Why Composables for reusable logic in Vue? - Purpose & Use Cases

The Big Idea

Discover how a simple function can save you hours of repetitive work and bugs!

The Scenario

Imagine you write the same code to handle user input validation in multiple Vue components. Every time you want to change the logic, you must update it in many places.

The Problem

Copying and pasting code everywhere makes your app hard to maintain. It's easy to forget one spot, causing bugs. It also wastes time and clutters your components.

The Solution

Composables let you write reusable logic once and share it across components. You keep your code clean, easy to update, and components focused on UI.

Before vs After

✗ Before

function validate() { /* repeated in many components */ }

✓ After

export function useValidation() { /* reusable logic here */ }

What It Enables

You can build cleaner, more maintainable Vue apps by sharing logic easily without repeating yourself.

Real Life Example

Imagine a form validation logic used in signup, login, and profile update forms—all using the same composable to keep validation consistent.

Key Takeaways

Manual code repetition causes bugs and wastes time.

Composables let you share logic cleanly across components.

This makes your app easier to maintain and update.

Practice

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

easy

A. To reuse reactive logic across multiple components

B. To define global CSS styles

C. To create new Vue directives

D. To register Vue components globally

Solution

1. Step 1: Understand composables role

   Composables are functions that encapsulate reactive logic to be reused.

2. Step 2: Identify main purpose

   They help share logic like state or effects across components without duplication.

3. Final Answer:

   To reuse reactive logic across multiple components -> Option A

4. Quick Check:

   Composable = reusable logic [OK]

Hint: Composables share logic, not styles or components [OK]

Common Mistakes:

• Confusing composables with CSS or templates
• Thinking composables register components
• Assuming composables create directives

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

easy

A. function useCounter() { return reactive({ count: 0 }) }

B. const useCounter = new Vue({ data() { return { count: 0 } } })

C. const useCounter = reactive({ count: 0 })

D. function useCounter() { const count = ref(0); return { count }; }

Solution

1. Step 1: Check composable function syntax

   A composable is a function that returns reactive state or methods.

2. Step 2: Validate options

   function useCounter() { const count = ref(0); return { count }; } defines a function returning a ref wrapped count, which is correct. const useCounter = new Vue({ data() { return { count: 0 } } }) wrongly uses Vue instance syntax. const useCounter = reactive({ count: 0 }) returns reactive object directly without function. function useCounter() { return reactive({ count: 0 }) } returns reactive but misses ref usage and best practice.

3. Final Answer:

   function useCounter() { const count = ref(0); return { count }; } -> Option D

4. Quick Check:

   Composable = function returning reactive state [OK]

Hint: Composable must be a function returning reactive data [OK]

Common Mistakes:

• Defining composables as Vue instances
• Returning reactive objects without a function
• Not using ref or reactive inside composables

3. Given this composable:

import { ref } from 'vue';
export function useToggle() {
  const isOn = ref(false);
  function toggle() { isOn.value = !isOn.value; }
  return { isOn, toggle };
}

What will be the value of isOn.value after calling toggle() twice starting from initial state?

medium

A. true

B. undefined

C. false

D. null

Solution

1. Step 1: Analyze initial state and toggle function

   Initially, isOn.value is false. Each toggle flips it to the opposite.

2. Step 2: Apply two toggles

   First toggle: false -> true. Second toggle: true -> false.

3. Final Answer:

   false -> Option C

4. Quick Check:

   Two toggles flip false twice back to false [OK]

Hint: Toggle flips boolean; even toggles return original [OK]

Common Mistakes:

• Assuming toggle sets true always
• Forgetting to access .value of ref
• Confusing initial state with toggled state

4. What is wrong with this composable code?

import { ref } from 'vue';
export function useCount() {
  const count = ref(0);
  function increment() {
    count++;
  }
  return { count, increment };
}

medium

A. The count variable should be declared with reactive()

B. You cannot increment a ref directly with ++ operator

C. ref should be imported from 'vue/reactive' instead

D. The function should be named useCounter, not useCount

Solution

1. Step 1: Understand ref usage

   Ref values are accessed and mutated via the .value property.

2. Step 2: Identify error in increment function

   The code uses count++ which tries to increment the ref object itself, not its value. It should be count.value++.

3. Final Answer:

   You cannot increment a ref directly with ++ operator -> Option B

4. Quick Check:

   Use count.value++ to update ref [OK]

Hint: Always use .value to read or write ref values [OK]

Common Mistakes:

• Incrementing ref without .value
• Confusing reactive and ref imports
• Thinking function name affects behavior

5. You want to create a composable that tracks window width and updates reactively on resize. Which approach correctly implements this?

hard

A. function useWindowWidth() {
const width = ref(window.innerWidth);
onMounted(() => {
window.addEventListener('resize', () => {
width.value = window.innerWidth;
});
});
onUnmounted(() => {
window.removeEventListener('resize', () => {
width.value = window.innerWidth;
});
});
return { width };
}

B. function useWindowWidth() {
const width = ref(window.innerWidth);
window.addEventListener('resize', () => {
width.value = window.innerWidth;
});
return { width };
}

C. function useWindowWidth() {
const width = reactive({ value: window.innerWidth });
window.onresize = () => {
width.value = window.innerWidth;
};
return width;
}

D. function useWindowWidth() {
const width = ref(0);
window.addEventListener('resize', () => {
width = window.innerWidth;
});
return { width };
}

Solution

1. Step 1: Understand reactive window width tracking

   We need a ref to hold width and update it on resize events.

2. Step 2: Check event listener lifecycle management

   function useWindowWidth() { const width = ref(window.innerWidth); onMounted(() => { window.addEventListener('resize', () => { width.value = window.innerWidth; }); }); onUnmounted(() => { window.removeEventListener('resize', () => { width.value = window.innerWidth; }); }); return { width }; } correctly adds listener on mount and removes it on unmount to avoid leaks. function useWindowWidth() { const width = ref(window.innerWidth); window.addEventListener('resize', () => { width.value = window.innerWidth; }); return { width }; } adds listener immediately without cleanup. function useWindowWidth() { const width = reactive({ value: window.innerWidth }); window.onresize = () => { width.value = window.innerWidth; }; return width; } uses reactive incorrectly and overwrites window.onresize. function useWindowWidth() { const width = ref(0); window.addEventListener('resize', () => { width = window.innerWidth; }); return { width }; } tries to assign to ref variable directly, which is invalid.

3. Final Answer:

   function useWindowWidth() { const width = ref(window.innerWidth); onMounted(() => { window.addEventListener('resize', () => { width.value = window.innerWidth; }); }); onUnmounted(() => { window.removeEventListener('resize', () => { width.value = window.innerWidth; }); }); return { width }; } -> Option A

4. Quick Check:

   Manage listeners with lifecycle hooks [OK]

Hint: Add and remove listeners inside lifecycle hooks [OK]

Common Mistakes:

• Not removing event listeners causing memory leaks
• Assigning directly to ref variable instead of .value
• Overwriting window.onresize instead of adding listener