React Chinese network has written a class on the clock example https://react.docschina.org/docs/state-and-lifecycle.html ,
React now launched a new hook api, they use hook easily rewritten at this example:
import React, { useState, useEffect} from 'react'; import ReactDOM from 'react-dom'; function Clock(){ const [date,setDate]=useState(new Date()); useEffect(()=>{ function tick(){ setDate(new Date()); } const timerID = setInterval(tick,1000); return function clearTick(){ clearInterval(timerID); }; }); return( <div> <h1>Hello, world!</h1> <h2>It is {date.toLocaleTimeString()}.</h2> </div> ); } ReactDOM.render( <Clock/>, document.getElementById('root') );