Vue.js: What is the difference between v-for key and index?

1<template>
2  <!-- Good -->
3  <div v-for="item in items" :key="item.id">
4    {{ item.name }}
5  </div>
6
7  <!-- Bad (unless static) -->
8  <div v-for="(item, index) in items" :key="index">
9    {{ item.name }}
10  </div>
11</template>