Typing computed properties in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a computed property in Vue?

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

beginner

Why do we type computed properties in Vue with TypeScript?

Typing computed properties helps catch errors early, provides better code completion, and ensures the computed value has the expected type.

intermediate

How do you type a computed property that returns a string in Vue 3 with TypeScript?

Use the generic type parameter: const myComputed = computed<string>(() => { return 'hello'; });

intermediate

What happens if you don't type a computed property explicitly in Vue with TypeScript?

TypeScript will infer the type from the return value, but explicit typing improves clarity and helps avoid mistakes when the return type is complex.

beginner

Show a simple example of typing a computed property that returns a number in Vue 3.

import { ref, computed } from 'vue';
const count = ref(0);
const doubleCount = computed<number>(() => count.value * 2);

How do you specify the return type of a computed property in Vue 3 with TypeScript?

A Use computed<Type>(() => ...)

B Use ref<Type>(() => ...)

C Use reactive<Type>(() => ...)

D Use watch<Type>(() => ...)

The computed function accepts a generic type parameter to specify the return type.

What does a computed property do in Vue?

A Caches a value and updates it when dependencies change

B Directly modifies the DOM

C Runs only once when the component mounts

D Stores global state

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

If a computed property returns a string, how should you type it?

A ref<string>(() => ...)

B computed<number>(() => ...)

C computed<string>(() => ...)

D reactive<string>(() => ...)

Use computed to indicate the computed property returns a string.

What benefit does typing computed properties provide?

A Faster runtime performance

B Better error checking and code completion

C Automatically updates the DOM

D Prevents all bugs

Typing helps catch errors early and improves developer experience.

Which Vue 3 API is used to create computed properties?

A watch

B ref

C reactive

D computed

The computed function creates computed properties.

Explain how to type a computed property in Vue 3 using TypeScript and why it is useful.

Think about how TypeScript helps with catching errors and improving developer tools.

Describe the behavior of computed properties in Vue and how typing them affects development.

Consider both runtime behavior and development experience.

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