<div class="totop" v-show="toTopShow" @click="toTop()">top</div>

.totop {
  width: 50px;
  height: 50px;
  line-height: 50px;
  border-radius: 25px;
  background-color: white;
  position: fixed;
  bottom: 75px;
  right: 10px;
  text-align: center;
}

 data() {
    return {
  toTopShow: false,
      srcoll: 0,
}
},

  watch: {
    srcoll() {
      if (this.srcoll > 400) {
        this.toTopShow = true;
      } else {
        this.toTopShow = false;
      }
    },
  },

 mounted() {
    window.addEventListener("scroll", this.srcollShow);
  },

 methods: {
    srcollShow() {
      this.srcoll =
        window.pageYOffset ||
        document.documentElement.scrollTop ||
        document.body.scrollTop;
    },
 
 
 toTop() {
      this.toTopSpeed = setInterval(() => {
        document.documentElement.scrollTop =
          document.documentElement.scrollTop - 20;
//通过改变数字实现动画延迟滚动
        if (this.srcoll < 10) {
          clearInterval(this.toTopSpeed);
        }
      }, 1);
    },
}