Computed in Composition API in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a computed property in Vue's Composition API?

A computed property is a special reactive value that automatically updates when its dependencies change. It caches its result until those dependencies change again.

beginner

How do you create a computed property in Vue's Composition API?

You import <code>computed</code> from 'vue' and call it with a function that returns the value you want to compute. Example: <code>const double = computed(() =&gt; count.value * 2)</code>.

intermediate

Why use computed instead of a regular function in Vue's Composition API?

Because computed caches its result and only recalculates when dependencies change, making your app more efficient and reactive.

intermediate

What happens if you try to assign a value directly to a computed property without a setter?

You will get an error because computed properties without a setter are read-only. To assign values, you must define a setter function.

advanced

Show a simple example of a writable computed property in Vue's Composition API.

Example:
<pre>const fullName = computed({
  get: () =&gt; firstName.value + ' ' + lastName.value,
  set: (val) =&gt; {
    const parts = val.split(' ')
    firstName.value = parts[0]
    lastName.value = parts[1] || ''
  }
})</pre>

What does a computed property in Vue's Composition API do?

A Caches a value and updates only when dependencies change

B Runs a function every time the component renders

C Stores a static value that never changes

D Directly modifies reactive state

Computed properties cache their result and only update when their reactive dependencies change.

How do you import the computed function in Vue 3 Composition API?

A import computed from 'vue3'

B import computed from 'vue-computed'

C import { Computed } from 'vue'

D import { computed } from 'vue'

The correct import is import { computed } from 'vue'.

What will happen if you try to assign a value to a read-only computed property?

A It will update the value silently

B It will throw an error

C It will ignore the assignment

D It will convert to a reactive ref

Read-only computed properties throw an error if you try to assign a value to them.

Which of these is a correct way to define a computed property?

A const total = computed(() => price.value + tax.value)

B const total = computed(price.value + tax.value)

C const total = computed = () => price + tax

D const total = computed { return price + tax }

Computed properties must be created by calling computed with a function.

What is the main benefit of using computed over a method in Vue?

A Methods are reactive but computed are not

B Methods automatically update when dependencies change

C Computed properties cache results for better performance

D Computed properties can only be used in templates

Computed properties cache their results and only recompute when dependencies change, improving performance.

Explain how to create and use a computed property in Vue's Composition API.

Think about how you make a reactive value that updates automatically.

Describe the difference between a computed property and a method in Vue's Composition API.

Consider performance and when values are recalculated.

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