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注意setTimeout是异步执行的，因此需要用递归的办法实现

const TIME0_in = 800  //ms
const TIME_delay = 40 //ms
const opacityDelay = TIME_delay/TIME0_in
let that = this
let i = 0;
function changeOpacity(){ 
  if(i>=TIME0_in/TIME_delay){
    console.log("2")
    this.opacity = 1
    return ;
  }
  that.opacity = 1-opacityDelay*i
  setTimeout(()=>{
    changeOpacity()
  },TIME_delay)
  i++
  console.log("1")
}
changeOpacity()

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