README.html

<script type="text/babel">
// React

// Server start : npx browser-sync start --server --files "./*.html" --no-open --no-notify --directory

// — Writing react similar to javascript

const element = React.createElement(‘div’, { children: "Hello World"})
ReactDOM.render(element, rootElement)


// Writing jsx code -> writing react similar to html tags and javascript

const element = <div className="container">Hello World</div>

// This jsx code is compiled using Babel - a javascript compiler  https://babeljs.io/
// Learn how it compiles jsx

// React.Fragment is used to add more tags side by side, without using div
ReactDOM.render(element, rootElement) //accepts only one element

<React.Fragment></React.Fragment>
// is written as

    const element = <> 
        <span/> <span/> 
    </>


// Writing functions with props as arguments

const Message = props => <h1 {...props}/>
    
const element = (
    <>
        <Message className='container'>BTS</Message>
        <Message>Army</Message>
    </>)
ReactDOM.render(element, rootElement)


// PropTypes
// PropTypes are defined as functions in prop-types.js.
// There is a production.min version to remove the warnings during compilation
// babel-plugin-transform-react-remove-prop-types removes the propTypes code from source code
// Remove React propTypes from the production build, as they are only used in development.

// Interpolation
// One can write html, javascript, jsx, css in one file
// Going back and forth javascript and jsx

// Re-render
// React will do a diff between the dom elements before and after the render.
// Then it will surgically render only the changed element, without rendering the whole dom element.
// This makes it very efficient than usual javascript rendering.

// Styling
// style={{backGroundColor: "blue"}}
// Remember to separate the components incase of overriding

// EventHandlers


function App() {
    function handleClick() {
    setState({eventCount: state.eventCount + 1})
    }

    function handleChange(event) {
    setState({username: event.target.value})
    }

    return (
    <div>
        <p>There have been {state.eventCount} events.</p>
        <p>
            <button onClick={handleClick}>Click Me</button>
        </p>
        <p>You typed: {state.username}</p>
        <p>
            <input onChange={handleChange} />
        </p>
    </div>
    )
}

function setState(newState) {
    Object.assign(state, newState)
    renderApp()
}

function renderApp() {
    ReactDOM.render(<App />, document.getElementById('root'))
}

renderApp()



// Synthetic Event Vs Native Event
// React does performance optimization to the native events thats is what Synthetic Event is.

e.preventDefault();

// Setting state
// React keeps track of the states. React.useState can be called as many times 
const [name, setName] = React.useState('')
const [name2, setName2] = React.useState('')

const handleChange = event => setName(event.target.value)
const handleChange2 = event => setName2(event.target.value)


// Local storage
// LocalStorage retains the value even after refresh
const [name, setName] = React.useState(
    window.localStorage.getItem('name') || '',
)
React.useEffect(() => {
    window.localStorage.setItem('name', name)
})
const handleChange = event => setName(event.target.value)

// Lazy initialization
// The above code useState hook access localStorage every time as one types (onchange).
// This should be avoided as follows by converting into a function.
// The function call will happen only if its absolutely needed.
// This is called as lazy initialization

const [name, setName] = React.useState(() =>
    window.localStorage.getItem('name') || '',
)

// Effect dependency
// The useEffect hook is also called everytime rendering happens.
// The useEffect should be called only when name is changed
// To avoid that, the dependent variable to call the useEffect is passed as follows.
// The dependency array will keep state of the world is sync-ed with the state of the application.
// eslint-plugin-react-hooks is provided to enforce this and it automatically updates the state.
React.useEffect(() => {
    window.localStorage.setItem('name', name)
}, [name])

// custom hooks - by convention starts with 'use'.
// The localStorage dependent codes are put in a method.
// 

const useLocalStorage = (key, defaultValues = {}) => {
    const [state, setState] = React.useState(() =>
        window.localStorage.getItem(key) || defaultValues,
    )
    React.useEffect((key, state) => {
        window.localStorage.setItem(key, state)
    }, [key, state])

    return [state, setState]
}
const handleChange = event => setName(event.target.value)

// useEffect is used to obtain any values after the rendering has happened


// hook workflow
// https://github.com/donavon/hook-flow
</script>

// Forms
<form onSubmit={submitHandler}>
</form>

<script>
const submitHandler = (event) => {
    event.preventDefault()
    alert(`Hi ${event.target.userName.value} `)
}

// Error Boundary
// Error boundaries are React components that catch JavaScript errors
// anywhere in their child component tree, log those errors, and 
// display a fallback UI instead of the component tree that crashed.

// Error boundaries catch errors during rendering, in lifecycle methods, 
// and in constructors of the whole tree below them.
// Handles the errors in react call stack, but not the async, event handlers and server side rendering
import {ErrorBoundary} from 'react-error-boundary'
const ErrorBoundary = ReactErrorBoundary.ErrorBoundary

// Using Kep prop to render lists/ array
// React will do a diff between the dom elements before and after the render.
// If key prop is not used, the React cannot keep track of the items properly if the array was modified

</script>