methods: {
  update() {
    for (let i = 0; i < 10; i++) {
      this.testNum = this.testNum + i;
    }
  },
},

<template>
 <div id="app">
  <ul ref="ul1">
   <li v-for="(v, i) in list" :key="i">{{ v }}</li>
  </ul>
  <button @click="add">add DOM</button>
 </div>
</template>
<script>
export default {
 name: "app",
 data() {
  return {
   list: ["a", "b", "c"],
  };
 },
 methods: {
  add() {
   this.list.push(`${Date.now()}`);
   this.list.push(`${Date.now()}`);
   this.list.push(`${Date.now()}`);
   const ulElem = this.$refs.ul1;
   console.log(ulElem.childNodes.length);
  },
 },
};
</script>

  add() {
   this.list.push(`${Date.now()}`);
   this.list.push(`${Date.now()}`);
   this.list.push(`${Date.now()}`);
   const ulElem = this.$refs.ul1;
   this.$nextTick(() => {
    console.log(ulElem.childNodes.length);
   });
  },