React Quick Reference

• Components
• Components
• Stateless Components
• Functional Components
• Pure Components
• Properties
• Props
• States
• Children
• Nesting
• Transferring props
• Portals
• Hydration
• PropTypes
• Basic types
• Required Types
• Elements
• Enumerables
• Custom validation
• Arrays and Objects
• Lifecycle
• Mounting
• Updating
• Errors
• Hooks
• State Hook
• Effect Hook
• References
• DOM Events
• JSX patterns
• Style shorthand
• Conditionals
• Lists
• Short-Circuit Evaluation
• Fragments and Arrays

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='Alex' />, el)

Stateless Components

// Stateless React Component
const Headline = () => {
return <h1>React Quick Reference</h1>
}
// Component that receives props
const Greetings = (props) => {
return <p>You will love it {props.name}.</p>
}
// Component must only return ONE element (eg. DIV)
const Intro = () => {
return (
<div>
<Headline />
<p>Welcome to the React world!</p>
<Greetings name="Alex" />
</div>
)
}
ReactDOM.render(
<Intro />,
document.getElementById('root')
);

Properties

<Video fullscreen={true} autoplay={false} />
render () {
this.props.fullscreen;
// Use `this.props` to access properties passed to the component.
const { fullscreen, autoplay } = this.props;
}

States

constructor(props) {
super(props)
this.state = { username: undefined }
}
this.setState({ username: 'Alex' })
render () {
this.state.username;
// Use states `this.state` to manage dynamic data.
const { username } = this.state
}

Children

<AlertBox>
<h1>You have pending notifications</h1>
</AlertBox>
class AlertBox extends Component {
render () {
return (
// Children are passed as the children property.
<div className='alert-box'>
{this.props.children}
</div>
)
}
}

Nesting

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>
)
}
}

Functional components

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

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

Pure Components

Performance-optimized version of React.Component.

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

Mounting

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

constructor (props)	      # Before rendering
componentWillMount()	      # Avoid using it
render()	              # Render
componentDidMount()	      # After rendering (DOM available)
componentWillUnmount()	      # Before DOM removal
componentDidCatch()	      # Catch errors (16+)

Updating

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

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

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>
);
}

Effect Hook

The useEffect() Hook can be used as componentDidMount(), componentDidUpdate(), and componentWillUnmount() combined.

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>
);
}

References

Allows access to DOM nodes.

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

DOM Events

Pass functions to attributes like onChange().

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 })
}
}

Transferring props

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

<VideoPlayer src="video.mp4" />
class VideoPlayer extends Component {
render () {
return <VideoEmbed {...this.props} />
}
}

JSX patterns

Style shorthand

Inline styles

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

Conditionals

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

Lists

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

Short-circuit evaluation

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

Fragments and Arrays

// 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>
)
}

Errors

Catch errors via componentDidCatch().

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

Portals

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

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

Hydration

Use ReactDOM.hydrate() instead of using ReactDOM.render() if you are rendering over the output of ReactDOMServer.

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

PropTypes

Typechecking with PropTypes

import PropTypes from 'prop-types';

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'
])
}

Custom validation

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

Arrays and Objects

Use .arrayOf(), .objectOf(), .instanceOf(), .shape().

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
})
}