Typing composables in Vue

Introduction

Typing composables helps you catch mistakes early and understand your code better by telling Vue what kind of data your composable works with.

When you create reusable logic in Vue using composables and want to ensure the data types are correct.

When you want your code editor to give helpful hints and catch errors while writing Vue composables.

When sharing composables with others to make it clear what inputs and outputs are expected.

When working on larger Vue projects where clear data types improve maintainability.

Syntax

Vue

import { ref, Ref } from 'vue'

function useCounter(): { count: Ref<number>, increment: () => void } {
  const count = ref(0)
  function increment() {
    count.value++
  }
  return { count, increment }
}

Use TypeScript to add types to your composable function's inputs and outputs.

Use Vue's Ref type to type reactive references.

Examples

This composable manages a string message with proper typing for the reactive ref and the setter function.

Vue

import { ref, Ref } from 'vue'

function useMessage(): { message: Ref<string>, setMessage: (newMsg: string) => void } {
  const message = ref('Hello')
  function setMessage(newMsg: string) {
    message.value = newMsg
  }
  return { message, setMessage }
}

This composable toggles a boolean state with typed input and output.

Vue

import { ref, Ref } from 'vue'

function useToggle(initial: boolean = false): { state: Ref<boolean>, toggle: () => void } {
  const state = ref(initial)
  function toggle() {
    state.value = !state.value
  }
  return { state, toggle }
}

Sample Program

This example shows a typed composable useCounter that returns a number ref and an increment function. We use it and print the count before and after incrementing.

Vue

import { ref, Ref } from 'vue'

// Composable to manage a number counter
function useCounter(): { count: Ref<number>, increment: () => void } {
  const count = ref(0)
  function increment() {
    count.value++
  }
  return { count, increment }
}

// Using the composable
const { count, increment } = useCounter()
console.log(count.value) // 0
increment()
console.log(count.value) // 1

Important Notes

Always import Vue types like Ref when typing reactive variables.

Typing composables improves code safety and editor support.

You can also type parameters your composable accepts for more flexibility.

Summary

Typing composables means adding TypeScript types to your Vue composable functions.

This helps catch errors and makes your code easier to understand.

Use Vue's Ref type for reactive data inside composables.

Practice

1. What is the main purpose of typing composables in Vue with TypeScript?

easy

A. To catch errors early and improve code understanding

B. To make the app run faster

C. To reduce the size of the final bundle

D. To avoid using reactive data

Solution

1. Step 1: Understand typing benefits

   Typing helps catch mistakes before running the code and clarifies what data types are used.

2. Step 2: Compare options

   Only To catch errors early and improve code understanding correctly states the purpose of typing composables; others are unrelated benefits.

3. Final Answer:

   To catch errors early and improve code understanding -> Option A

4. Quick Check:

   Typing composables = catch errors early [OK]

Hint: Typing helps find mistakes before running code [OK]

Common Mistakes:

• Thinking typing improves app speed
• Confusing typing with bundle size reduction
• Believing typing removes reactive data

2. Which of the following is the correct way to type a reactive number in a Vue composable using TypeScript?

easy

A. const count: Ref<number> = ref(0);

B. const count: reactive<number> = reactive(0);

C. const count: number = ref(0);

D. const count: Ref = ref('0');

Solution

1. Step 1: Identify correct typing for reactive refs

   Vue uses Ref<Type> to type reactive refs, so Ref<number> is correct for a number.

2. Step 2: Check each option

   const count: Ref<number> = ref(0); correctly types count as Ref<number>. const count: reactive<number> = reactive(0); uses wrong syntax, C assigns ref to number type, D uses wrong type and string value.

3. Final Answer:

   const count: Ref<number> = ref(0); -> Option A

4. Quick Check:

   Use Ref<Type> for reactive refs [OK]

Hint: Use Ref<Type> to type reactive refs in composables [OK]

Common Mistakes:

• Using reactive instead of Ref for refs
• Assigning ref() to plain types
• Mismatching value type and Ref generic

3. Given this composable code, what is the type of message inside the component?

import { ref } from 'vue';

export function useMessage() {
  const message = ref('Hello');
  return { message };
}

medium

A. string

B. Ref<string>

C. Ref<any>

D. any

Solution

1. Step 1: Understand ref return type

   The ref function returns a Ref object wrapping the value type, here a string.

2. Step 2: Determine message type

   Since message is created by ref('Hello'), its type is Ref<string>.

3. Final Answer:

   Ref<string> -> Option B

4. Quick Check:

   ref('text') returns Ref<string> [OK]

Hint: ref('text') returns Ref<string> type [OK]

Common Mistakes:

• Assuming message is plain string
• Using Ref<any> without specifying type
• Confusing Ref<string> with string

4. Identify the error in this typed composable code:

import { ref, Ref } from 'vue';

export function useCounter(): { count: Ref<number> } {
  const count = ref('0');
  return { count };
}

medium

A. Ref type is not imported

B. Return type is missing

C. count should be reactive, not ref

D. ref is called with a string instead of a number

Solution

1. Step 1: Check ref argument type

   The function expects count to be Ref<number>, but ref is called with a string '0'.

2. Step 2: Identify mismatch error

   This causes a type mismatch error because the initial value type does not match the declared Ref generic.

3. Final Answer:

   ref is called with a string instead of a number -> Option D

4. Quick Check:

   ref argument type must match Ref generic [OK]

Hint: Check ref initial value matches declared Ref type [OK]

Common Mistakes:

• Passing wrong type to ref()
• Ignoring return type mismatch
• Confusing reactive and ref usage

5. How would you correctly type a composable that returns a reactive object with a string title and a number count using Vue's Ref type?

import { ref, Ref } from 'vue';

export function useData() {
  const title = ref('My Title');
  const count = ref(10);
  return { title, count };
}

hard

A. export function useData(): { title: reactive<string>; count: reactive<number> }

B. export function useData(): { title: string; count: number }

C. export function useData(): { title: Ref<string>; count: Ref<number> }

D. export function useData(): { title: Ref<any>; count: Ref<any> }

Solution

1. Step 1: Identify types of returned refs

   Both title and count are refs holding string and number respectively, so their types are Ref<string> and Ref<number>.

2. Step 2: Match return type signature

   The function return type should reflect these as an object with keys typed as Ref<string> and Ref<number>.

3. Final Answer:

   export function useData(): { title: Ref<string>; count: Ref<number> } -> Option C

4. Quick Check:

   Return type matches Ref types of reactive data [OK]

Hint: Return type must match Ref types of reactive variables [OK]

Common Mistakes:

• Typing return values as plain string or number
• Using Ref<any> instead of specific types
• Using reactive instead of Ref in type