Lifecycle hooks in Composition API in Vue - Step-by-Step Execution

Concept Flow - Lifecycle hooks in Composition API

Setup function runs

↓

onBeforeMount hook

↓

Component mounts to DOM

↓

onMounted hook

↓

Reactive data changes?

↓

Component updates

↓

onBeforeUpdate hook

↓

DOM updates

↓

onUpdated hook

↓

Component unmounts

↓

onBeforeUnmount hook

↓

onUnmounted hook

↓

Cleanup done

This flow shows how Vue Composition API lifecycle hooks run in order during component setup, mounting, updating, and unmounting.

Execution Sample

Vue

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

export default {
  setup() {
    const count = ref(0);
    onMounted(() => console.log('Mounted'));
    onBeforeUnmount(() => console.log('Before Unmount'));
    return { count };
  }
}

This code sets up a reactive count and logs messages when the component mounts and before it unmounts.

Execution Table

Step	Hook Triggered	Action	Console Output	Component State
1	setup()	Initialize count=0		count=0
2	onMounted	Run mounted callback	Mounted	count=0
3	Reactive change	count incremented to 1		count=1
4	onBeforeUpdate	Before DOM updates		count=1
5	DOM updates	Render new count		count=1
6	onUpdated	After DOM updates		count=1
7	onBeforeUnmount	Run before unmount callback	Before Unmount	count=1
8	onUnmounted	Cleanup after unmount		count=1

💡 Component unmounted, lifecycle ends

Variable Tracker

Variable	Start	After Step 3	Final
count	0	1	1

Key Moments - 3 Insights

Why does onMounted run after setup but before reactive changes?

Because setup initializes state first, then onMounted runs after the component is attached to the DOM but before reactive changes trigger updates, as shown in steps 1 and 2.

What happens if reactive data changes after mounting?

The onBeforeUpdate and onUpdated hooks run around the DOM update to reflect the new state, as seen in steps 3 to 6.

When do onBeforeUnmount and onUnmounted run?

They run just before and after the component is removed from the DOM, allowing cleanup, as shown in steps 7 and 8.

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 2

💡 Hint

Check the 'Component State' column at step 3 in the execution_table.

At which step does the component first log 'Mounted'?

A Step 1

B Step 2

C Step 5

D Step 7

💡 Hint

Look at the 'Console Output' column to find when 'Mounted' appears.

If the count never changes, which hooks related to updates will NOT run?

A onMounted and onBeforeUnmount

B onBeforeUnmount and onUnmounted

C onBeforeUpdate and onUpdated

D setup and onMounted

💡 Hint

Refer to steps 4 and 6 where update hooks run after reactive changes.

Concept Snapshot

Vue Composition API lifecycle hooks:
- setup() runs first to initialize state
- onMounted runs after component mounts
- onBeforeUpdate and onUpdated run around reactive updates
- onBeforeUnmount and onUnmounted run during unmount
Use these hooks to run code at key component moments.

Full Transcript

This visual trace shows how Vue's Composition API lifecycle hooks work step-by-step. First, the setup function runs to create reactive data. Then onMounted runs after the component is attached to the page. When reactive data changes, onBeforeUpdate runs before the DOM updates, and onUpdated runs after. Finally, when the component is removed, onBeforeUnmount and onUnmounted run to allow cleanup. The execution table tracks these steps with state and console outputs, helping beginners see exactly when each hook triggers during the component's life.