Computed properties for derived state in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Vue computed property?

Consider this Vue component using the Composition API. What will be displayed inside the paragraph when the component renders?

Vue

<script setup>
import { ref, computed } from 'vue'
const count = ref(3)
const doubleCount = computed(() => count.value * 2)
</script>
<template>
  <p>{{ doubleCount }}</p>
</template>

A undefined

B 3

C 6

D NaN

💡 Hint

Remember that computed properties return a reactive value based on other reactive sources.

The computed property doubleCount multiplies count.value (3) by 2, resulting in 6. This value is displayed in the paragraph.

❓ state_output

intermediate

What is the value of fullName computed property?

Given this Vue component, what is the value of the computed property fullName?

Vue

<script setup>
import { reactive, computed } from 'vue'
const user = reactive({ firstName: 'Jane', lastName: 'Doe' })
const fullName = computed(() => `${user.firstName} ${user.lastName}`)
</script>

A undefined

B "Jane Doe"

C "Doe Jane"

D "JaneDoe"

💡 Hint

Look at the template literal combining firstName and lastName with a space.

The computed property fullName concatenates user.firstName and user.lastName with a space, resulting in "Jane Doe".

📝 Syntax

advanced

Which option correctly defines a computed property that returns the square of a number?

Select the option that correctly defines a computed property named squared that returns the square of a reactive number value.

A const squared = computed(() => number.value ^ 2)

B const squared = computed(() => number * 2)

C const squared = computed(() => number.value * 2)

D const squared = computed(() => number.value ** 2)

💡 Hint

Remember to access the reactive value with .value and use the exponentiation operator.

Option D correctly accesses number.value and uses the exponentiation operator ** to square it. Option D and C multiply by 2 instead of squaring. Option D uses ^ which is a bitwise XOR, not exponentiation.

🔧 Debug

advanced

Why does this computed property not update when the reactive source changes?

Given this Vue component, why does the computed property fullName not update when user.firstName changes?

Vue

<script setup>
import { reactive, computed } from 'vue'
const user = { firstName: 'John', lastName: 'Smith' }
const fullName = computed(() => `${user.firstName} ${user.lastName}`)
user.firstName = 'Mike'
</script>

A Because user is not reactive, so changes to firstName are not tracked.

B Because computed properties cannot depend on objects.

C Because fullName needs to be a ref, not computed.

D Because user.firstName is a constant and cannot be changed.

💡 Hint

Check how user is declared and whether Vue can track its changes.

The user object is a plain object, not reactive. Vue cannot track changes to its properties, so fullName does not update when firstName changes.

🧠 Conceptual

expert

How does Vue's computed property caching work?

Which statement best describes how Vue caches computed properties?

A Vue caches computed properties and only recalculates them when their reactive dependencies change.

B Vue recalculates computed properties on every component render regardless of dependencies.

C Vue caches computed properties permanently after the first calculation and never updates them.

D Vue does not cache computed properties; they behave like methods.

💡 Hint

Think about efficiency and when Vue should update computed values.

Vue tracks reactive dependencies of computed properties and only recalculates them when those dependencies change, improving performance.

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