Why composables matter in Vue - See It in Action

Why composables matter

📖 Scenario: You are building a simple Vue 3 app that tracks a user's online status. You want to reuse this online status logic in multiple components without repeating code.

🎯 Goal: Create a composable function to manage online status and use it inside a Vue component to display if the user is online or offline.

📋 What You'll Learn

Create a composable function called useOnlineStatus that returns a reactive isOnline variable

Add a configuration variable checkInterval inside the composable to set how often to check the online status

Use setInterval inside the composable to update isOnline every checkInterval milliseconds

Create a Vue component that uses useOnlineStatus and shows Online or Offline text based on isOnline

💡 Why This Matters

🌍 Real World

Many apps need to know if the user is online or offline to adjust behavior, like disabling buttons or showing warnings.

💼 Career

Understanding composables helps you write clean, reusable Vue 3 code, a key skill for frontend developer roles.

1

Create the composable function

Create a composable function called useOnlineStatus that imports ref from 'vue' and defines a reactive variable isOnline initialized to navigator.onLine.

Vue

import { ref } from 'vue'

// Your code here

Hint

Use ref to create a reactive variable and initialize it with navigator.onLine.

2

Add a check interval variable

Inside the useOnlineStatus function, add a constant checkInterval and set it to 5000 (milliseconds).

Vue

import { ref } from 'vue'

export function useOnlineStatus() {
  const isOnline = ref(navigator.onLine)
  const checkInterval = 5000
  // Your code here
  return { isOnline }
}

Hint

Just add a constant named checkInterval with value 5000 inside the function.

3

Update online status periodically

Inside useOnlineStatus, use setInterval with checkInterval to update isOnline.value to navigator.onLine every interval.

Vue

import { ref } from 'vue'

export function useOnlineStatus() {
  const isOnline = ref(navigator.onLine)
  const checkInterval = 5000
  setInterval(() => {
    // Your code here
  }, checkInterval)
  return { isOnline, checkInterval }
}

Hint

Use setInterval to update isOnline.value inside the callback function.

4

Use the composable in a Vue component

Create a Vue component using <script setup> that imports useOnlineStatus, calls it, and displays Online if isOnline.value is true, else Offline inside a <p> tag.

Vue

import { useOnlineStatus } from './useOnlineStatus'

// Your code here



  

Hint

Use useOnlineStatus inside <script setup> and bind isOnline in the template with a conditional text.

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