Vue Tip: Avoid Side Effects in Computed Properties

It is considered bad practice to introduce side effects inside computed properties and functions because it makes the code unpredictable and hard to understand.

What is a side effect? In the context of computed properties, a side effect is a modification of the state's global state or the internal component state.

Let's take a look at an example with side effects:

1<script setup lang="ts">
2import { computed, ref } from 'vue'
3
4const firstName = ref('Michael')
5const lastName = ref('Hoffmann')
6const array = ref([])
7
8const fullName = computed(() => {
9  firstName.value = 'Mike' // side effect
10  return `${firstName.value} ${lastName.value}`
11})
12
13const reversedArray = computed(() => array.value.reverse()) // side effect, original array is mutated
14</script>

Now let's change the code and remove the side effects:

1<script setup lang="ts">
2import { computed, ref } from 'vue'
3
4const firstName = ref('Michael')
5const lastName = ref('Hoffmann')
6const array = ref([])
7
8const fullName = computed(() => `${firstName.value} ${lastName.value}`)
9const reversedArray = computed(() => array.value.slice(0).reverse()) // .slice creates a copy of the array
10</script>

Now the code is predictable and easy to understand. The computed properties fullName and reversedArray only depend on the values of firstName, lastName, and array. They don't modify the global state or the internal component state.