import React, {useEffect, useState} from 'react'; function App () { const [n, setN] = useState ( 0 ) const onClick = () => { setN (i => i + 1 ) useEffect (()=>{ console .log (n) }, [] ) useEffect (()=>{ if (n> 0 ){ console .log ("n变化了") }, [n] ) return ( <div className="App"> <button onClick={onClick}>+ 1 </button> export default App; Cindy React.js