Computed in Composition API in Vue

Introduction

Computed properties automatically update when their dependencies change. They help keep your UI in sync without extra work.

You want to show a value based on other reactive data.

You need to perform simple calculations or formatting for display.

You want to avoid repeating the same logic in your template.

You want to cache a value until its dependencies change.

You want to keep your template clean and easy to read.

Syntax

Vue

import { computed } from 'vue';

const myComputed = computed(() => {
  // return a value based on reactive data
});

Use computed inside the setup() function of your component.

The function inside computed should return the value you want to track.

Examples

This creates a computed property doubleCount that is always twice the count value.

Vue

import { ref, computed } from 'vue';
const count = ref(0);
const doubleCount = computed(() => count.value * 2);

Combines two reactive values into one computed full name.

Vue

import { ref, computed } from 'vue';
const firstName = ref('Jane');
const lastName = ref('Doe');
const fullName = computed(() => `${firstName.value} ${lastName.value}`);

Sample Program

This Vue component shows a count and its double. When you click the button, the count increases and the double updates automatically.

Vue

<template>
  <div>
    <p>Count: {{ count }}</p>
    <p>Double Count: {{ doubleCount }}</p>
    <button @click="increment">Increase</button>
  </div>
</template>

<script setup>
import { ref, computed } from 'vue';

const count = ref(0);
const doubleCount = computed(() => count.value * 2);

function increment() {
  count.value++;
}
</script>

Important Notes

Computed properties are cached and only recalculate when their dependencies change.

Always access reactive values inside computed using .value.

Summary

Computed properties keep your UI reactive and efficient.

They automatically update when their dependencies change.

Use them inside setup() with the computed function.

Practice

1. What is the main purpose of computed properties in Vue's Composition API?

easy

A. To store static data that never changes

B. To define methods that run only once

C. To replace the data option in Options API

D. To create reactive values that update automatically when dependencies change

Solution

1. Step 1: Understand computed properties role

   Computed properties are reactive and update automatically when their dependencies change.

2. Step 2: Compare with other options

   Methods run on demand, static data doesn't update, and computed doesn't replace data but complements it.

3. Final Answer:

   To create reactive values that update automatically when dependencies change -> Option D

4. Quick Check:

   Computed = reactive auto-update [OK]

Hint: Computed = reactive values that auto-update [OK]

Common Mistakes:

• Confusing computed with methods
• Thinking computed stores static data
• Believing computed replaces data option

2. Which of the following is the correct syntax to create a computed property named fullName inside setup()?

easy

A. const fullName = computed(() => firstName + ' ' + lastName);

B. const fullName = computed { return firstName + ' ' + lastName };

C. const fullName = computed(function() { firstName + ' ' + lastName });

D. const fullName = computed(() => { firstName + ' ' + lastName });

Solution

1. Step 1: Recall computed syntax

   Computed takes a function returning a value, usually an arrow function with parentheses and curly braces if needed.

2. Step 2: Check each option

   const fullName = computed(() => fullName + ' ' + lastName); uses arrow function with parentheses and returns expression correctly. const fullName = computed { return firstName + ' ' + lastName }; misses parentheses and arrow. const fullName = computed(function() { firstName + ' ' + lastName }); lacks return statement. const fullName = computed(() => { firstName + ' ' + lastName }); has curly braces but no return, so returns undefined.

3. Final Answer:

   const fullName = computed(() => firstName + ' ' + lastName); -> Option A

4. Quick Check:

   Correct arrow function syntax = const fullName = computed(() => firstName + ' ' + lastName); [OK]

Hint: Use arrow function with parentheses and return expression [OK]

Common Mistakes:

• Omitting parentheses in arrow function
• Forgetting return statement with curly braces
• Using wrong function syntax inside computed

3. Given the code inside setup():

const count = ref(2);
const double = computed(() => count.value * 2);
count.value = 5;
console.log(double.value);

What will be logged to the console?

medium

A. 2

B. 5

C. 10

D. 4

Solution

1. Step 1: Understand reactive refs and computed

   Initially, count.value is 2, so double.value is 4. Then count.value is updated to 5.

2. Step 2: Compute updated value

   Computed property recalculates automatically: 5 * 2 = 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   Updated count * 2 = 10 [OK]

Hint: Computed updates automatically when dependencies change [OK]

Common Mistakes:

• Thinking computed doesn't update after ref changes
• Confusing count value with computed value
• Ignoring .value when accessing refs

4. Identify the error in this computed property code inside setup():

const firstName = ref('John');
const lastName = ref('Doe');
const fullName = computed(() => {
  firstName + ' ' + lastName
});

medium

A. Missing return statement inside computed function

B. Refs should not be used inside computed

C. Computed must be declared outside setup()

D. firstName and lastName should not be refs

Solution

1. Step 1: Analyze computed function body

   The function uses curly braces but does not return any value explicitly.

2. Step 2: Understand arrow function behavior

   With curly braces, arrow functions need an explicit return; otherwise, they return undefined.

3. Final Answer:

   Missing return statement inside computed function -> Option A

4. Quick Check:

   Curly braces need return [OK]

Hint: Use return with curly braces in arrow functions [OK]

Common Mistakes:

• Omitting return in arrow function with braces
• Thinking refs can't be used in computed
• Believing computed must be outside setup

5. You want to create a computed property statusMessage that returns "Active" if isActive is true, "Inactive" if false, and "Unknown" if isActive is null or undefined. Which code correctly implements this inside setup()?

hard

A. const statusMessage = computed(() => isActive.value ? 'Active' : 'Inactive');

B. const statusMessage = computed(() => isActive.value === true ? 'Active' : isActive.value === false ? 'Inactive' : 'Unknown');

C. const statusMessage = computed(() => { if (isActive.value) return 'Active'; else return 'Inactive'; });

D. const statusMessage = computed(() => isActive ? 'Active' : 'Inactive');

Solution

1. Step 1: Understand the conditions

   We need to check three cases: true, false, and null/undefined (unknown).

2. Step 2: Evaluate each option

   const statusMessage = computed(() => isActive.value ? 'Active' : 'Inactive'); only handles true/false, ignoring null/undefined. const statusMessage = computed(() => isActive.value === true ? 'Active' : isActive.value === false ? 'Inactive' : 'Unknown'); uses nested ternary to cover all three cases correctly. const statusMessage = computed(() => { if (isActive.value) return 'Active'; else return 'Inactive'; }); misses the unknown case and uses if-else without else if. const statusMessage = computed(() => isActive ? 'Active' : 'Inactive'); checks isActive directly without .value and misses unknown case.

3. Final Answer:

   const statusMessage = computed(() => isActive.value === true ? 'Active' : isActive.value === false ? 'Inactive' : 'Unknown'); -> Option B

4. Quick Check:

   All cases handled with nested ternary [OK]

Hint: Use nested ternary to cover all true, false, unknown cases [OK]

Common Mistakes:

• Ignoring null or undefined cases
• Not using .value to access ref
• Using simple if-else missing else if