React.js

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

A cheatsheet by @rstacruz|Refreshed 10 months ago.Refresh|View source on Github

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Components

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 multiple exports

import React, {Component} from 'react'
import ReactDOM from 'react-dom'
class Hello extends Component {
  ...
}

Properties

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

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

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

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.

Setting default props

Setting default props

Hello.defaultProps = {
  color: 'blue'
}

See: defaultProps

Setting default state

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

Functional 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

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

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

Mounting

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

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

State Hook

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

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

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

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

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

References

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

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

Transferring props

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

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

Style shorthand

Style shorthand

const style = { height: 10 }
return <div style={style}></div>
return <div style={{ margin: 0, padding: 0 }}></div>

See: Inline styles

Inner HTML

Inner HTML

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

See: Dangerously set innerHTML

Lists

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

Conditionals

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

Short-circuit evaluation

Short-circuit evaluation

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

Returning multiple elements

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

Returning strings

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

You can return just a string.

See: Fragments and strings

Errors

Errors

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

Catch errors via componentDidCatch. (React 16+)

See: Error handling in React 16

Portals

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

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

PropTypes

PropTypes

import PropTypes from 'prop-types'

See: Typechecking with PropTypes

Key Description
any Anything

Basic

Key Description
string  
number  
func Function
bool True or false

Enum

Key Description
oneOf(any) Enum types
oneOfType(type array) Union

Array

Key Description
array  
arrayOf(…)  

Object

Key Description
object  
objectOf(…) Object with values of a certain type
instanceOf(…) Instance of a class
shape(…)  

Elements

Key Description
element React element
node DOM node

Required

Key Description
(···).isRequired Required

Basic types

Basic types

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

Required types

Required types

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

Elements

Elements

MyCo.propTypes = {
  // React element
  element: PropTypes.element,

  // num, string, element, or an array of those
  node: PropTypes.node
}

Enumerables (oneOf)

Enumerables (oneOf)

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

Arrays and objects

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

Custom validation

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

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