Rails UJS for the react-rails gem
Last updated 6 months ago by bookofgreg .
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React-Rails is a flexible tool to use React with Rails. The benefits:

  • Automatically renders React server-side and client-side
  • Supports Webpacker 4.x, 3.x, 2.x, 1.1+
  • Supports Sprockets 4.x, 3.x, 2.x
  • Lets you use JSX, ES6, TypeScript, CoffeeScript

A source code example utilizing React-Rails:


After reading this README file, additional information about React-Rails can be found in the Wiki page: The Wiki page features a significant amount of additional information about React-Rails which includes instructional articles and answers to the most frequently asked questions.

Get started with Webpacker

Alternatively, get started with Sprockets

Webpacker provides modern JS tooling for Rails. Here are the listed steps for integrating Webpacker and Rails-React with Rails:

1) Create a new Rails app:
$ rails new my-app
$ cd my-app
2) Add webpacker and react-rails to your gemfile:
gem 'webpacker'
gem 'react-rails'
3) Now run the installers:
Rails 6.x:
$ bundle install
$ rails webpacker:install
$ rails webpacker:install:react
$ rails generate react:install

Note: For Rails 6, You don't need to add javascript_pack_tag as in Step 4. Since its already added by default.

Rails 5.x:
$ bundle install
$ rails webpacker:install       # OR (on rails version < 5.0) rake webpacker:install
$ rails webpacker:install:react # OR (on rails version < 5.0) rake webpacker:install:react
$ rails generate react:install

This gives you:

  • app/javascript/components/ directory for your React components
  • ReactRailsUJS setup in app/javascript/packs/application.js
  • app/javascript/packs/server_rendering.js for server-side rendering
4) Link the JavaScript pack in Rails view using javascript_pack_tag helper:
<!-- application.html.erb in Head tag below turbolinks -->
<%= javascript_pack_tag 'application' %>
5) Generate your first component:
$ rails g react:component HelloWorld greeting:string
6) You can also generate your component in a subdirectory:
$ rails g react:component my_subdirectory/HelloWorld greeting:string

Note: Your component is added to app/javascript/components/ by default.

7) Render it in a Rails view:
<!-- erb: paste this in view -->
<%= react_component("HelloWorld", { greeting: "Hello from react-rails." }) %>
8) Lets Start the app:
$ rails s

output: greeting: Hello from react-rails", inspect webpage in your browser too see change in tag props.

Component name

The component name tells react-rails where to load the component. For example:

react_component call component require
react_component("Item") require("Item")
react_component("items/index") require("items/index")
react_component("items.Index") require("items").Index
react_component("items.Index.Header") require("items").Index.Header

This way, you can access top-level, default, or named exports.

The require.context inserted into packs/application.js is used to load components. If you want to load components from a different directory, override it by calling ReactRailsUJS.useContext:

var myCustomContext = require.context("custom_components", true)
var ReactRailsUJS = require("react_ujs")
// use `custom_components/` for <%= react_component(...) %> calls

If require fails to find your component, ReactRailsUJS falls back to the global namespace, described in Use with Asset Pipeline.

File naming

React-Rails supports plenty of file extensions such as: .js, .jsx.js, .js.jsx, .es6.js, .coffee, etcetera! Sometimes this will cause a stumble when searching for filenames.

Component File Name react_component call
app/javascript/components/samplecomponent.js react_component("samplecomponent")
app/javascript/components/sample_component.js react_component("sample_component")
app/javascript/components/SampleComponent.js react_component("SampleComponent")
app/javascript/components/SampleComponent.js.jsx Has to be renamed to SampleComponent.jsx, then use react_component("SampleComponent")

Typescript support

If you want to use React-Rails with Typescript, simply run the installer and add @types:

$ bundle exec rails webpacker:install:typescript
$ yarn add @types/react @types/react-dom

Doing this will allow React-Rails to support the .tsx extension.

Test component

You can use assert_react_component to test component render:


<%= react_component("HelloWorld", { greeting: "Hello from react-rails.", info: { name: "react-rails" } }, { class: "hello-world" }) %>
class WelcomeControllerTest < ActionDispatch::IntegrationTest
  test 'assert_react_component' do
    get "/welcome"
    assert_equal 200, response.status

    # assert rendered react component and check the props
    assert_react_component "HelloWorld" do |props|
      assert_equal "Hello from react-rails.", props[:greeting]
      assert_equal "react-rails", props[:info][:name]
      assert_select "[class=?]", "hello-world"

    # or just assert component rendered
    assert_react_component "HelloWorld"

Use with Asset Pipeline

react-rails provides a pre-bundled React.js & a UJS driver to the Rails asset pipeline. Get started by adding the react-rails gem:

gem 'react-rails'

And then install the react generator:

$ rails g react:install

Then restart your development server.

