fade动画

const fade = (el, type = 'in') {
     el.style.opacity = (type === 'in' ? 0 : 1)
     let last = +new Date()
     const tick = () => {
         const opacityValue = (type === 'in' 
                             ? (new Date() - last) / 400
                             : -(new Date() - last) / 400)
         el.style.opacity = +el.style.opacity + opacityValue
         last = +new Date()
         if (type === 'in'
           ? (+el.style.opacity < 1)
           : (+el.style.opacity > 0)) {
             requestAnimationFrame(tick)
         }
     }
     tick()
 }