It can be tempting to use v-if with v-for in these common scenarios: Filtering items in a list and avoiding rendering a list if it should be hidden.
Let's take a detailed look at these two scenarios.
Filtering items in a list
You might want to write the following code to filter items in a list:
<script setup lang="ts">
const todos = [
{ id: 1, isComplete: false, name: 'Buy Milk' },
// ...
]
</script>
<template>
<ul>
<li v-for="todo in todos" v-if="!todo.isComplete">
{{ todo.name }}
</li>
</ul>
</template>
This code will throw an error because the v-if directive will be evaluated first and the iteration variable todo does not exist at this moment. When Vue processes directives, v-if has a higher priority than v-for.
You can fix the code by using a computed property that returns your filtered list:
<script setup lang="ts">
import { computed } from 'vue'
const todos = [
{ id: 1, isComplete: false, name: 'Buy Milk' },
// ...
]
const uncompletedTodos = computed(() => todos.filter((todo) => !todo.isComplete))
</script>
<template>
<ul>
<li v-for="todo in uncompletedTodos" :key="todo.id">
{{ todo.name }}
</li>
</ul>
</template>
In this example, the code doesn't loop through the entire array, only the filtered computed property, which is also cached. In case you have massive arrays, this can be a huge performance improvement.
Avoid rendering a list if it should be hidden
In some cases, you want to render a list only if a certain variable is set:
<script setup lang="ts">
import { ref } from 'vue'
const todos = [
{ id: 1, isComplete: false, name: 'Buy Milk' },
// ...
]
const showTodos = ref(false)
</script>
<template>
<ul>
<li v-for="todo in todos" v-if="!showTodos">
{{ todo.name }}
</li>
</ul>
</template>
A better solution would be to move the v-if to a container element (e.g. ul, ol):
<script setup lang="ts">
import { ref } from 'vue'
const todos = [
{ id: 1, isComplete: false, name: 'Buy Milk' },
// ...
]
const showTodos = ref(false)
</script>
<template>
<ul v-if="!showTodos">
<li v-for="todo in todos">
{{ todo.name }}
</li>
</ul>
</template>
StackBlitz Demo
The code of this article is interactively available at StackBlitz:
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