Computed in Composition API in Vue - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the computed function from Vue.

Vue

import { [1] } from 'vue';

Drag options to blanks, or click blank then click option'

Acomputed

Bref

Creactive

Dwatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ref' instead of 'computed' to create computed properties.

Forgetting to import 'computed' from 'vue'.

The computed function is imported from Vue to create computed properties in the Composition API.

2fill in blank

medium

Complete the code to define a computed property that returns the double of count.

Vue

const doubleCount = computed(() => [1] * 2);

Drag options to blanks, or click blank then click option'

Athis.count

Bcount.value

Ccount

Dcount()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'count' without '.value' which returns the ref object.

Trying to call 'count()' as if it were a function.

In Composition API, to access the value inside a ref, you use .value. So count.value is correct.

3fill in blank

hard

Fix the error in the computed property by completing the blank.

Vue

const greeting = computed(() => `Hello, [1]!`);

Drag options to blanks, or click blank then click option'

Aname

Bname()

Cthis.name

Dname.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name' without '.value' inside the template string.

Trying to use 'this.name' which is not available in Composition API functions.

Since name is a ref, you must use name.value inside computed to get the actual string.

4fill in blank

hard

Fill both blanks to create a computed property that returns the full name by joining firstName and lastName.

Vue

const fullName = computed(() => [1].value + ' ' + [2].value);

Drag options to blanks, or click blank then click option'

AfirstName

BlastName

CfullName

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the computed property name inside its own function causing infinite recursion.

Forgetting to add '.value' to the refs.

The computed property accesses firstName.value and lastName.value to join them with a space.

5fill in blank

hard

Fill all three blanks to create a computed property that filters an array of numbers to only positive even numbers.

Vue

const evenNumbers = computed(() => [1].value.filter(num => num [2] 2 === 0 && num [3] 0));

Drag options to blanks, or click blank then click option'

Anumbers

B%

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '===' for strict equality.

Forgetting to use '.value' on the ref array.

Using '<' instead of '>' to filter positive numbers.

The computed property accesses numbers.value and filters for numbers where num % 2 === 0 and num > 0.

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