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Table of Contents
Basic writing method: use computed to create read-only attributes
Readable and writable computed properties (with getter and setter)
Notes on using computed in templates
The difference between watch: When should I use computed?
Home Web Front-end Vue.js How to use computed properties with Composition API?

How to use computed properties with Composition API?

Jul 03, 2025 am 12:47 AM

In the Composition API of Vue 3, the computed function can be used to implement responsive computed properties; 1. The basic writing method is to pass a function that returns a value to create read-only properties, such as computed(() => count.value * 2); 2. When it is necessary to be readable and writable, you can use the object form with get and set, such as updating firstName and lastName by assigning fullName.value; 3. When using it in a template, no .value is required, Vue will be automatically unpacked; computed is more suitable for derived values, while watch is suitable for side-effect operations. Reasonable choice can improve the simplicity of the code and the clarity of responsive logic.

Using computed attributes in Vue 3's Composition API is actually similar to the Options API, but the writing method is more flexible and the organizational logic is clearer. You can use computed function to create a responsive calculated value, which will be automatically updated based on the dependent responsive data.

Below are some common usage methods and scenarios.


Basic writing method: use computed to create read-only attributes

The easiest way to use it is to introduce computed and then pass in a function that returns a value:

 import { ref, calculated } from 'vue'

export default {
  setup() {
    const count = ref(1)
    const double = computed(() => count.value * 2)

    return { count, double }
  }
}

double here is a calculation property. When count changes, double will automatically recalculate. This writing is suitable for most "read-only" situations.


Readable and writable computed properties (with getter and setter)

If you need to modify the value of the calculated attribute and inversely affect the original data, you can write it as an object with get and set :

 const firstName = ref('John')
const lastName = ref('Doe')

const fullName = computed({
  get: () => `${firstName.value} ${lastName.value}`,
  set: (value) => {
    const parts = value.split(' ')
    firstName.value = parts[0]
    lastName.value = parts[1]
  }
})

This way you can update both firstName and lastName by assigning fullName.value = 'Jane Smith' . This trick is useful in form linkage or two-way binding.


Notes on using computed in templates

When using the value returned computed directly in the template, be careful to call .value like using a normal variable, because it is implemented based on ref :

 <template>
  <div>Count: {{ count }}</div>
  <div>Double: {{ double }}</div>
</template>

Although double is a ref , there is no need to add .value to the template, Vue will automatically unpack.


The difference between watch: When should I use computed?

  • computered is more suitable for derive new values ??based on existing responsive data.
  • Watch is more suitable for executing side effects, such as asynchronous requests, operating DOMs, etc.

For example, if you want to do a search filtering function:

  • Use computed to filter the list based on the input content;
  • Use watch to listen for input changes and initiate API requests.

So remember: try not to use watch if you can solve problems with computed , because it is more efficient and more declarative.


Basically that's it. Using computed well can make your code more concise and responsive logic clearer. What is not complicated but easy to ignore is to remember to distinguish between read-only and writable usage scenarios, and whether .value is needed in the template.

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