Typing computed properties in Vue - Mini Project: Build & Apply

Typing computed properties

📖 Scenario: You are building a simple Vue 3 app that shows a user's full name by combining their first and last names.

🎯 Goal: Create a Vue 3 component using the Composition API where you define firstName and lastName as reactive variables, then create a typed computed property fullName that combines them. Display the full name in the template.

📋 What You'll Learn

Use Vue 3 Composition API with <script setup>

Create reactive variables firstName and lastName with initial values

Create a typed computed property fullName that returns a string combining first and last names

Display fullName in the template inside a <p> tag

💡 Why This Matters

🌍 Real World

Typing computed properties helps you build reactive user interfaces that update automatically and safely with type checking.

💼 Career

Understanding typed computed properties is essential for Vue developers to write maintainable and bug-free code in professional projects.

1

Setup reactive variables

Inside the <script setup> block, import ref from 'vue' and create two reactive variables called firstName and lastName with initial values 'John' and 'Doe' respectively.

Vue

// Import ref and create firstName and lastName reactive variables
// Your code here



  

Hint

Use import { ref } from 'vue' and then const firstName = ref('John') and const lastName = ref('Doe').

2

Add typed computed property

Import computed from 'vue' and create a computed property called fullName that returns a string combining firstName.value and lastName.value separated by a space. Add a TypeScript type annotation to fullName to indicate it is a computed ref of type string.

Vue

import { ref, computed } from 'vue'

const firstName = ref('John')
const lastName = ref('Doe')

// Create typed computed property fullName here
// Your code here



  

Hint

Use computed and add the type ComputedRef<string> to fullName.

3

Display computed property in template

In the <template> section, add a <p> tag that displays the value of fullName using Vue's mustache syntax.

Vue

import { ref, computed, ComputedRef } from 'vue'

const firstName = ref('John')
const lastName = ref('Doe')

const fullName: ComputedRef = computed(() => `${firstName.value} ${lastName.value}`)



   tag to show fullName here -->
  

Hint

Use <p>{{ fullName }}</p> inside the template.

4

Add accessibility and responsiveness

Add an aria-label attribute to the <p> tag with the value 'User full name'. Also, add a class attribute with the value 'full-name' for styling. Then add a <style scoped> block that sets the font size of .full-name to 1.5rem and color to #333. This ensures the full name is accessible and looks good on all screen sizes.

Vue

import { ref, computed, ComputedRef } from 'vue'

const firstName = ref('John')
const lastName = ref('Doe')

const fullName: ComputedRef = computed(() => `${firstName.value} ${lastName.value}`)



   tag -->
  

Hint

Add aria-label="User full name" and class="full-name" to the <p> tag. Then add scoped CSS for .full-name.

Practice

1. What is the main benefit of typing computed properties in Vue?

easy

A. It disables reactivity for the computed property.

B. It helps catch errors early and improves code clarity.

C. It automatically updates the DOM without reactivity.

D. It makes the computed property run faster.

Solution

1. Step 1: Understand typing purpose

   Typing in computed properties helps the developer know what type of value to expect and prevents mistakes.

2. Step 2: Recognize benefits

   Typing improves code clarity and helps catch errors before running the code.

3. Final Answer:

   It helps catch errors early and improves code clarity. -> Option B

4. Quick Check:

   Typing computed properties = better error detection [OK]

Hint: Typing improves clarity and error detection [OK]

Common Mistakes:

• Thinking typing affects runtime speed
• Believing typing disables reactivity
• Confusing typing with DOM updates

2. Which of the following is the correct syntax to type a computed property returning a number in Vue 3 with TypeScript?

easy

A. const total = computed<number>(() => 42);

B. const total = computed((): string => 42);

C. const total = computed<string>(() => 42);

D. const total = computed(() => '42' as number);

Solution

1. Step 1: Identify correct typing syntax

   The correct way to type a computed property is using computed<Type>(() => value).

2. Step 2: Match return type and value

   The value 42 is a number, so computed<number>(() => 42) matches correctly.

3. Final Answer:

   const total = computed<number>(() => 42); -> Option A

4. Quick Check:

   Use computed<Type> for typing [OK]

Hint: Use computed<Type>(() => value) to type computed [OK]

Common Mistakes:

• Using wrong generic type like string for number value
• Typing the function return instead of computed generic
• Casting inside the function instead of typing computed

3. Given the code below, what is the type of fullName.value?

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

medium

A. string

B. Ref<string>

C. undefined

D. number

Solution

1. Step 1: Understand computed typing

   The computed property is typed as computed<string>, so its value is a string.

2. Step 2: Check .value type

   Accessing fullName.value gives the computed value, which is a string.

3. Final Answer:

   string -> Option A

4. Quick Check:

   Typed computed value = string [OK]

Hint: Access .value to get typed computed result [OK]

Common Mistakes:

• Confusing computed ref type with its value type
• Assuming fullName.value is a Ref<string>
• Thinking fullName.value is undefined before assignment

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

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

medium

A. The computed function should not use an arrow function.

B. The ref should be typed as string, not number.

C. The computed type should be number, not string.

D. The computed property cannot depend on a ref.

Solution

1. Step 1: Check the computed return value type

   The expression count.value * 2 results in a number.

2. Step 2: Match computed generic type

   The computed is typed as string, which mismatches the number result.

3. Final Answer:

   The computed type should be number, not string. -> Option C

4. Quick Check:

   Computed type must match return value type [OK]

Hint: Match computed generic type to returned value type [OK]

Common Mistakes:

• Typing computed as string when returning number
• Thinking arrow functions are invalid in computed
• Believing computed can't depend on refs

5. You want to create a typed computed property that returns the count of items in a reactive array items. Which is the correct way to type it in Vue 3 with TypeScript?

const items = ref<string[]>([]);
const itemCount = computed<???>(() => items.value.length);

hard

A. Array<string>

B. string

C. Ref<number>

D. number

Solution

1. Step 1: Determine the return type of computed

   items.value.length returns a number representing the count.

2. Step 2: Use computed generic type matching return

   The computed should be typed as computed<number> to match the number return.

3. Final Answer:

   number -> Option D

4. Quick Check:

   Computed type = return value type [OK]

Hint: Type computed with the exact return value type [OK]

Common Mistakes:

• Typing computed as Ref<number> instead of number
• Using string type for a numeric length
• Confusing array type with count type