This will:

  • add some //= requires to application.js
  • add a components/ directory for React components
  • add server_rendering.js for server-side rendering

Now, you can create React components in .jsx files:

// app/assets/javascripts/components/post.jsx

window.Post = createReactClass({
  render: function() {
    return <h1>{this.props.title}</h1>

// or, equivalent:
class Post extends React.Component {
  render() {
    return <h1>{this.props.title}</h1>

Then, you can render those components in views:

<%= react_component("Post", {title: "Hello World"}) %>

Components must be accessible from the top level, but they may be namespaced, for example:

<%= react_component("Comments.NewForm", {post_id:}) %>
<!-- looks for `window.Comments.NewForm` -->

Custom JSX Transformer

react-rails uses a transformer class to transform JSX in the asset pipeline. The transformer is initialized once, at boot. You can provide a custom transformer to config.react.jsx_transformer_class. The transformer must implement:

  • #initialize(options), where options is the value passed to config.react.jsx_transform_options
  • #transform(code_string) to return a string of transformed code

react-rails provides two transformers, React::JSX::BabelTransformer (which uses ruby-babel-transpiler) and React::JSX::JSXTransformer (which uses the deprecated JSXTransformer.js).

Transform Plugin Options

To supply additional transform plugins to your JSX Transformer, assign them to config.react.jsx_transform_options

react-rails uses the Babel version of the babel-source gem.

For example, to use babel-plugin-transform-class-properties :

config.react.jsx_transform_options = {
  optional: ['es7.classProperties']

React.js versions

//= require react brings React into your project.

By default, React's [development version] is provided to Rails.env.development. You can override the React build with a config:

# Here are the defaults:
# config/environments/development.rb
MyApp::Application.configure do
  config.react.variant = :development

# config/environments/production.rb
MyApp::Application.configure do
  config.react.variant = :production

Be sure to restart your Rails server after changing these files. See to learn which version of React.js is included with your react-rails version. In some edge cases you may need to bust the sprockets cache with rake tmp:clear

View Helper

react-rails includes a view helper and an unobtrusive JavaScript driver which work together to put React components on the page.

The view helper (react_component) puts a div on the page with the requested component class & props. For example:

<%= react_component('HelloMessage', name: 'John') %>
<!-- becomes: -->
<div data-react-class="HelloMessage" data-react-props="{&quot;name&quot;:&quot;John&quot;}"></div>

On page load, the react_ujs driver will scan the page and mount components using data-react-class and data-react-props.

The view helper's signature is:

react_component(component_class_name, props={}, html_options={})
  • component_class_name is a string which identifies a component. See getConstructor for details.
  • props is either:
    • an object that responds to #to_json; or
    • an already-stringified JSON object (see JBuilder note below).
  • html_options may include:
    • tag: to use an element other than a div to embed data-react-class and data-react-props.
    • prerender: true to render the component on the server.
    • camelize_props to transform a props hash
    • **other Any other arguments (eg class:, id:) are passed through to content_tag.

Custom View Helper

react-rails uses a "helper implementation" class to generate the output of the react_component helper. The helper is initialized once per request and used for each react_component call during that request. You can provide a custom helper class to config.react.view_helper_implementation. The class must implement:

  • #react_component(name, props = {}, options = {}, &block) to return a string to inject into the Rails view
  • #setup(controller_instance), called when the helper is initialized at the start of the request
  • #teardown(controller_instance), called at the end of the request

react-rails provides one implementation, React::Rails::ComponentMount.


react-rails's JavaScript is available as "react_ujs" in the asset pipeline or from NPM. It attaches itself to the window as ReactRailsUJS.

Mounting & Unmounting

Usually, react-rails mounts & unmounts components automatically as described in Event Handling below.

You can also mount & unmount components from <%= react_component(...) %> tags using UJS:

// Mount all components on the page:
// Mount components within a selector:
// Mount components within a specific node:

// Unmounting works the same way:

You can use this when the DOM is modified by AJAX calls or modal windows.

Event Handling

ReactRailsUJS checks for various libraries to support their page change events:

  • Turbolinks
  • pjax
  • jQuery
  • Native DOM events

ReactRailsUJS will automatically mount components on <%= react_component(...) %> tags and unmount them when appropriate.

If you need to re-detect events, you can call detectEvents:

// Remove previous event handlers and add new ones:

For example, if Turbolinks is loaded after ReactRailsUJS, you'll need to call this again. This function removes previous handlers before adding new ones, so it's safe to call as often as needed.

