Vue 中为什么不推荐用index 做 key属性值

if (patchFlag > 0) {
      if (patchFlag & PatchFlags.KEYED_FRAGMENT) { 
         
        patchKeyedChildren(
         ...
        )
        return
      } else if (patchFlag & PatchFlags.UNKEYED_FRAGMENT) {
         
        patchUnkeyedChildren( 
          ...
        )
        return
      }
    }

let i = 0              
const l2 = c2.length   
let e1 = c1.length - 1 
let e2 = l2 - 1        

//(a b) c
//(a b) d e

    while (i <= e1 && i <= e2) {
      const n1 = c1[i]
      const n2 = (c2[i] = optimized
        ? cloneIfMounted(c2[i] as VNode)
        : nORMalizeVNode(c2[i]))
        
      if (isSameVNodeType(n1, n2)) {
        patch(
          ...
        )
      } else {
        break
      }
      i++
    }

export function isSameVNodeType(n1: VNode, n2: VNode): boolean {
  return n1.type === n2.type && n1.key === n2.key
}

//a (b c)
//d e (b c)

    while (i <= e1 && i <= e2) {
      const n1 = c1[e1]
      const n2 = (c2[e2] = optimized
        ? cloneIfMounted(c2[e2] as VNode)
        : normalizeVNode(c2[e2]))
      if (isSameVNodeType(n1, n2)) {
        patch(
         ...
        )
      } else {
        break
      }
      e1--
      e2--
    }

//(a b)
//(a b) c
//i = 2, e1 = 1, e2 = 2
//(a b)
//c (a b)
//i = 0, e1 = -1, e2 = 0

    if (i > e1) {
      if (i <= e2) {
        const nextPos = e2 + 1
        const anchor = nextPos < l2 ? (c2[nextPos] as VNode).el : parentAnchor
        while (i <= e2) {
          patch( 
            ...
          )
          i++
        }
      }
    }

//i > e2
//(a b) c
//(a b)
//i = 2, e1 = 2, e2 = 1
//a (b c)
//(b c)
//i = 0, e1 = 0, e2 = -1
else if (i > e2) {
   while (i <= e1) {
      unmount(c1[i], parentComponent, parentSuspense, true)
      i++
   }
}

<template>
  <div class="hello">
    <ul>
      <li v-for="(item,index) in studentList" :key="index">{{item.name}}</li>
      <br>
      <button @click="addStudent">添加一条数据</button>
    </ul>
 
  </div>
</template>
 
<script>
export default {
  name: 'HelloWorld',
  data() {
    return {
      studentList: [
        { id: 1, name: '张三', age: 18 },
        { id: 2, name: '李四', age: 19 },
      ],
    };
  },
  methods:{
    addStudent(){
      const studentObj = { id: 3, name: '王五', age: 20 };
      this.studentList=[studentObj,...this.studentList]
    }
  }
}
</script>

<template>
  <div class="hello">
    <ul>
      <button @click="addStudent">添加一条数据</button>
      <br>
      <li v-for="(item,index) in studentList" :key="index">{{item.id}}</li>
    </ul>
  </div>
</template>
 
<script>
import uuidv1 from 'uuid/v1'
export default {
  name: 'HelloWorld',
  data() {
    return {
      studentList: [{id:uuidv1()}],
    };
  },
  created(){
    for (let i = 0; i < 1000; i++) {
      this.studentList.push({
        id: uuidv1(),
      });
    }
  },
  beforeUpdate(){
    console.time('for');
  },
  updated(){
    console.timeEnd('for')//for: 75.259033203125 ms
  },
  methods:{
    addStudent(){
      const studentObj = { id: uuidv1() };
      this.studentList=[studentObj,...this.studentList]
    }
  }
}
</script>

<template>
  <div class="hello">
    <ul>
      <button @click="addStudent">添加一条数据</button>
      <br>
      <li v-for="(item,index) in studentList" :key="item.id">{{item.id}}</li>
    </ul>
  </div>
</template>
  beforeUpdate(){
    console.time('for');
  },
  updated(){
    console.timeEnd('for')//for: 42.200927734375 ms
  },

<template>
  <div class="hello">
    <ul>
      <li v-for="(item,index) in studentList" :key="index">{{item.name}}<input /></li>
      <br>
      <button @click="addStudent">添加一条数据</button>
    </ul>
  </div>
</template>
 
<script>
export default {
  name: 'HelloWorld',
  data() {
    return {
      studentList: [
        { id: 1, name: '张三', age: 18 },
        { id: 2, name: '李四', age: 19 },
      ],
    };
  },
  methods:{
    addStudent(){
      const studentObj = { id: 3, name: '王五', age: 20 };
      this.studentList=[studentObj,...this.studentList]
    }
  }
}
</script>

let a=Symbol('测试')
let b=Symbol('测试')
console.log(a===b)//false