Composable with lifecycle hooks in Vue

Introduction

Composable functions help you reuse code in Vue components. Lifecycle hooks inside composables let you run code at key moments like when a component starts or stops.

You want to share logic like fetching data or timers between components.

You need to run setup or cleanup code when a component mounts or unmounts.

You want to keep your component code clean by moving side effects to composables.

You want to react to component lifecycle events inside reusable functions.

Syntax

Vue

import { onMounted, onUnmounted } from 'vue';

export function useExample() {
  onMounted(() => {
    // code to run when component mounts
  });

  onUnmounted(() => {
    // code to run when component unmounts
  });

  return {
    // reactive data or methods
  };
}

Use onMounted to run code after the component appears on screen.

Use onUnmounted to clean up when the component is removed.

Examples

This composable logs a message when the component mounts.

Vue

import { onMounted } from 'vue';

export function useLogger() {
  onMounted(() => {
    console.log('Component is ready');
  });
}

This composable counts seconds while the component is visible and stops when it unmounts.

Vue

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

export function useTimer() {
  const seconds = ref(0);
  let intervalId;

  onMounted(() => {
    intervalId = setInterval(() => {
      seconds.value++;
    }, 1000);
  });

  onUnmounted(() => {
    clearInterval(intervalId);
  });

  return { seconds };
}

Sample Program

This Vue component uses a composable useTimer that starts counting seconds when the component mounts and stops when it unmounts. The seconds count updates every second and shows on screen.

Vue

<template>
  <div>
    <p>Seconds passed: {{ seconds }}</p>
  </div>
</template>

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

function useTimer() {
  const seconds = ref(0);
  let intervalId;

  onMounted(() => {
    intervalId = setInterval(() => {
      seconds.value++;
    }, 1000);
  });

  onUnmounted(() => {
    clearInterval(intervalId);
  });

  return { seconds };
}

const { seconds } = useTimer();
</script>

Important Notes

Always clear timers or subscriptions in onUnmounted to avoid memory leaks.

Composable lifecycle hooks run in the component that calls them.

Summary

Composable functions let you reuse logic across components.

Use lifecycle hooks inside composables to run code at component start and cleanup.

This keeps your components clean and your code organized.

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