Answers for "react use state"

react useEffect

import React, { useEffect } from 'react';

export const App: React.FC = () => {
  
  useEffect(() => {
        
  }, [/*Here can enter some value to call again the content inside useEffect*/])
  
  return (
    <div>Use Effect!</div>
  );
}

state with react functions

class Example extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    };
  }

  render() {
    return (
      <div>
        <p>You clicked {this.state.count} times</p>
        <button onClick={() => this.setState({ count: this.state.count + 1 })}>
          Click me
        </button>
      </div>
    );
  }
}

react useEffect

import React, { useEffect, useState } from 'react';
import ReactDOM from 'react-dom';

function LifecycleDemo() {
  // It takes a function
  useEffect(() => {
    // This gets called after every render, by default
    // (the first one, and every one after that)
    console.log('render!');

    // If you want to implement componentWillUnmount,
    // return a function from here, and React will call
    // it prior to unmounting.
    return () => console.log('unmounting...');
  }, [ // dependencies to watch = leave blank to run once or you will get a stack overflow  ]);

  return "I'm a lifecycle demo";
}

function App() {
  // Set up a piece of state, just so that we have
  // a way to trigger a re-render.
  const [random, setRandom] = useState(Math.random());

  // Set up another piece of state to keep track of
  // whether the LifecycleDemo is shown or hidden
  const [mounted, setMounted] = useState(true);

  // This function will change the random number,
  // and trigger a re-render (in the console,
  // you'll see a "render!" from LifecycleDemo)
  const reRender = () => setRandom(Math.random());

  // This function will unmount and re-mount the
  // LifecycleDemo, so you can see its cleanup function
  // being called.
  const toggle = () => setMounted(!mounted);

  return (
    <>
      <button onClick={reRender}>Re-render</button>
      <button onClick={toggle}>Show/Hide LifecycleDemo</button>
      {mounted && <LifecycleDemo/>}
    </>
  );
}

ReactDOM.render(<App/>, document.querySelector('#root'));

hooks in react

import React, { useState, useEffect } from "react";
export default props => {
  console.log("componentWillMount");
  console.log("componentWillReceiveProps", props);
  const [x, setX] = useState(0);
  const [y, setY] = useState(0);
  const [moveCount, setMoveCount] = useState(0);
  const [cross, setCross] = useState(0);
  const mouseMoveHandler = event => {
    setX(event.clientX);
    setY(event.clientY);
  };
  useEffect(() => {
    console.log("componentDidMount");
    document.addEventListener("mousemove", mouseMoveHandler);
    return () => {
      console.log("componentWillUnmount");
      document.removeEventListener("mousemove", mouseMoveHandler);
    };
  }, []); // empty-array means don't watch for any updates
  useEffect(
    () => {
      // if (componentDidUpdate & (x or y changed))
      setMoveCount(moveCount + 1);
    },
    [x, y]
  );
useEffect(() => {
    // if componentDidUpdate or componentDidMount
    if (x === y) {
      setCross(x);
    }
  });
  return (
    <div>
      <p style={{ color: props.color }}>
        Your mouse is at {x}, {y} position.
      </p>
      <p>Your mouse has moved {moveCount} times</p>
      <p>
        X and Y positions were last equal at {cross}, {cross}
      </p>
    </div>
  );
};

use state

var fruitStateVariable = useState('banana'); // Returns a pair
  var fruit = fruitStateVariable[0]; // First item in a pair
  var setFruit = fruitStateVariable[1]; // Second item in a pair

how to use hooks react

const App = () => {
const [students , setStudents] = useState([]);
  
  return (
// put in the jsx code here
  )
}