearn.info/2018/05/20/how-to-use-webpack-and-react-with-phoenix-1-3.html#configure-react

install npm packages

$ cd assets
$ yarn add react react-dom prop-types babel-preset-react

NOTE: add babel-preset-react package as normal (not development) dependency or else it may be not installed in production if installing npm packages with yarn install --production command.


it’s possible to run webpacker:install:react Webpacker generator to install required npm packages, add react preset to Babel config and create a sample React component app/assets/packs/hello_react.jsx but generally it’s better to do everything manually.

add react preset to Babel config

  // assets/.babelrc

  "presets": [
    // ...
+   "react"

don’t install @babel/preset-react package - it requires Babel 7 (while Babel 6 is currently used in my project).

add jsx extension to be resolved automatically

// assets/webpack.config.js

resolve: {
  // ...
  extensions: ['.js', '.jsx', /* ... */],


jsx extension is added in Webpacker config.

update app.js to load page-specific JS files recursively

  1. https://webpack.js.org/guides/dependency-management/#context-module-api

page-specific JS file is JS file used to load JS for specific page (all pages should have unique body classes like p-user-new where p is a page prefix, user is controller name and new is action name).

load all JS files in assets/js/pages/ recursively:

// assets/js/app.js

const r = require.context('./pages', true, /\.js$/);

add HTML element with react ID to existing page template

of course you can choose any other ID name and insert this element wherever you want inside the template:

// lib/my_app_web/templates/user/new.html.slime

// it's better to serialize user outside the template
// (say, in a view helper)
#react data-user=Poison.encode!(@user)

React component props are passed via data attributes.

create page-specific JS file

say, JS file for page to create new user:

import React from 'react';
import ReactDOM from 'react-dom';

import User from 'js/react/components/User';

document.addEventListener('DOMContentLoaded', () => {
  if (!document.body.classList.contains('p-user-new')) {

  const node = document.getElementById('react');
  const user = JSON.parse(node.dataset.user);

  ReactDOM.render(<User.NewPage user={user} />, node);

create React component

// assets/js/react/components/User/index.js

import NewPage from './NewPage';

export default {
// assets/js/react/components/User/NewPage.js

import React from 'react';

class NewPage extends React.Component {
  render () {
    return (
      <div>Hello {this.props.user.name}!</div>

export default NewPage;


[Rails] ReferenceError: Unknown option: …react/index.js.Children

$ cap production deploy
01 ERROR in ./app/assets/packs/app.js
01 Module build failed: ReferenceError: [BABEL]
  Unknown option: /home/sith/production/releases/20180626151539/node_modules/react/index.js.Children.
  Check out http://babeljs.io/docs/usage/options/ for more information about options.


  1. https://github.com/rails/webpacker/issues/1330
  2. https://github.com/rails/webpacker/issues/1460
  3. https://github.com/rails/webpacker/issues/1441#issuecomment-383328538
  4. https://github.com/rails/webpacker/issues/1037#issuecomment-347610374


The error is unhelpful, but the issue is that your config has react in the preset list, but it can’t find the babel-preset-react module in your node_modules, so instead it is loading the react module itself as if it were a preset. But since the “react” module isn’t a preset, Babel throws.

babel-preset-react npm package is added as development dependency to package.json - it’s removed by webpacker:yarn_install task which is run as depedency of webpacker:compile task:


Webpacker hooks up a new webpacker:compile task to assets:precompile, which gets run whenever you run assets:precompile.

webpacker:yarn_install runs this command:

yarn install --no-progress --frozen-lockfile --production

--production flag means that development dependencies won’t be installed.

surprisingly babel-preset-react package is not installed even when it’s added as normal depedency - maybe it’s some caching problem or bug in Yarn itself.

my current solution (workraround to be precise) is to run yarn install manually (without --production flag) and skip webpacker:yarn_install task at all:

# lib/tasks/webpacker.rake


namespace :webpacker do
  desc 'Skip default webpacker yarn install'
  task :yarn_install do
    puts 'Skipping webpacker yarn install'
  # config/deploy.rb

  namespace :deploy do
+   task :yarn_install do
+     on roles(:app) do
+       within release_path do
+         execute "cd #{release_path} && yarn install"
+       end
+     end
+   end

    # ...

+ before 'deploy:assets:precompile', 'deploy:yarn_install'

or else it’s possible to compile assets locally and copy them to production server with rsync (link) - this way you don’t have to install Node.js and Yarn on production server at all.


In the end I reverted changes above and started to precompile assets locally.