Composable with lifecycle hooks in Vue - Performance & Optimization

Performance: Composable with lifecycle hooks

MEDIUM IMPACT

This affects how efficiently Vue components manage side effects and resource cleanup during rendering and interaction.

Managing side effects and cleanup in Vue components using composables

Vue

import { onMounted, onUnmounted } from 'vue';

export function useTimer() {
  let intervalId = null;
  onMounted(() => {
    intervalId = setInterval(() => {
      console.log('tick');
    }, 1000);
  });
  onUnmounted(() => {
    clearInterval(intervalId);
  });
}

Properly clears the interval when the component unmounts, freeing resources and avoiding unnecessary work.

📈 Performance GainPrevents memory leaks and reduces CPU usage, improving interaction responsiveness.

Managing side effects and cleanup in Vue components using composables

Vue

import { onMounted, onUnmounted } from 'vue';

export function useTimer() {
  let intervalId = null;
  onMounted(() => {
    intervalId = setInterval(() => {
      console.log('tick');
    }, 1000);
  });
  // Missing cleanup on unmount
}

Not cleaning up the interval on component unmount causes memory leaks and unnecessary CPU usage.

📉 Performance CostCauses continuous background work, increasing CPU usage and reducing responsiveness.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Composable without cleanup	Minimal	0	Low but ongoing CPU usage	[X] Bad
Composable with proper lifecycle cleanup	Minimal	0	Low and stops when unused	[OK] Good

Rendering Pipeline

Lifecycle hooks in composables run during component setup and teardown, affecting when side effects start and stop in the rendering process.

→Setup

→Cleanup

→Reactive Updates

⚠️ BottleneckUnmanaged side effects causing background work after component unmount

Core Web Vital Affected

INP

This affects how efficiently Vue components manage side effects and resource cleanup during rendering and interaction.

Optimization Tips

1Always pair side effects with cleanup in lifecycle hooks to avoid leaks.

2Use onMounted and onUnmounted to start and stop timers or subscriptions.

3Proper lifecycle management improves interaction responsiveness (INP).

Performance Quiz - 3 Questions

What is the main performance risk of not using onUnmounted in a composable with side effects?

A Increased DOM nodes

B Memory leaks and unnecessary CPU usage

C Slower initial page load

D Longer CSS calculation times

DevTools: Performance

How to check: Record a session while mounting and unmounting the component using the composable. Look for ongoing CPU activity after unmount.

What to look for: CPU usage spikes or continuous timers running after component unmount indicate missing cleanup.

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