Composable with lifecycle hooks in Vue - Step-by-Step Execution

Concept Flow - Composable with lifecycle hooks

Start Component Setup

↓

Call Composable Function

↓

Inside Composable: Setup Reactive State

↓

Register Lifecycle Hook (e.g., onMounted)

↓

Return State and Methods

↓

Component Uses Returned State

↓

Component Mounts

↓

Lifecycle Hook Runs

↓

Effect Happens (e.g., console log, data fetch)

↓

Component Renders with Updated State

The component calls a composable that sets up reactive state and registers lifecycle hooks. When the component mounts, the lifecycle hook inside the composable runs, triggering side effects.

Execution Sample

Vue

import { ref, onMounted } from 'vue'

function useCounter() {
  const count = ref(0)
  onMounted(() => {
    console.log('Mounted!')
  })
  return { count }
}

This composable creates a reactive count and logs 'Mounted!' when the component using it mounts.

Execution Table

Step	Action	State Before	State After	Effect
1	Component setup starts	No state	No state	No effect
2	Call useCounter composable	No state	count = 0	No effect
3	Register onMounted hook	count = 0	count = 0	Hook registered
4	Composable returns { count }	count = 0	count = 0	No effect
5	Component mounts	count = 0	count = 0	Trigger onMounted hook
6	onMounted hook runs	count = 0	count = 0	Console logs 'Mounted!'
7	Component renders with count=0	count = 0	count = 0	UI shows count 0

💡 Lifecycle hook runs once on component mount, then normal rendering continues.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
count	undefined	0	0	0	0

Key Moments - 2 Insights

Why does the console.log inside onMounted run only after the component mounts?

Because onMounted registers a hook that runs only when the component finishes mounting, as shown in step 5 and 6 of the execution_table.

Is the reactive state 'count' available before the component mounts?

Yes, 'count' is created during composable call in setup (step 2), but lifecycle hooks run later (step 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'count' after step 3?

A 1

B undefined

C 0

D null

💡 Hint

Check the 'State After' column for step 3 in execution_table.

At which step does the onMounted hook actually run?

A Step 2

B Step 6

C Step 4

D Step 7

💡 Hint

Look for the step where the effect 'Console logs "Mounted!"' happens in execution_table.

If we add a reactive variable inside onMounted instead of setup, when will it be available?

A Only after component mounts

B Immediately during setup

C Before component setup

D Never

💡 Hint

Recall that onMounted runs after mounting, so variables created there appear later.

Concept Snapshot

Composable with lifecycle hooks in Vue:
- Define reactive state inside composable using ref/reactive.
- Register lifecycle hooks like onMounted inside composable.
- Return state and methods from composable.
- Hooks run when component lifecycle reaches that phase.
- Enables reusable logic with side effects tied to component lifecycle.

Full Transcript

In Vue, a composable is a function that sets up reactive state and can register lifecycle hooks like onMounted. When a component calls this composable during its setup phase, the reactive state is created immediately. However, lifecycle hooks registered inside the composable only run when the component reaches that lifecycle phase, for example, when it mounts. This allows the composable to perform side effects such as logging or fetching data exactly when the component is ready. The execution table shows the step-by-step process: the component starts setup, calls the composable which creates state and registers hooks, then when the component mounts, the onMounted hook runs and triggers effects. This pattern helps organize reusable logic with lifecycle awareness.

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