Why composables matter in Vue

Introduction

Composables help you reuse code easily in Vue. They keep your code clean and organized by sharing logic across components.

You want to share the same logic between multiple components without repeating code.

You need to separate complex logic from the UI to make your components simpler.

You want to test your logic independently from the component.

You want to keep your code organized as your app grows bigger.

You want to improve readability by grouping related logic in one place.

Syntax

Vue

import { ref } from 'vue'

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

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

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

Examples

This composable manages a simple toggle state that can be reused in any component.

Vue

import { ref } from 'vue'

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

This composable tracks the window width reactively and updates 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 the useCounter composable to add a counter button. Clicking the button increases the count.

Vue

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

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

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

Important Notes

Always name composables starting with 'use' to keep your code clear.

Composables help separate logic from UI, making components easier to read and maintain.

Use composables to share logic without copying code between components.

Summary

Composables let you reuse logic easily in Vue apps.

They keep your components simple and clean.

Use composables to organize and share code better.

Practice

1. Why are composables important in Vue applications?

easy

A. They compile Vue templates into JavaScript.

B. They automatically style components without CSS.

C. They replace the need for Vue Router.

D. They help reuse logic across multiple components easily.

Solution

1. Step 1: Understand composables' purpose

   Composables are functions that let you reuse logic in Vue components.

2. Step 2: Identify the correct benefit

   They help keep components clean by sharing logic, not styling or routing.

3. Final Answer:

   They help reuse logic across multiple components easily. -> Option D

4. Quick Check:

   Reusing logic = composables importance [OK]

Hint: Composables = reusable logic functions in Vue [OK]

Common Mistakes:

• Thinking composables handle styling
• Confusing composables with routing
• Believing composables compile templates

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

easy

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

B. const useCounter = new VueComponent() { count: 0 }

C. class useCounter { count = 0; }

D. const useCounter = () => { this.count = 0; }

Solution

1. Step 1: Recall composable syntax

   Composables are functions that return reactive state or methods.

2. Step 2: Check each option

   function useCounter() { const count = ref(0); return { count }; } defines a function returning a reactive ref, which is correct. Others misuse class, VueComponent, or this context.

3. Final Answer:

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

4. Quick Check:

   Composable = function returning reactive state [OK]

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

Common Mistakes:

• Using class syntax for composables
• Trying to use 'this' inside arrow functions
• Confusing composables with components

3. Given this composable:

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

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

medium

A. undefined

B. true

C. false

D. null

Solution

1. Step 1: Understand initial state

   The ref on starts as false.

2. Step 2: Apply toggle twice

   First toggle flips false to true. Second toggle flips true back to false.

3. Final Answer:

   false -> Option C

4. Quick Check:

   Toggle twice returns to initial false [OK]

Hint: Toggle flips boolean; two flips return to start [OK]

Common Mistakes:

• Assuming toggle only flips once
• Confusing ref.value with ref itself
• Expecting undefined or null values

4. What is wrong with this composable code?

function useData() {
  const data = ref(null);
  fetch('api/data').then(response => {
    data.value = response.json();
  });
  return { data };
}

medium

A. fetch returns a promise, so data.value is set to a promise, not actual data.

B. ref cannot hold null values initially.

C. The composable should not return data.

D. fetch cannot be used inside composables.

Solution

1. Step 1: Analyze fetch usage

   fetch returns a promise; response.json() also returns a promise.

2. Step 2: Identify mistake in assigning data.value

   Assigning data.value = response.json() sets it to a promise, not resolved data.

3. Final Answer:

   fetch returns a promise, so data.value is set to a promise, not actual data. -> Option A

4. Quick Check:

   response.json() returns promise, must await [OK]

Hint: Remember to await response.json() before assigning [OK]

Common Mistakes:

• Not awaiting response.json()
• Thinking ref can't start as null
• Believing fetch is disallowed in composables

5. You want to share a composable that tracks window width and updates reactively on resize. Which approach correctly uses Vue composables to achieve this?

hard

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

B. 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 };
}

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

D. function useWindowWidth() {
const width = ref(0);
setInterval(() => {
width.value = window.innerWidth;
}, 1000);
return { width };
}

Solution

1. Step 1: Understand reactive state and lifecycle

   We need reactive width and proper event listener cleanup to avoid leaks.

2. Step 2: Evaluate options for 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 }; } uses onMounted and onUnmounted to add and remove listeners correctly, ensuring cleanup.

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 B

4. Quick Check:

   Use lifecycle hooks to add/remove listeners [OK]

Hint: Add and remove listeners in lifecycle hooks for cleanup [OK]

Common Mistakes:

• Not removing event listeners causing memory leaks
• Using non-reactive variables for width
• Using setInterval instead of event listeners