React.js

React is a JavaScript library for building user interfaces. This guide targets React v15 to v16.

React.Component
render()
componentDidMount()
props/state

Components

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

class Hello extends React.Component {
  render () {
    return <div className='message-box'>
      Hello {this.props.name}
    </div>
  }
}

const el = document.body
ReactDOM.render(<Hello name='John' />, el)

Use the React.js jsfiddle to start hacking. (or the unofficial jsbin)

Import multiple exports

import React, {Component} from 'react'
import ReactDOM from 'react-dom'

class Hello extends Component {
  ...
}

Properties

<Video fullscreen={true} autoplay={false} />

render () {
  this.props.fullscreen
  const { fullscreen, autoplay } = this.props
  ···
}

Use this.props to access properties passed to the component.

See: Properties

States

constructor(props) {
  super(props)
  this.state = { username: undefined }
}

this.setState({ username: 'rstacruz' })

render () {
  this.state.username
  const { username } = this.state
  ···
}

Use states (this.state) to manage dynamic data.

With Babel you can use proposal-class-fields and get rid of constructor

class Hello extends Component {
  state = { username: undefined };
  ...
}

See: States

Nesting

class Info extends Component {
  render () {
    const { avatar, username } = this.props
    return <div>
      <UserAvatar src={avatar} />
      <UserProfile username={username} />
    </div>
  }
}

As of React v16.2.0, fragments can be used to return multiple children without adding extra wrapping nodes to the DOM.

import React, {
  Component,
  Fragment
} from 'react'
class Info extends Component {
  render () {
    const { avatar, username } = this.props
    return (
      <Fragment>
        <UserAvatar src={avatar} />
        <UserProfile username={username} />
      </Fragment>
    )
  }
}

Nest components to separate concerns.

See: Composing Components

Children

<AlertBox>
  <h1>You have pending notifications</h1>
</AlertBox>

class AlertBox extends Component {
  render () {
    return <div className='alert-box'>
      {this.props.children}
    </div>
  }
}

Children are passed as the children property.

Defaults

Setting default props

Hello.defaultProps = {
  color: 'blue'
}

See: defaultProps

Setting default state

class Hello extends Component {
  constructor (props) {
    super(props)
    this.state = { visible: true }
  }
}

Set the default state in the constructor().

And without constructor using Babel with proposal-class-fields.

class Hello extends Component {
    state = { visible: true }
  }
}

See: Setting the default state

Other components

Functional components

function MyComponent ({ name }) {
  return <div className='message-box'>
    Hello {name}
  </div>
}

Functional components have no state. Also, their props are passed as the first parameter to a function.

See: Function and Class Components

Pure components

import React, {PureComponent} from 'react'
class MessageBox extends PureComponent {
  ···
}

Performance-optimized version of React.Component. Doesn't rerender if props/state hasn't changed.

See: Pure components

Component API

this.forceUpdate()

this.setState({ ... })
this.setState(state => { ... })

this.state
this.props

These methods and properties are available for Component instances.

See: Component API

Lifecycle

Mounting

Method	Description
`constructor` (props)	Before rendering #
`componentWillMount()`	Don't use this #
`render()`	Render #
`componentDidMount()`	After rendering (DOM available) #
---	---
`componentWillUnmount()`	Before DOM removal #
---	---
`componentDidCatch()`	Catch errors (16+) #

Set initial the state on constructor(). Add DOM event handlers, timers (etc) on componentDidMount(), then remove them on componentWillUnmount().

Updating

Method	Description
`componentDidUpdate` (prevProps, prevState, snapshot)	Use `setState()` here, but remember to compare props
`shouldComponentUpdate` (newProps, newState)	Skips `render()` if returns false
`render()`	Render
`componentDidUpdate` (prevProps, prevState)	Operate on the DOM here

Called when parents change properties and .setState(). These are not called for initial renders.

See: Component specs

Hooks (New)

State Hook

import React, { useState } from 'react';
function Example() {
  // Declare a new state variable, which we'll call "count"
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

Hooks are a new addition in React 16.8.

