Composable with lifecycle hooks in Vue - Mini Project: Build & Apply

Composable with lifecycle hooks

📖 Scenario: You are building a Vue 3 app that needs to track the window width and update it reactively when the window is resized.

🎯 Goal: Create a composable function that uses Vue lifecycle hooks to track and update the window width reactively.

📋 What You'll Learn

Create a reactive variable to store the window width

Use the onMounted lifecycle hook to add a window resize event listener

Use the onUnmounted lifecycle hook to remove the event listener

Return the reactive window width variable from the composable

💡 Why This Matters

🌍 Real World

Tracking window size is common in responsive design to adapt UI elements dynamically.

💼 Career

Understanding composables and lifecycle hooks is essential for building maintainable Vue 3 applications.

1

Create a reactive variable for window width

Create a composable function called useWindowWidth that defines a reactive variable width initialized to window.innerWidth using Vue's ref.

Vue

import { ref } from 'vue'

// Your code here

Hint

Use ref from Vue to create a reactive variable.

2

Add onMounted lifecycle hook to listen for resize

Inside the useWindowWidth function, import and use Vue's onMounted hook to add a resize event listener on window. The listener should update width.value to window.innerWidth.

Vue

import { ref, onMounted } from 'vue'

export function useWindowWidth() {
  const width = ref(window.innerWidth)
  // Your code here
}

Hint

Use onMounted to add the event listener only after the component is mounted.

3

Add onUnmounted lifecycle hook to remove listener

Import Vue's onUnmounted hook and use it inside useWindowWidth to remove the resize event listener from window when the component unmounts. Store the listener function in a variable called updateWidth.

Vue

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

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

  onMounted(() => {
    // Your code here
  })

  // Your code here
}

Hint

Define the listener function separately so you can remove it later.

4

Return the reactive width variable

At the end of the useWindowWidth function, return an object with the width reactive variable so components can use it.

Vue

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

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

  const updateWidth = () => {
    width.value = window.innerWidth
  }

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

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

  // Your code here
}

Hint

Return the reactive variable inside an object for easy destructuring.

Practice

1. What is the main purpose of using lifecycle hooks inside a Vue composable?

easy

A. To run code automatically when a component starts or stops

B. To define the component's template structure

C. To style the component with CSS

D. To create new Vue components

Solution

1. Step 1: Understand lifecycle hooks role

   Lifecycle hooks let you run code at specific times in a component's life, like when it starts or stops.

2. Step 2: Connect lifecycle hooks with composables

   Using lifecycle hooks inside composables allows shared logic to run automatically when components use the composable.

3. Final Answer:

   To run code automatically when a component starts or stops -> Option A

4. Quick Check:

   Lifecycle hooks run code at component start/stop = A [OK]

Hint: Lifecycle hooks run code on component start or cleanup [OK]

Common Mistakes:

• Thinking lifecycle hooks define templates
• Confusing styling with lifecycle hooks
• Believing composables create components

2. Which of the following is the correct way to import and use the onMounted lifecycle hook inside a Vue 3 composable?

easy

A. import { onMounted } from 'vue'; onMounted = () => { /* code */ };

B. import onMounted from 'vue'; onMounted(() => { /* code */ });

C. import { onMounted } from 'vue'; onMounted(() => { /* code */ });

D. import { onMounted } from 'vue'; onMounted.function(() => { /* code */ });

Solution

1. Step 1: Check import syntax

   Vue lifecycle hooks like onMounted are named exports, so use curly braces in import.

2. Step 2: Check usage syntax

   onMounted is a function that takes a callback, so call it with a function argument.

3. Final Answer:

   import { onMounted } from 'vue'; onMounted(() => { /* code */ }); -> Option C

4. Quick Check:

   Correct import and call syntax = A [OK]

Hint: Use curly braces to import lifecycle hooks from 'vue' [OK]

Common Mistakes:

• Importing without curly braces
• Assigning onMounted to a function instead of calling it
• Using incorrect method like onMounted.function

3. Given this composable code, what will be logged when a component using it is mounted?

import { ref, onMounted } from 'vue';
export function useLogger() {
  const message = ref('Hello');
  onMounted(() => {
    console.log(message.value);
  });
  return { message };
}

medium

A. Hello

B. undefined

C. null

D. An error is thrown

Solution

1. Step 1: Understand ref initialization

   The message ref is initialized with the string 'Hello'.

2. Step 2: Check onMounted callback

   When the component mounts, the callback logs message.value, which is 'Hello'.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Ref value logged on mount = B [OK]

Hint: Ref values accessed with .value inside lifecycle hooks [OK]

Common Mistakes:

• Logging ref without .value
• Expecting undefined before mount
• Thinking onMounted runs before ref initialization

4. Identify the error in this composable using lifecycle hooks:

import { onMounted, onUnmounted } from 'vue';
export function useTimer() {
  let timer = null;
  onMounted(() => {
    timer = setInterval(() => console.log('tick'), 1000);
  });
  onUnmounted(() => {
    clearInterval(timer);
  });
  return {};
}

medium

A. timer should be a ref to persist between hooks

B. onUnmounted should be onBeforeUnmount

C. setInterval callback syntax is wrong

D. No error, code works correctly

Solution

1. Step 1: Check variable scope and usage

   Variable timer is declared in composable scope and updated in onMounted, accessible in onUnmounted.

2. Step 2: Verify lifecycle hooks usage

   onMounted sets interval, onUnmounted clears it correctly to avoid memory leaks.

3. Final Answer:

   No error, code works correctly -> Option D

4. Quick Check:

   Proper timer setup and cleanup = D [OK]

Hint: Variables in composable scope persist across lifecycle hooks [OK]

Common Mistakes:

• Thinking timer must be a ref
• Confusing onUnmounted with onBeforeUnmount
• Misunderstanding setInterval callback syntax

5. You want to create a composable that fetches data when a component mounts and cancels the fetch if the component unmounts before completion. Which lifecycle hooks and pattern should you use inside the composable?

hard

A. Use onUnmounted to start fetch and onMounted to abort fetch

B. Use onMounted to start fetch and onUnmounted to abort fetch with an AbortController

C. Use onBeforeMount to start fetch and onMounted to abort fetch

D. Use onMounted to start fetch and no cleanup is needed

Solution

1. Step 1: Determine when to start fetch

   Fetching data should start when the component is fully mounted, so use onMounted.

2. Step 2: Handle cleanup on unmount

   To cancel fetch if component unmounts early, use onUnmounted to abort the fetch using an AbortController.

3. Final Answer:

   Use onMounted to start fetch and onUnmounted to abort fetch with an AbortController -> Option B

4. Quick Check:

   Fetch start on mount, abort on unmount = C [OK]

Hint: Start fetch on mount, abort on unmount with AbortController [OK]

Common Mistakes:

• Starting fetch too early or too late
• Not aborting fetch on unmount causing leaks
• Using wrong lifecycle hooks for cleanup