Composables for reusable logic in Vue

Introduction

Composables help you share and reuse code easily in Vue. They keep your code clean and organized.

You want to share a piece of logic between multiple components.

You need to keep your component code simple and focused.

You want to separate concerns like fetching data or handling user input.

You want to test logic separately from UI.

You want to avoid repeating the same code in different places.

Syntax

Vue

import { ref } from 'vue';

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

  function increment() {
    state.value++;
  }

  return { state, increment };
}

Composables are regular JavaScript functions that use Vue's reactive features.

They start with 'use' by convention to show they provide reusable logic.

Examples

This composable provides a simple counter with a number and a function to increase it.

Vue

import { ref } from 'vue';

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

This composable tracks the window width and updates reactively when the window resizes.

Vue

import { ref, onMounted, onUnmounted } from 'vue';

export function useWindowWidth() {
  const width = ref(window.innerWidth);

  function updateWidth() {
    width.value = window.innerWidth;
  }

  onMounted(() => {
    window.addEventListener('resize', updateWidth);
  });

  onUnmounted(() => {
    window.removeEventListener('resize', updateWidth);
  });

  return { width };
}

Sample Program

This Vue component uses a composable useToggle to manage a toggle state. The button text changes between ON and OFF when clicked. The aria-pressed attribute improves accessibility by indicating the toggle state to screen readers.

Vue

<script setup>
import { ref } from 'vue';

function useToggle(initial = false) {
  const isOn = ref(initial);
  function toggle() {
    isOn.value = !isOn.value;
  }
  return { isOn, toggle };
}

const { isOn, toggle } = useToggle();
</script>

<template>
  <button @click="toggle" :aria-pressed="isOn">
    {{ isOn ? 'ON' : 'OFF' }}
  </button>
</template>

Important Notes

Always name composables starting with use to keep code clear.

Composables can use Vue's reactive features like ref and reactive.

Use composables to keep components focused on UI, not logic.

Summary

Composables let you reuse logic easily across Vue components.

They are simple functions using Vue's reactive tools.

Use composables to keep your code clean and maintainable.

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