See: Hooks at a Glance

Declaring multiple state variables

function ExampleWithManyStates() {
  // Declare multiple state variables!
  const [age, setAge] = useState(42);
  const [fruit, setFruit] = useState('banana');
  const [todos, setTodos] = useState([{ text: 'Learn Hooks' }]);
  // ...
}

Effect hook

import React, { useState, useEffect } from 'react';
function Example() {
  const [count, setCount] = useState(0);
  // Similar to componentDidMount and componentDidUpdate:
  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;
  }, [count]);
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

If you’re familiar with React class lifecycle methods, you can think of useEffect Hook as componentDidMount, componentDidUpdate, and componentWillUnmount combined.

By default, React runs the effects after every render — including the first render.

Building your own hooks

Define FriendStatus

import React, { useState, useEffect } from 'react';
function FriendStatus(props) {
  const [isOnline, setIsOnline] = useState(null);
  useEffect(() => {
    function handleStatusChange(status) {
      setIsOnline(status.isOnline);
    }
    ChatAPI.subscribeToFriendStatus(props.friend.id, handleStatusChange);
    return () => {
      ChatAPI.unsubscribeFromFriendStatus(props.friend.id, handleStatusChange);
    };
  }, [props.friend.id]);
  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

Effects may also optionally specify how to “clean up” after them by returning a function.

Use FriendStatus

function FriendStatus(props) {
  const isOnline = useFriendStatus(props.friend.id);
  if (isOnline === null) {
    return 'Loading...';
  }
  return isOnline ? 'Online' : 'Offline';
}

See: Building Your Own Hooks

Hooks API Reference

Also see: Hooks FAQ

Basic Hooks

Hook	Description
`useState`(initialState)
`useEffect`(() => { ... })
`useContext`(MyContext)	value returned from `React.createContext`

Full details: Basic Hooks

Additional Hooks

Hook	Description
`useReducer`(reducer, initialArg, init)
`useCallback`(() => { ... })
`useMemo`(() => { ... })
`useRef`(initialValue)
`useImperativeHandle`(ref, () => { ... })
`useLayoutEffect`	identical to `useEffect`, but it fires synchronously after all DOM mutations
`useDebugValue`(value)	display a label for custom hooks in React DevTools

Full details: Additional Hooks

DOM nodes

References

class MyComponent extends Component {
  render () {
    return <div>
      <input ref={el => this.input = el} />
    </div>
  }
  componentDidMount () {
    this.input.focus()
  }
}

Allows access to DOM nodes.

See: Refs and the DOM

DOM Events

class MyComponent extends Component {
  render () {
    <input type="text"
        value={this.state.value}
        onChange={event => this.onChange(event)} />
  }
  onChange (event) {
    this.setState({ value: event.target.value })
  }
}

Pass functions to attributes like onChange.

See: Events

Other features

Transferring props

<VideoPlayer src="video.mp4" />

class VideoPlayer extends Component {
  render () {
    return <VideoEmbed {...this.props} />
  }
}

Propagates src="..." down to the sub-component.

See Transferring props

Top-level API

React.createClass({ ... })
React.isValidElement(c)

ReactDOM.render(<Component />, domnode, [callback])
ReactDOM.unmountComponentAtNode(domnode)

ReactDOMServer.renderToString(<Component />)
ReactDOMServer.renderToStaticMarkup(<Component />)

There are more, but these are most common.

See: React top-level API

JSX patterns

Style shorthand

const style = { height: 10 }
return <div style={style}></div>

return <div style={{ margin: 0, padding: 0 }}></div>

See: Inline styles

Inner HTML

function markdownify() { return "<p>...</p>"; }
<div dangerouslySetInnerHTML={{__html: markdownify()}} />

See: Dangerously set innerHTML

Lists

class TodoList extends Component {
  render () {
    const { items } = this.props
    return <ul>
      {items.map(item =>
        <TodoItem item={item} key={item.key} />)}
    </ul>
  }
}

Always supply a key property.

