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.bodyReactDOM.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.statethis.propsThese methods and properties are available for Component instances.
See: Component API
#Lifecycle
Mounting
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
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';} Hooks API Reference
Also see: Hooks FAQ
Basic Hooks
useState(initialState) | |
useEffect(() => { … }) | |
useContext(MyContext) | value returned from React.createContext |
Full details: Basic Hooks
Additional Hooks
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.
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> )} Returning strings
render() { return 'Look ma, no spans!';} You can return just a string.
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
string | |
number | |
func | Function |
bool | True or false |
Enum
oneOf(any) | Enum types |
oneOfType(type array) | Union |
Array
array | |
arrayOf(…) |
Object
object | |
objectOf(…) | Object with values of a certain type |
instanceOf(…) | Instance of a class |
shape(…) |
Elements
element | React element |
node | DOM node |
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.