Computed properties for derived state in Vue

Introduction

Computed properties help you create values that automatically update when your data changes. They keep your code simple and efficient.

You want to show a value based on other data, like a full name from first and last names.

You need to filter or sort a list based on user input.

You want to calculate a total price from item quantities and prices.

You want to format a date or number dynamically.

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

Syntax

Vue

import { computed, ref } from 'vue';

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

Use computed to define a property that updates automatically.

Access reactive values inside computed using .value.

Examples

Combine two reactive values into one computed full name.

Vue

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

Filter a list reactively to get only even numbers.

Vue

const numbers = ref([1, 2, 3, 4]);
const evenNumbers = computed(() => numbers.value.filter(n => n % 2 === 0));

Calculate total price from price and quantity.

Vue

const price = ref(10);
const quantity = ref(3);
const total = computed(() => price.value * quantity.value);

Sample Program

This Vue component lets you type your first and last name. The full name updates automatically using a computed property.

Vue

<template>
  <div>
    <input v-model="firstName" aria-label="First name" />
    <input v-model="lastName" aria-label="Last name" />
    <p>Your full name is: {{ fullName }}</p>
  </div>
</template>

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

const firstName = ref('John');
const lastName = ref('Smith');

const fullName = computed(() => `${firstName.value} ${lastName.value}`);
</script>

Important Notes

Computed properties cache their result until dependencies change, so they are efficient.

Do not use computed for side effects; use watch instead.

Always access reactive refs inside computed with .value.

Summary

Computed properties create values that update automatically when data changes.

They help keep templates clean and avoid repeated calculations.

Use computed with reactive refs and access values with .value.

Practice

1. What is the main purpose of a computed property in Vue?

easy

A. To directly modify the DOM elements

B. To store static data that never changes

C. To create a value that updates automatically when its dependencies change

D. To replace methods for event handling

Solution

1. Step 1: Understand computed property role

   Computed properties automatically update when their reactive dependencies change, unlike static data.

2. Step 2: Compare with other options

   Static data does not update automatically, DOM manipulation is done differently, and event handling uses methods or event listeners.

3. Final Answer:

   To create a value that updates automatically when its dependencies change -> Option C

4. Quick Check:

   Computed properties = auto-updating values [OK]

Hint: Computed properties auto-update when data changes [OK]

Common Mistakes:

• Thinking computed properties store static data
• Confusing computed with methods for events
• Assuming computed directly changes the DOM

2. Which of the following is the correct syntax to define a computed property named fullName using Vue's Composition API?

easy

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

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

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

D. const fullName = computed(() => firstName.concat(lastName))

Solution

1. Step 1: Check correct computed syntax

   Vue's computed expects a function returning a value, so arrow function with parentheses and return expression is correct.

2. Step 2: Analyze each option

   const fullName = computed(() => fullName + ' ' + lastName) uses arrow function returning concatenated string correctly. const fullName = computed { return firstName + ' ' + lastName } has wrong syntax (missing parentheses). const fullName = computed(() => { firstName + ' ' + lastName }) lacks return statement in block. const fullName = computed(() => firstName.concat(lastName)) uses concat without space.

3. Final Answer:

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

4. Quick Check:

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

Hint: Use arrow function with return expression inside computed [OK]

Common Mistakes:

• Omitting parentheses in computed call
• Forgetting return in block arrow function
• Using string concat without space

3. Given the following Vue setup code, what will be the output of fullName.value?

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

firstName.value = 'John'

medium

A. undefined

B. "Jane Doe"

C. "Jane John"

D. "John Doe"

Solution

1. Step 1: Understand reactive refs and computed

   firstName and lastName are reactive refs. Changing firstName.value updates computed fullName automatically.

2. Step 2: Evaluate fullName after update

   After setting firstName.value = 'John', fullName.value becomes 'John Doe' because computed uses current ref values.

3. Final Answer:

   "John Doe" -> Option D

4. Quick Check:

   Computed updates with ref changes = "John Doe" [OK]

Hint: Computed reflects latest ref values automatically [OK]

Common Mistakes:

• Assuming computed keeps old value
• Ignoring .value when accessing refs
• Confusing variable names in output

4. Identify the error in this Vue computed property code:

import { ref, computed } from 'vue'
const count = ref(0)
const doubleCount = computed(() => {
  count + 2
})

medium

A. Missing return statement inside computed function

B. count should not be a ref

C. computed should be imported from 'vuex'

D. doubleCount should be a method, not computed

Solution

1. Step 1: Check computed function body

   The arrow function uses braces but does not return a value explicitly, so it returns undefined.

2. Step 2: Correct usage

   Either remove braces or add a return statement to return count.value + 2.

3. Final Answer:

   Missing return statement inside computed function -> Option A

4. Quick Check:

   Computed with braces needs return [OK]

Hint: Add return when using braces in arrow function [OK]

Common Mistakes:

• Forgetting return in block arrow functions
• Importing computed from wrong package
• Misusing ref as normal variable

5. You have a list of products with prices and quantities as refs. How would you create a computed property totalPrice that sums the product of price and quantity for all products? Choose the correct computed property code snippet:

hard

A. const totalPrice = computed(() => products.reduce((sum, p) => sum + p.price * p.quantity, 0))

B. const totalPrice = computed(() => products.value.reduce((sum, p) => sum + p.price.value * p.quantity.value, 0))

C. const totalPrice = computed(() => { products.value.forEach(p => sum += p.price.value * p.quantity.value); return sum })

D. const totalPrice = computed(() => products.value.map(p => p.price.value * p.quantity.value))

Solution

1. Step 1: Understand data structure and refs

   Each product has price and quantity as refs, so access with .value. products is a ref array, so use products.value.

2. Step 2: Choose correct aggregation method

   Use reduce to sum all price * quantity values. const totalPrice = computed(() => products.value.reduce((sum, p) => sum + p.price.value * p.quantity.value, 0)) correctly uses reduce with .value access and initial sum 0.

3. Step 3: Analyze incorrect options

   const totalPrice = computed(() => products.reduce((sum, p) => sum + p.price * p.quantity, 0)) misses .value on refs. const totalPrice = computed(() => { products.value.forEach(p => sum += p.price.value * p.quantity.value); return sum }) uses forEach but sum is undefined. const totalPrice = computed(() => products.value.map(p => p.price.value * p.quantity.value)) returns array, not sum.

4. Final Answer:

   const totalPrice = computed(() => products.value.reduce((sum, p) => sum + p.price.value * p.quantity.value, 0)) -> Option B

5. Quick Check:

   Use reduce with .value refs to sum totals [OK]

Hint: Use reduce with .value to sum reactive refs [OK]

Common Mistakes:

• Forgetting .value on refs inside products
• Using map instead of reduce for sum
• Not initializing sum in reduce