React is a component based front end librry, all parts of web application are divided into small components.

components is a small piece of the user interface. Every react.js application is a tree of components, components allow us to split the UI into independent reusable parts

Why react can allow all page loaded in a html。

React plays a major role in the most popular programming stack. Like MERN(a poppular javascript stack)

While we browsing the page, react updates what we see on a document object model(DOM), But we don't update the DOM directly like in html. React use a virtual dom.It's a lightweight representation of dom. Updating the virtual dom is much faster than updating the real dom.Thus react only update the page where the change was made instead of the entire page

functional components and class-based components

• class based components(This is already the history of react.js)

import React,{components} from 'React'
class Example extends Components{
    render ()
}

• Functional components (we build a function if we are using funcitonal components)

import React from 'react' //This is no longer needed
const Example=() =>{ //This is a function
    return <div>This is a JSX object</div>
}

While talking about Functional components. The object the functional components return is a JSX object. Jsx is used in react to describle what the user interface should look like? It comes with the full power of javascript , jsx produces react elements, it form the core of react syntax.

1. package json. You can see all dependencies or packages that your application is currently installed. The core react.js package are react, react-dom,react-scripts.
2. The other npm packages that you installed latter on will be added in package.json as well. The code of those package will be in node_modules.
3. we do most of our development work in src folder . In public folder, we can only see one index.html. There is only a single div with the id of root.All of our components are going to get injected inside of this div

// index.html
<div id='root'></div>

4. src/index.js ----> This is the starting point of every react.js application. We have react DOM in index.js. And we only call it once in our entire react application, no m atter how big it is. React dom is used to render our components and our entire application into the real dom.More specifically, into a div with an id of root.

** This is how the react dom in index.js looks like.**

// index.js
import ReactDOM from 'react-dom/client';
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

const Person=(props)=> {
  return (
    <>
      <h1>Name: {props.name}</h1>
      <h2>Last Name: {props.ln}</h2>
      <h2>Age :{props.age}</h2>
    </>
)
}
 // 后面的App functional component
      <div className='App'>
        <Person
          name={'who is your daddy'}
          ln={'Yune'}
        />
      </div>

state in react is a plain javascript object used by react to represent a piece of information, about the components current situation, it is completey managed by the component itself.

  const [counter, setCounter] = useState(0) // 0 is the default value of count

• The first part in the square brackets pair is going to be the name of the state. The second part is gping to be a setter function and we can call it setstatename.
• A good rule of thumb is that you call the second varibale, the same as you call the first one, but add 'set' in front, because it is the setter function for the first variable.

useEffect(()=>{
counter=100  //counter is a state, this is going to be an error
})

• one important rule of react is that never modify state manually, never mutate the state. counter is not just a normal variable , it is a part of the react state, and a react state can only be changed using its own setter function

Whenever you call a function and it starts with use like const [counter, setCounter] = useState(0), we call that a hook.

an event is an action that can be triggered as a result of the user action or some kind of a system generated event. E.X. a mouse clikc or a button press ia an event. How we handle evnets in react?

<button onClick={()=>{}}></button>

you can call some code whenever something happens in your react application by simple providing that variable here, and whenever that variable changed you can call the code inside of the use effect.

  useEffect(() => {
    alert("you have change the counter to "+counter);
  }, [counter]);