Typing computed properties in Vue

Introduction

Typing computed properties helps your code know what kind of value to expect. This makes your app safer and easier to understand.

When you want to create a value that changes based on other data in your Vue component.

When you want to make sure TypeScript knows the exact type of the computed value.

When you want to avoid mistakes by catching type errors early.

When you want better code suggestions and auto-completion in your editor.

Syntax

Vue

import { computed } from 'vue'

const myComputed = computed<Type>(() => {
  // return a value of type Type
})

You add the type inside angle brackets <Type> right after computed.

The function inside computed returns the value you want, and TypeScript checks it matches the type.

Examples

This example types the computed property as a number because it returns a number.

Vue

import { computed, ref } from 'vue'

const count = ref(5)
const doubleCount = computed<number>(() => count.value * 2)

Here, the computed property returns a string, so we type it as string.

Vue

import { computed, ref } from 'vue'

const firstName = ref('Jane')
const lastName = ref('Doe')
const fullName = computed<string>(() => `${firstName.value} ${lastName.value}`)

This example shows typing with an interface and a computed property returning a string.

Vue

import { computed, ref } from 'vue'

interface User {
  name: string
  age: number
}

const user = ref<User>({ name: 'Sam', age: 30 })
const userName = computed<string>(() => user.value.name)

Sample Program

This Vue component uses typed computed properties to calculate the total price. The total is typed as a number, so TypeScript helps ensure the calculation is correct.

Vue

<script setup lang="ts">
import { ref, computed } from 'vue'

const price = ref(100)
const quantity = ref(3)

const total = computed<number>(() => price.value * quantity.value)
</script>

<template>
  <div>
    <p>Price: {{ price }}</p>
    <p>Quantity: {{ quantity }}</p>
    <p><strong>Total: {{ total }}</strong></p>
  </div>
</template>

Important Notes

Typing computed properties is optional but recommended for better code safety.

If you don't specify a type, TypeScript tries to guess it from the return value.

Use ref for reactive data and computed for values derived from that data.

Summary

Typing computed properties helps catch errors early and improves code clarity.

Use computed<Type>(() => value) to specify the type.

This works well with ref and other reactive Vue features.

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