Creating a custom composable in Vue

Introduction

A custom composable lets you reuse logic easily in Vue components. It keeps your code clean and organized.

You want to share reactive state or functions between multiple components.

You need to separate complex logic from the template for clarity.

You want to keep your components small and focused.

You want to reuse code like fetching data or handling user input.

You want to test logic separately from UI.

Syntax

Vue

import { ref, computed } from 'vue';

export function useCustomLogic() {
  const state = ref(0);

  function updateState(newValue) {
    state.value = newValue;
  }

  const derivedValue = computed(() => state.value * 2);

  return { state, updateState, derivedValue };
}

A composable is a regular function that uses Vue's reactive APIs like ref or computed.

It returns reactive data and functions to be used in components.

Examples

A simple counter composable that tracks a number and increases it.

Vue

import { ref } from 'vue';

export function useCounter() {
  const count = ref(0);
  function increment() {
    count.value++;
  }
  return { count, increment };
}

This composable logs input changes to the console.

Vue

import { ref, watch } from 'vue';

export function useInputLogger() {
  const input = ref('');
  watch(input, (newVal) => {
    console.log('Input changed to:', newVal);
  });
  return { input };
}

Sample Program

This Vue component uses the useCounter composable to show a button. Clicking it increases the count shown.

It uses semantic HTML and an ARIA label for accessibility.

Vue

<script setup>
import { useCounter } from './useCounter';

const { count, increment } = useCounter();
</script>

<template>
  <button @click="increment" aria-label="Increase count">Count is: {{ count }}</button>
</template>

Important Notes

Always prefix composable functions with use to follow Vue community conventions.

Keep composables focused on one task for easier reuse and testing.

Use ref for reactive variables and computed for derived values inside composables.

Summary

Custom composables help reuse reactive logic across Vue components.

They are simple functions using Vue's reactive APIs and return reactive data and methods.

Use composables to keep components clean, testable, and organized.

Practice

1. What is the main purpose of creating a custom composable in Vue?

easy

A. To define global CSS styles

B. To reuse reactive logic across multiple components

C. To create new Vue directives

D. To register components globally

Solution

1. Step 1: Understand what composables do

   Custom composables are functions that encapsulate reactive logic to be reused.

2. Step 2: Identify the purpose in Vue components

   They help share logic without repeating code in multiple components.

3. Final Answer:

   To reuse reactive logic across multiple components -> Option B

4. Quick Check:

   Reusable reactive logic = B [OK]

Hint: Composables = reusable reactive logic functions [OK]

Common Mistakes:

• Confusing composables with styling or directives
• Thinking composables register components
• Assuming composables handle templates

2. Which of the following is the correct way to start a custom composable function in Vue 3?

easy

A. function useData() { const count = ref(0); return { count }; }

B. const useData = () => { const count = reactive(0); return count; }

C. function useData() { const count = ref(0); }

D. const useData = () => { return ref({ count: 0 }); }

Solution

1. Step 1: Check function declaration and return

   function useData() { const count = ref(0); return { count }; } declares a function named with 'use' prefix and returns reactive refs.

2. Step 2: Validate reactive usage and return object

   function useData() { const count = ref(0); return { count }; } returns an object with the reactive ref, which is correct pattern.

3. Final Answer:

   function useData() { const count = ref(0); return { count }; } -> Option A

4. Quick Check:

   Composable returns reactive refs in object = D [OK]

Hint: Composable must return reactive data inside an object [OK]

Common Mistakes:

• Not returning reactive data
• Returning reactive primitive directly instead of object
• Missing 'use' prefix in function name

3. Given this composable code, what will be logged when calling increment() twice?

import { ref } from 'vue';

export function useCounter() {
  const count = ref(0);
  function increment() {
    count.value++;
    console.log(count.value);
  }
  return { count, increment };
}

medium

A. Undefined and then 1

B. 0 and then 1

C. 2 and then 3

D. 1 and then 2

Solution

1. Step 1: Understand initial count value

   The count starts at 0 as a ref.

