官网示例:

<el-backtop target=".page-component__scroll .el-scrollbar__wrap" :bottom="100">
    <div
      style="{
        height: 100%;
        width: 100%;
        background-color: #f2f5f6;
        box-shadow: 0 0 6px rgba(0,0,0, .12);
        text-align: center;
        line-height: 40px;
        color: #1989fa;
      }"
    >
      UP
    </div>
  </el-backtop>

注意：target属性是为了获取产生滚动条的dom元素，所以target属性里面的类，应该要精准指向产生滚动条的dom对象，要不然就无法生效，参考el-backtop组件源码：

<template>
  <transition name="el-fade-in">
    <div
      v-if="visible"
      @click.stop="handleClick"
      :style="{
        'right': styleRight,
        'bottom': styleBottom
      }"
      class="el-backtop">
      <slot>
        <el-icon name="caret-top"></el-icon>
      </slot>
    </div>
  </transition>
</template>
 
<script>
import throttle from 'throttle-debounce/throttle';
 
export default {
  name: 'ElBacktop',
 
  props: {
    visibilityHeight: {
      type: Number,
      default: 200
    },
    target: [String],
    right: {
      type: Number,
      default: 40
    },
    bottom: {
      type: Number,
      default: 40
    }
  },
 
  data() {
    return {
      el: null,
      container: null,
      visible: false
    };
  },
 
  computed: {
    styleBottom() {
      return `${this.bottom}px`;
    },
    styleRight() {
      return `${this.right}px`;
    }
  },
 
  mounted() {
    this.init();
    this.throttledScrollHandler = throttle(300, this.onScroll);
    this.container.addEventListener('scroll', this.throttledScrollHandler);
  },
 
  methods: {
    init() {
      this.container = document;
      this.el = document.documentElement;
      if (this.target) {
        this.el = document.querySelector(this.target);
        if (!this.el) {
          throw new Error(`target is not existed: ${this.target}`);
        }
        this.container = this.el;
      }
    },
    onScroll() {
      const scrollTop = this.el.scrollTop;
      this.visible = scrollTop >= this.visibilityHeight;
    },
    handleClick(e) {
      this.scrollToTop();
      this.$emit('click', e);
    },
    scrollToTop() {
      let el = this.el;
      let step = 0;
      let interval = setInterval(() => {
        if (el.scrollTop <= 0) {
          clearInterval(interval);
          return;
        }
        step += 10;
        el.scrollTop -= step;
      }, 20);
    }
  },
 
  beforeDestroy() {
    this.container.removeEventListener('scroll', this.throttledScrollHandler);
  }
};
</script>

根据源码中init()函数可知，当target未设置时，当前滚动操作对象默认为document.documentElement对象,即html根节点，当设置了target时，会根据target里面填写的类名去querySelector查找该元素，而且根据该类应只能精准查找到一个元素，注意querySelector和querySelectorAll的区别，前者只返回对应查找条件找到的第一个元素，所以target类名要精准且唯一，否则backtop组件则无效。