Composable with lifecycle hooks in Vue - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composable in Vue 3?

A composable is a reusable function that uses Vue's Composition API to encapsulate and share reactive state and logic across components.

beginner

Which Vue lifecycle hook is used to run code when a component is mounted?

The onMounted lifecycle hook runs code after the component is inserted into the DOM.

intermediate

How do you use lifecycle hooks inside a composable?

You import the lifecycle hook functions like <code>onMounted</code> from Vue and call them inside the composable function to run code at specific component lifecycle stages.

intermediate

Why use lifecycle hooks inside composables?

Using lifecycle hooks inside composables lets you keep related logic and side effects together, making your code cleaner and easier to reuse across components.

intermediate

Example: What does this composable do?
<pre>import { ref, onMounted } from 'vue';
export function useTimer() {
  const time = ref(0);
  onMounted(() => {
    setInterval(() => {
      time.value++;
    }, 1000);
  });
  return { time };
}</pre>

This composable useTimer creates a reactive time value that increases by 1 every second after the component using it is mounted.

What is the purpose of the onUnmounted lifecycle hook in a composable?

A To update the template

B To run code before the component is mounted

C To initialize reactive state

D To run code when the component is removed from the DOM

onUnmounted runs cleanup code when the component is removed from the DOM, such as clearing timers or event listeners.

Where do you import lifecycle hooks like onMounted when creating a composable?

A From 'axios'

B From 'vue-router'

C From 'vue' package

D From 'vuex'

Lifecycle hooks are imported directly from the core vue package.

What happens if you call onMounted outside a composable or component setup function?

A It will cause an error or not work properly

B It will run immediately

C It will delay the component mounting

D It will update the DOM

Lifecycle hooks must be called inside setup functions or composables used in setup; otherwise, Vue cannot link them to the component lifecycle.

Which of these is a benefit of using composables with lifecycle hooks?

A Better code reuse and organization

B Slower app performance

C More complex templates

D Less reactive state

Composables with lifecycle hooks help organize logic and reuse it easily across components.

In the example composable useTimer, when does the timer start counting?

A Immediately when the composable is imported

B After the component is mounted

C Before the component is created

D When the component is unmounted

The timer starts inside the onMounted hook, so it begins after the component is mounted.

Explain how to create a composable in Vue that uses lifecycle hooks to start and stop a timer.

Think about how to manage side effects with lifecycle hooks inside a reusable function.

Describe why lifecycle hooks are important when writing composables in Vue.

Consider what happens when components mount and unmount.

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