2. Step 2: Trace increment calls and console logs

   Each increment increases count.value by 1 and logs it immediately.

3. Final Answer:

   1 and then 2 -> Option D

4. Quick Check:

   Increment twice logs 1 then 2 [OK]

Hint: Ref value increments and logs immediately [OK]

Common Mistakes:

• Confusing ref.value with ref itself
• Assuming count starts at 1
• Thinking console logs before increment

4. Identify the error in this composable code:

import { ref } from 'vue';

export function useToggle() {
  const state = ref(false);
  function toggle() {
    state = !state;
  }
  return { state, toggle };
}

medium

A. ref should be reactive instead

B. Missing return statement

C. Cannot reassign a const ref variable directly

D. toggle function should be async

Solution

1. Step 1: Check how ref is updated

   The code tries to assign 'state = !state', which reassigns the const variable.

2. Step 2: Correct way to update ref value

   Should update 'state.value = !state.value' instead of reassigning 'state'.

3. Final Answer:

   Cannot reassign a const ref variable directly -> Option C

4. Quick Check:

   Update ref.value, not ref variable [OK]

Hint: Update ref.value, never reassign ref variable [OK]

Common Mistakes:

• Reassigning ref variable instead of its value
• Forgetting to use .value to access ref
• Assuming reactive is needed instead of ref

5. You want to create a composable that fetches user data and exposes loading state. Which approach correctly combines reactive state and async fetch inside a composable?

hard

A. function useUser() {
const user = ref(null);
const loading = ref(true);
async function fetchUser() {
loading.value = true;
user.value = await fetch('/api/user').then(r => r.json());
loading.value = false;
}
fetchUser();
return { user, loading };
}

B. function useUser() {
const user = reactive(null);
const loading = false;
async function fetchUser() {
loading = true;
user = await fetch('/api/user').then(r => r.json());
loading = false;
}
fetchUser();
return { user, loading };
}

C. function useUser() {
const user = ref(null);
const loading = ref(false);
function fetchUser() {
loading.value = true;
fetch('/api/user').then(r => r.json()).then(data => {
user.value = data;
loading.value = false;
});
}
return { user, loading, fetchUser };
}

D. function useUser() {
const user = ref(null);
const loading = ref(true);
fetch('/api/user').then(r => r.json()).then(data => {
user = data;
loading.value = false;
});
return { user, loading };
}

Solution

1. Step 1: Check reactive state initialization

   function useUser() { const user = ref(null); const loading = ref(true); async function fetchUser() { loading.value = true; user.value = await fetch('/api/user').then(r => r.json()); loading.value = false; } fetchUser(); return { user, loading }; } correctly uses ref for user and loading states.

2. Step 2: Verify async fetch and state updates

   function useUser() { const user = ref(null); const loading = ref(true); async function fetchUser() { loading.value = true; user.value = await fetch('/api/user').then(r => r.json()); loading.value = false; } fetchUser(); return { user, loading }; } uses async function to set loading true, await fetch, then set loading false.

3. Step 3: Confirm fetch is called inside composable

   function useUser() { const user = ref(null); const loading = ref(true); async function fetchUser() { loading.value = true; user.value = await fetch('/api/user').then(r => r.json()); loading.value = false; } fetchUser(); return { user, loading }; } calls fetchUser() immediately to start loading data.

4. Final Answer:

   function useUser() { const user = ref(null); const loading = ref(true); async function fetchUser() { loading.value = true; user.value = await fetch('/api/user').then(r => r.json()); loading.value = false; } fetchUser(); return { user, loading }; } -> Option A

5. Quick Check:

   Async fetch with reactive refs = A [OK]

Hint: Use ref for state and async function with await inside composable [OK]

Common Mistakes:

• Reassigning reactive or ref variables directly
• Not using async/await properly
• Not updating loading state correctly