Javascript .push( ) 没有更新变量

App将在每次更改状态时重新渲染，因此const datas=[];将被初始化为一个新array.

我可以预见的另一个问题是，在同一个函数上调用setData(day)，然后调用datas.push(data)，因为状态的更改是异步的，所以该函数不会应用于最新的数据

const handlePress=(day)=>{
   setData(day); //`day` is set here but not applied immediately
   setName(`${events[rand]}`)
   datas.push(data); //data is the previous data, not `day`
   console.log(datas);
 }

完整的修改可以是

const datas=[]; //move your `datas` to the global scope

export default function App() {
 const [data,setData]= useState([]);
 const [activity, setActivity]= useState([])
 const [name, setName] = useState("")

 
 useEffect(()=>{
  // handlePress()
 }, [setData, setName])

 const rand= Math.floor(Math.random(1,5)*4)+1
 const events=["Event A","Event B","Event C","Event D","Event E"]

 const handlePress=(day)=>{
   setData(day); //`data` will be changed later
   setName(`${events[rand]}`)
   datas.push(day); //push `day` directly instead of data
   console.log(datas);
 }
  return (
    <div className="App">
      <Calendar 
        onDayPress={day => {
          {handlePress(day)}
        }}
        
      />
      
    </div>
  );
}

您也不应该将useEffect与setter(setData和setName)一起使用.这些二传者与副作用无关.您应该在useEffect中将状态值作为依赖项传递.

//whenever `name` or `data` change, `useEffect` will be triggered
useEffect(()=>{
  //TODO: Do your logic here
 }, [data, name])