Conditionals

<Fragment>
  {showMyComponent
    ? <MyComponent />
    : <OtherComponent />}
</Fragment>

Short-circuit evaluation

<Fragment>
  {showPopup && <Popup />}
  ...
</Fragment>

New features

Returning multiple elements

You can return multiple elements as arrays or fragments.

Arrays

render () {
  // Don't forget the keys!
  return [
    <li key="A">First item</li>,
    <li key="B">Second item</li>
  ]
}

Fragments

render () {
  // Fragments don't require keys!
  return (
    <Fragment>
      <li>First item</li>
      <li>Second item</li>
    </Fragment>
  )
}

See: Fragments and strings

Returning strings

render() {
  return 'Look ma, no spans!';
}

You can return just a string.

See: Fragments and strings

Errors

class MyComponent extends Component {
  ···
  componentDidCatch (error, info) {
    this.setState({ error })
  }
}

Catch errors via componentDidCatch. (React 16+)

See: Error handling in React 16

Portals

render () {
  return React.createPortal(
    this.props.children,
    document.getElementById('menu')
  )
}

This renders this.props.children into any location in the DOM.

See: Portals

Hydration

const el = document.getElementById('app')
ReactDOM.hydrate(<App />, el)

Use ReactDOM.hydrate instead of using ReactDOM.render if you're rendering over the output of ReactDOMServer.

See: Hydrate

Property validation

PropTypes

import PropTypes from 'prop-types'

See: Typechecking with PropTypes

| any | Anything |

Basic

Enum

Array

Object

Elements

Required

| (···).isRequired | Required |

Basic types

MyComponent.propTypes = {
  email:      PropTypes.string,
  seats:      PropTypes.number,
  callback:   PropTypes.func,
  isClosed:   PropTypes.bool,
  any:        PropTypes.any
}

Required types

MyCo.propTypes = {
  name:  PropTypes.string.isRequired
}

Elements

MyCo.propTypes = {
  // React element
  element: PropTypes.element,
  // num, string, element, or an array of those
  node: PropTypes.node
}

Enumerables (oneOf)

MyCo.propTypes = {
  direction: PropTypes.oneOf([
    'left', 'right'
  ])
}

Arrays and objects

MyCo.propTypes = {
  list: PropTypes.array,
  ages: PropTypes.arrayOf(PropTypes.number),
  user: PropTypes.object,
  user: PropTypes.objectOf(PropTypes.number),
  message: PropTypes.instanceOf(Message)
}

MyCo.propTypes = {
  user: PropTypes.shape({
    name: PropTypes.string,
    age:  PropTypes.number
  })
}

Use .array[Of], .object[Of], .instanceOf, .shape.

Custom validation

MyCo.propTypes = {
  customProp: (props, key, componentName) => {
    if (!/matchme/.test(props[key])) {
      return new Error('Validation failed!')
    }
  }
}

Also see

React website (reactjs.org)
React cheatsheet (reactcheatsheet.com)
Awesome React (github.com)
React v0.14 cheatsheet Legacy version

Edit this page on GitHub

Table of Contents

React.js

Components

Import multiple exports

Properties

States

Nesting

Children

Defaults

Setting default props

Setting default state

Other components

Functional components

Pure components

Component API

Lifecycle

Mounting

Updating

Hooks (New)

State Hook

Declaring multiple state variables

Effect hook

Building your own hooks

Define FriendStatus

Use FriendStatus

Hooks API Reference

Basic Hooks

Additional Hooks

DOM nodes

References

DOM Events

Other features

Transferring props

Top-level API

JSX patterns

Style shorthand

Inner HTML

Lists

Conditionals

Short-circuit evaluation

New features

Returning multiple elements

Arrays

Fragments

Returning strings

Errors

Portals

Hydration

Property validation

PropTypes

Basic

Enum

Array

Object

Elements

Required

Basic types

Required types

Elements

Enumerables (oneOf)

Arrays and objects

Custom validation

Also see

JavaScript

AngularJS