在寫vue項目時，瀏覽器的console出現如下警告信息：
[Vue warn]: Property or method "index" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties. found in ---> <Navheader> at E:\GitRepository\companyProject\src\components\crimalcheck\crimalcheck.vue <App> at E:\GitRepository\companyProject\src\App.vue <Root>
原因很簡單：不熟悉或者粗心的情況下很容易把v-for循環寫成如下形式
<td v-for="item in xxx" :key="index" >
正確的寫法應為<td v-for="(item,index) in xxx" :key="index" >
ps:把:key="index"刪掉也可以解決此警告，但是這樣就違反了Vue文檔當中的推薦寫法，v-for當中最好還是把:key寫上