If Turbolinks is imported via Webpacker (and thus not available globally), ReactRailsUJS will be unable to locate it. To fix this, you can temporarily add it to the global namespace:

// Order is particular. First start Turbolinks:
// Add Turbolinks to the global namespace:
window.Turbolinks = Turbolinks;
// Remove previous event handlers and add new ones:
// (Optional) Clean up global namespace:
delete window.Turbolinks;


Components are loaded with ReactRailsUJS.getConstructor(className). This function has two built-in implementations:

  • On the asset pipeline, it looks up className in the global namespace.
  • On Webpacker, it requires files and accesses named exports, as described in Get started with Webpacker.

You can override this function to customize the mapping of name-to-constructor. Server-side rendering also uses this function.

Server-Side Rendering

You can render React components inside your Rails server with prerender: true:

<%= react_component('HelloMessage', {name: 'John'}, {prerender: true}) %>
<!-- becomes: -->
<div data-react-class="HelloMessage" data-react-props="{&quot;name&quot;:&quot;John&quot;}">
  <h1>Hello, John!</h1>

(It will also be mounted by the UJS on page load.)

Server rendering is powered by ExecJS and subject to some requirements:

  • react-rails must load your code. By convention, it uses server_rendering.js, which was created by the install task. This file must include your components and their dependencies (eg, Underscore.js).
  • Your code can't reference document or window. Prerender processes don't have access to document or window, so jQuery and some other libs won't work in this environment :(

ExecJS supports many backends. CRuby users will get the best performance from mini_racer.


Server renderers are stored in a pool and reused between requests. Threaded Rubies (eg jRuby) may see a benefit to increasing the pool size beyond the default 0.

These are the default configurations:

# config/application.rb
# These are the defaults if you don't specify any yourself
module MyApp
  class Application < Rails::Application
    # Settings for the pool of renderers:
    config.react.server_renderer_pool_size  ||= 1  # ExecJS doesn't allow more than one on MRI
    config.react.server_renderer_timeout    ||= 20 # seconds
    config.react.server_renderer = React::ServerRendering::BundleRenderer
    config.react.server_renderer_options = {
      files: ["server_rendering.js"],       # files to load for prerendering
      replay_console: true,                 # if true, console.* will be replayed client-side
    # Changing files matching these dirs/exts will cause the server renderer to reload:
    config.react.server_renderer_extensions = ["jsx", "js"]
    config.react.server_renderer_directories = ["/app/assets/javascripts", "/app/javascript/"]

JavaScript State

Some of ExecJS's backends are stateful (eg, mini_racer, therubyracer). This means that any side-effects of a prerender will affect later renders with that renderer.

To manage state, you have a couple options:

  • Make a custom renderer with #before_render / #after_render hooks as described below
  • Use per_request_react_rails_prerenderer to manage state for a whole controller action.

To check out a renderer for the duration of a controller action, call the per_request_react_rails_prerenderer helper in the controller class:

class PagesController < ApplicationController
  # Use the same React server renderer for the entire request:

Then, you can access the ExecJS context directly with react_rails_prerenderer.context:

def show
  react_rails_prerenderer           # => #<React::ServerRendering::BundleRenderer>
  react_rails_prerenderer.context   # => #<ExecJS::Context>

  # Execute arbitrary JavaScript code
  # `self` is the global context
  render :show

react_rails_prerenderer may also be accessed in before- or after-actions.

Custom Server Renderer

react-rails depends on a renderer class for rendering components on the server. You can provide a custom renderer class to config.react.server_renderer. The class must implement:

  • #initialize(options={}), which accepts the hash from config.react.server_renderer_options
  • #render(component_name, props, prerender_options) to return a string of HTML

react-rails provides two renderer classes: React::ServerRendering::ExecJSRenderer and React::ServerRendering::BundleRenderer.

ExecJSRenderer offers two other points for extension:

  • #before_render(component_name, props, prerender_options) to return a string of JavaScript to execute before calling React.render
  • #after_render(component_name, props, prerender_options) to return a string of JavaScript to execute after calling React.render

Any subclass of ExecJSRenderer may use those hooks (for example, BundleRenderer uses them to handle console.* on the server).

Controller Actions

Components can also be server-rendered directly from a controller action with the custom component renderer. For example:

class TodoController < ApplicationController
  def index
    @todos = Todo.all
    render component: 'TodoList', props: { todos: @todos }, tag: 'span', class: 'todo'

You can also provide the "usual" render arguments: content_type, layout, location and status. By default, your current layout will be used and the component, rather than a view, will be rendered in place of yield. Custom data-* attributes can be passed like data: {remote: true}.

