React Cheat sheet (Updated June 2021)

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That's why I decided to do this memory aid and given that my memory is not that good I thought why not make a monstrous memory aid with all the concepts I know about React.

So I can read it from time to time and thereby strengthen my knowledge of React.

If you have ideas or recommendation do not hesitate and do so in the comments section.

React Cheat Sheet

Create a React App

// Create a new app
npx create-react-app my-app-name

// Run the created app
cd my-app-name
yarn start

// http://localhost:3000

First React functional Component

No need to import React from 'react' (since React 17)
Must have uppercase first letter
Must return JSX

(src/App.js)

// React component
function App(){
  return <h1>Hello World</h1>
} 

export default App;

How this component get render to the browser? The main project file is src/index.js and in that file there are instruction to render the component

ReactDOM.render(<App />, document.getElementById('root'))

The App component will then be rendered inside public/index.html 'root' div

Import Component

Component will be created in separate files. Each component need to be export and then import

function Greeting(){
    return <h1>Hello World</h2>
}
export default Greeting

This component can then be import

import Greeting from './Gretting'

function App(){
    return <Greeting />
}

or name export...

export function Greeting(){
    return <h1>Hello World</h2>
}

This component can then be import

import {Greeting} from './Gretting'

BEM Naming Convention

return (
<div className="app">
  <h1 className="app_title">Welcome to my application: {appTitle}</h1>
  <div className="product">
    <h1 className="product__name--large">Product name: {product.name}</h1>
<h1 className="product__name--small">Nick name: {product.nickName}</h1>
    <p className="product__description">Product description: {product.description}
  </div>
<div>
)

JSX Rules

Return a single element (only one parent element)

// not valid
return <h1>Hello world</h1><h2>Hi!</h2>

// valid with fragment. 
return (
    <>
        <h1>Hello World</h1>
        <h2>Hi!</h2>
    </>
)
// Noted the parenthesis for multi-line formatting

Use className instead of class
Also all attribute name need to be camelCase

// not valid
return (
    <div class="title">
        Hello World
    </div>
)

// valid
return (
    <div className="title">
    </div>
)

Close every element

return (
    <img src="http:example.com/image.jpg" />
    <input type="text" name="first_name" />
)

Nested Components

// Arrow function shorthand component
const Person = () => <h1>Mike Taylor</h1>

// Arrow function component
const Message = () => {
    return <h1>Hello</h1>
}

// Function component
function HelloWorld(){
  return (
      <>
          <Message />
          <Person />
      </>
  )
}

Component CSS

(src/App.css)

h1 {
    color: red;
}

(src/App.js)
Import the CSS file

import './App.css'

function App(){
  return <h1>Hello World</h1>
}

Inline CSS

function App(){
  return <h1 style={{ color: 'red' }}>Hello World</h1>
}

Javascript in JSX

Enclose between {}
Must be an expression (return a value)

function App(){
    const name = 'Mike'
    return (
      <>
          <h1>Hello {name}</h1>
          <p>{name === 'Mike' ? '(admin)': '(user)'}</p>
      </>
    )
}

Component Properties (Props)

function App()
    return <Person name='Mike' age={29} />
} 

const Person = (props) => {
    return <h1>Name: {props.name}, Age: {props.age}</h1>
}

// or props object deconstructing
const Person = ({name, age}) => {
    return <h1>Name: {name} Age: {age}</h1>
}

Children Props (slot)

function App()
    return (
        <Person name='Mike' age={29}>
            Hi, this is a welcome message
        </Person>
    )
} 

const Person = (props) => {
    return (
        <h1>Name: {props.name}, Age: {props.age}</h1>
        <p>{props.children}</p>
    )
}

// or props object deconstructing
const Person = ({name, age, children}) => {
    return (
        <h1>Name: {name} Age: {age}</h1>
        <p>{children}</p>
    )
}

Default Props value

const Person = ({name, age, children}) => {
    return (
        <h1>Name: {name} Age: {age}</h1>
        <p>{children}</p>
    )
}

Person.defaultProps = {
    name: 'No name',
    age: 0,
}

List

const people = [
  {id: 1, name: 'Mike', age: 29},
  {id: 2, name: 'Peter', age: 24},
  {id: 3, name: 'John', age: 39},
]
function App(){
    return (
        people.map(person => {
            return <Person name={person.name} age={person.age}/>
        })
    )
} 

const Person = (props) => {
  return (
      <h1>Name: {props.name}, Age: {props.age}</h1>
  )
}

List with key (for React internal reference)

function App(){
    return (
        people.map(person => {
            return <Person key={person.id} name={person.name} age={person.age}/>
        })
     )
}

Props object destructuring

function App(){
  return people.map(person => <Person key={person.id} {...person} />)
}

const Person = ({name, age}) => {
  return (
      <h1>Name: {name}, Age: {age}</h1>
  )
}

Click Event

const clickHandler = () => alert('Hello World')
function App(){
    return (
        <>
            <h1>Welcome to my app</h1>
            <button onClick={clickHandler}>Say Hi</button>
        </> 
    )
}

or inline...

function App(){
    return (
        <>
            <h1>Welcome to my app</h1>
            <button onClick={ () => alert('Hello World') }>Say Hi</button>
        </>
     )
}

