SOURCE

let itemHtml = document.querySelector('#divBox')
    let i = 0
    let timer = setInterval(()=>{
      itemHtml.style.width = i+'px'
      if(i===400){
        //width增加到目标值则清除定时器
        clearInterval(timer)
      }
      i++
    },10)

<div id='divBox'>菜逼淇狗，在线倒垃圾！！！</div>

#divBox {
      height: 50px;
      width: 0px;
      background-color: red;
    }