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

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

/ http://localhost:3000

• 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(, document.getElementById('root'))

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

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

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

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>
)

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" />
)

// 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 />
	    </>
	)
}

(src/App.css)

h1 {
    color: red;
}

(src/App.js)
Import the CSS file

import './App.css'
function App(){
    return <h1>Hello World</h1>
}

• 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>
	</>
 )
}

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>
}

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>
     )
}

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

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

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>
    )
}

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

function App(){
    return people.map(person => )
}

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

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

or inline...

function App(){
    return (
        <>
            <h1>Welcome to my app</h1> <br>
            <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> <br>
            <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> <br>
            <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}/>
        }
    )
}

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;

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>
    </>
};

export default DisplayTitle;

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>
        </>
    )
}

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 need to be place inside useEffect function

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

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.

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)

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 {isLoggedIn ? 'currently' : 'not'} 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>}

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 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>
)
};