key
Attribute 破壊的変更
Overview
- NEW:
key
s are no longer necessary onv-if
/v-else
/v-else-if
branches, since Vue now automatically generates uniquekey
s.- BREAKING: If you manually provide
key
s, then each branch must use a uniquekey
. You can no longer intentionally use the samekey
to force branch reuse.
- BREAKING: If you manually provide
- BREAKING:
<template v-for>
key
should be placed on the<template>
tag (rather than on its children).
Background
The key
special attribute is used as a hint for Vue's virtual DOM algorithm to keep track of a node's identity. That way, Vue knows when it can reuse and patch existing nodes and when it needs to reorder or recreate them. For more information, see the following sections:
On conditional branches
In Vue 2.x, it was recommended to use key
s on v-if
/v-else
/v-else-if
branches.
<!-- Vue 2.x -->
<div v-if="condition" key="yes">Yes</div>
<div v-else key="no">No</div>
The example above still works in Vue 3.x. However, we no longer recommend using the key
attribute on v-if
/v-else
/v-else-if
branches, since unique key
s are now automatically generated on conditional branches if you don't provide them.
<!-- Vue 3.x -->
<div v-if="condition">Yes</div>
<div v-else>No</div>
The breaking change is that if you manually provide key
s, each branch must use a unique key
. In most cases, you can remove these key
s.
<!-- Vue 2.x -->
<div v-if="condition" key="a">Yes</div>
<div v-else key="a">No</div>
<!-- Vue 3.x (recommended solution: remove keys) -->
<div v-if="condition">Yes</div>
<div v-else>No</div>
<!-- Vue 3.x (alternate solution: make sure the keys are always unique) -->
<div v-if="condition" key="a">Yes</div>
<div v-else key="b">No</div>
With <template v-for>
In Vue 2.x, a <template>
tag could not have a key
. Instead, you could place the key
s on each of its children.
<!-- Vue 2.x -->
<template v-for="item in list">
<div :key="'heading-' + item.id">...</div>
<span :key="'content-' + item.id">...</span>
</template>
In Vue 3.x, the key
should be placed on the <template>
tag instead.
<!-- Vue 3.x -->
<template v-for="item in list" :key="item.id">
<div>...</div>
<span>...</span>
</template>
Similarly, when using <template v-for>
with a child that uses v-if
, the key
should be moved up to the <template>
tag.
<!-- Vue 2.x -->
<template v-for="item in list">
<div v-if="item.isVisible" :key="item.id">...</div>
<span v-else :key="item.id">...</span>
</template>
<!-- Vue 3.x -->
<template v-for="item in list" :key="item.id">
<div v-if="item.isVisible">...</div>
<span v-else>...</span>
</template>