Prerendering is set to true by default, but can be turned off with prerender: false.

Component Generator

You can generate a new component file with:

rails g react:component ComponentName prop1:type prop2:type ...

For example,

rails g react:component Post title:string published:bool published_by:instanceOf{Person}

would generate:

var Post = createReactClass({
  propTypes: {
    title: PropTypes.string,
    published: PropTypes.bool,
    publishedBy: PropTypes.instanceOf(Person)

  render: function() {
    return (
        Title: {this.props.title}
        Published: {this.props.published}
        Published By: {this.props.publishedBy}

The generator also accepts options:

  • --es6: use class ComponentName extends React.Component
  • --coffee: use CoffeeScript

Accepted PropTypes are:

  • Plain types: any, array, bool, element, func, number, object, node, shape, string
  • instanceOf takes an optional class name in the form of instanceOf{className}.
  • oneOf behaves like an enum, and takes an optional list of strings in the form of 'name:oneOf{one,two,three}'.
  • oneOfType takes an optional list of react and custom types in the form of 'model:oneOfType{string,number,OtherType}'.

Note that the arguments for oneOf and oneOfType must be enclosed in single quotes to prevent your terminal from expanding them into an argument list.

Use with JBuilder

If you use Jbuilder to pass a JSON string to react_component, make sure your JSON is a stringified hash, not an array. This is not the Rails default -- you should add the root node yourself. For example:

# BAD: returns a stringified array
json.array!(@messages) do |message|
  json.extract! message, :id, :name
  json.url message_url(message, format: :json)

# GOOD: returns a stringified hash
json.messages(@messages) do |message|
  json.extract! message, :id, :name
  json.url message_url(message, format: :json)

Camelize Props

You can configure camelize_props option:

MyApp::Application.configure do
  config.react.camelize_props = true # default false

Now, Ruby hashes given to react_component(...) as props will have their keys transformed from underscore- to camel-case, for example:

{ all_todos: @todos, current_status: @status }
# becomes:
{ "allTodos" => @todos, "currentStatus" => @status }

You can also specify this option in react_component:

<%= react_component('HelloMessage', {name: 'John'}, {camelize_props: true}) %>


2.3 to 2.4

Keep your react_ujs up to date, yarn upgrade

React-Rails 2.4.x uses React 16+ which no longer has React Addons. Therefore the pre-bundled version of react no longer has an addons version, if you need addons still, there is the 2.3.1+ version of the gem that still has addons.

If you need to make changes in your components for the prebundled react, see the migration docs here:

For the vast majority of cases this will get you most of the migration:

  • global find+replace React.Prop -> Prop
  • add import PropTypes from 'prop-types' (Webpacker only)
  • re-run bundle exec rails webpacker:install:react to update npm packages (Webpacker only)

Common Errors

During installation

  1. While using installers.(rails webpacker:install:react && rails webpacker:install) Error:
public/packs/manifest.json. Possible causes:
1. You want to set webpacker.yml value of compile to true for your environment
   unless you are using the `webpack -w` or the webpack-dev-server.
2. webpack has not yet re-run to reflect updates.
3. You have misconfigured Webpacker's config/webpacker.yml file.
4. Your webpack configuration is not creating a manifest.
yarn: error: no such option: --dev
ERROR: [Errno 2] No such file or directory: 'add'

Fix: Try updating yarn package.

sudo apt remove cmdtest
sudo apt remove yarn
curl -sS | sudo apt-key add -
echo "deb stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
sudo apt-get update && sudo apt-get install yarn

yarn install

Undefined Set

ExecJS::ProgramError (identifier 'Set' undefined):


If you see any variation of this issue, see Using TheRubyRacer

Using TheRubyRacer

TheRubyRacer hasn't updated LibV8 (The library that powers Node.js) from v3 in 2 years, any new features are unlikely to work.

LibV8 itself is already beyond version 7 therefore many serverside issues are caused by old JS engines and fixed by using an up to date one such as MiniRacer or TheRubyRhino on JRuby.


Hot Module Replacement is possible with this gem as it does just pass through to Webpacker. Please open an issue to let us know tips and tricks for it to add to the wiki.

Sample repo that shows HMR working with react-rails:

One caveat is that currently you cannot Server-Side Render along with HMR.

Related Projects


???? Thanks for taking the time to contribute! ????

With 5 Million+ downloads of the react-rails Gem and another 2 Million+ downloads of react_ujs on NPM, you're helping the biggest React + Rails community!

By contributing to React-Rails, you agree to abide by the code of conduct.

You can always help by submitting patches or triaging issues, even offering reproduction steps to issues is incredibly helpful!

Please see our Contribution guide for more info.

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