react 01

一，react 特点

1. 声明式设计

2.虚拟dom

3.jsx

4.组件化，模块化

5.单向数据流

二，脚手架 Create React App

npm i -g create-react-app

create-react-app --version

create-react-app 项目名称

三，渲染元素

let jsx = <h1>这是jsx 写法</h1>
ReactDom.render(jsx,document.getElementById('root'))

import React from 'react';
import ReactDOM from 'react-dom';

// 显示当前时间

// 写法一
function clock() {
  // let time = new Date().toLocaleDateString()   //2020/4/20
  let time = new Date().toLocaleTimeString()  //下午4:12:01
  ReactDOM.render(
  <h1>{time}</h1>,
    document.getElementById('root')
  );
}
clock()


// 写法二函数式组件
function Clock(props) {
  return (
  <h1>{props.time}</h1>
  )
}
function run() {
  ReactDOM.render(
    <Clock time={ new Date().toLocaleTimeString() }></Clock>,
    document.getElementById('root')
  )
}
setInterval(()=>{
  run()
},1000)

 四，jsx

　　1.有根节点

　　2.普通html 小写 ，大写会默认成组件

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
// import App from './App';
import * as serviceWorker from './serviceWorker';

let a = "hello";
let b = 12;
let c = "1";
let d = true;

ReactDOM.render(
  <div>
    <p>{a + 'world'}</p> 
    <p>{b + 'world'}</p>
    <p>{c === '1' ? <button>1</button> : 2}</p>
    <p>{d}</p>
    <p>{d.toString()}</p>

  </div>,
  document.getElementById('root')
);

 五，css