To pass arguments we need to use arrow function

const clickHandler = (message) => alert(message)
function App(){
    return (
        <>
            <h1>Welcome to my app</h1>
            <button onClick={() => clickHandler('Hello World')}>Say Hi</button>
        </> 
    )
}

e for event arguments

const clickHandler = (e) => console.log(e.target)
function App(){
    return (
        <>
            <h1>Welcome to my app</h1>
            <button onClick={clickHandler}>Say Hi</button>
        </> 
    )
}

Pass event from child to parent

function Todo({item, onDelete}) {
    return (
      <div>
        {item}
        <button onClick={() => onDelete(item)} 
      </div>
    )
}

function Todos() {
  const handleDelete = (todo) => {
    const newTodos = todos.filter(item => item !== todo)
    setTodos(() => newTodos)
  }

  return (
    {todos.map(todo => (
       <Todo item={todo} onDelete={handleDelete}/>
    }
  )
}

useState Hook

The purpose of useState is to handle reactive data. any data that changes in the application is called state. And when the state changes, you want react to update the UI.

Hook always start with 'use' prefix
Must be invoke only in a React component/function
Must be call at top level of a functional component
Declaration cannot be call conditionally
useState return an array of 2: [state value, set state function]

import React, {useState} from 'react';

const DisplayTitle = () => {
  const [title, setTitle] = useState('This is the Title')
  const handleClick = () => setTitle('New Title')
  return <>
    <h2>{title}</h2>
    <button type="button" className="btn" onClick={handleClick}>
      Change Title
    </button>
  </>
};

export default DisplayTitle;

useState with object

const DisplayTitle = () => {
  const [person, setPerson] = useState({name: 'Mike', age: 29})
  const handleClick = () => setPerson({...person, age: 35})
  return <>
    <h2>{title}</h2>
    <button type="button" className="btn" onClick={handleClick}>
      Change Age
    </button>
  </>
};

setState functional form

function Counter() {
  const [count, setCount] = useState(0)
  // Use a function to set State
  const increase = () => setCount(() => count + 1)
  return (
    <>
      <h1>Counter</h1>
      <p>{count}</p>
      <button onClick={increase} className='btn'> + </button>
      <button onClick={() => setCount(() => count - 1)} className='btn'> - </button>
    </>
  )
}

useEffect

In React you may want to execute code after lifecycle events or side effects.

By default useEffect function is execute after every re-render. You can then execute code everytime component update.

import React, { useEffect } from 'react';

function IncreaseValue() {
    const [value, setValue] = useState(0)
    useEffect(() => {
        document.title = `New value: ${value}` 
    })
    return <button onClick={() => setValue(value + 1)}>Increase</button>
}

Conditional useEffect

Conditional need to be place inside useEffect function

useEffect(() => {
    if (value > 0) {
        document.title = `New value: ${value}` 
    }
})

useEffect Dependency List

What if you want to execute code only on first render or only when a particular state change? You can use the useEffect function and send an array of dependencies as parameter.

useEffect will run only if state is in the Dependency List.
If the list is empty [] the useEffect will only run on initial render.

useEffect(() => {
    document.title = `New value: ${value}` 
}, [])
// Noted the empty array. useEffect will then only run once on initial render

useEffect(() => {
    document.title = `New value: ${value}` 
}, [value])
// Will run each time 'value' state change.

useEffect cleanup function

What if you want to execute code each time the component unmount?

To execute code only when a component is unmount/destroy you need to add a 'return' statement to your useEffect function.

useEffect(() =>  { 
    const timer = window.setInterval(() => { 
        setCount(count => count + 1)
    }, 1000)
    return () => clearInterval(timer)
}, [])

The code 'clearInterval(timer)' will only be execute before component is remove from UI (unmount)

Conditional Rendering

function DisplayGreeting() {
    const [name, setName] = useState('Mike')
    if (name === 'Mike') {
        return <h1>Hello admin {name}</h1> 
    }
    return <h1>Hello user {name}</h1> 
}

Inline If-Else

return (
    <div>
      The user is <b>{isLoggedIn ? 'currently' : 'not'}</b> logged in.
    </div>
  );
}

Inline Logical && Operator.
Display only if first expression is truthy
truthy = Not : 0, "", null, undefined, and NaN

function DisplayUserInfo({active}) {
    return (
      <div>
        { active && <h1>User is active</h1>}
      </div>
    );
}

Multiple inline If

<span className={count === 0 && 'text-gray-500' || count > 0 && 'text-green-500' || count < 0 && 'text-red-500'}>{count}</span>

Form

const UserForm = () => {
  const [userName, setUserName] = useState('')
  const handleSubmit = (e) => {
    e.preventDefault()
    console.log(userName)
  }
return (
<>
    <form onSubmit={handleSubmit}>
      <input 
          value={userName} 
          onChange={(e) => setUserName(e.target.value)} 
          type="text" id="userName" 
          name="userName"
      />
       <button type="submit">Submit</button>
    </form>
</>
)
};

export default UserForm;

useRef

useRef is mostly use to target a DOM element. But it can also be use to keep/preserve a mutable value between each render. useRef does not trigger a re-render (like a useState).

const UseRefBasics = () => {
  const refContainer = useRef(null)
  const handleSubmit = (e) => {
    e.preventDefault()
    console.log(refContainer.current.value)
  }

  useEffect(() => {
    refContainer.current.focus()
  }, [])

  return (
    <div>
      <form className="form" onSubmit={handleSubmit}>
        <div>
          <input ref={refContainer} type="text" />
          <button type="submit">Submit</button>
        </div>
      </form>
    </div>
  )
};