onBeforeMount and onBeforeUnmount in Vue - Step-by-Step Execution

Concept Flow - onBeforeMount and onBeforeUnmount

Component Setup

↓

onBeforeMount Hook

↓

Component Mounts to DOM

↓

Component Active

↓

Trigger Unmount

↓

onBeforeUnmount Hook

↓

Component Removed from DOM

This flow shows when Vue calls onBeforeMount before the component appears, and onBeforeUnmount just before it disappears.

Execution Sample

Vue

import { onBeforeMount, onBeforeUnmount } from 'vue';

export default {
  setup() {
    onBeforeMount(() => console.log('Before mount'));
    onBeforeUnmount(() => console.log('Before unmount'));
  }
};

This code logs messages right before the component mounts and right before it unmounts.

Execution Table

Step	Hook Triggered	Action	Console Output	Component State
1	Setup	Register hooks		Not mounted
2	onBeforeMount	Runs before mounting	Before mount	Not mounted yet
3	Mount	Component added to DOM		Mounted and visible
4	Active	Component running		Mounted and visible
5	Trigger unmount	Start unmount process		Mounted and visible
6	onBeforeUnmount	Runs before unmounting	Before unmount	Still mounted
7	Unmount	Component removed from DOM		Not mounted

💡 Component unmounted, lifecycle hooks completed

Variable Tracker

Variable	Start	After onBeforeMount	After Mount	After onBeforeUnmount	After Unmount
componentState	Not mounted	Not mounted yet	Mounted and visible	Still mounted	Not mounted

Key Moments - 2 Insights

Why does onBeforeMount run before the component is visible?

Because onBeforeMount is designed to run right before the component is added to the DOM, as shown in execution_table step 2.

Does onBeforeUnmount run after the component is removed from the DOM?

No, onBeforeUnmount runs just before the component is removed, as shown in execution_table step 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the component state when onBeforeMount runs?

A Mounted and visible

B Not mounted yet

C Still mounted

D Not mounted

💡 Hint

Check the 'Component State' column at step 2 in the execution_table.

At which step does the component get removed from the DOM?

A Step 7

B Step 5

C Step 6

D Step 4

💡 Hint

Look for the 'Unmount' action in the execution_table.

If onBeforeUnmount was removed, what would change in the execution table?

A onBeforeMount would not run

B Component would not mount

C No console output 'Before unmount' at step 6

D Component would never unmount

💡 Hint

Check the 'Console Output' column at step 6 in the execution_table.

Concept Snapshot

onBeforeMount and onBeforeUnmount are Vue lifecycle hooks.
onBeforeMount runs just before the component is added to the DOM.
onBeforeUnmount runs just before the component is removed from the DOM.
Use them to run code at these exact moments.
They help prepare or clean up resources.
Called only once per mount/unmount cycle.

Full Transcript

In Vue, the onBeforeMount hook runs right before the component appears on the page. This lets you prepare things before the user sees the component. After the component is visible and active, when it is about to be removed, Vue calls onBeforeUnmount. This hook lets you clean up or stop things before the component disappears. The execution table shows the order: first setup, then onBeforeMount runs, then the component mounts and is visible. Later, when unmounting starts, onBeforeUnmount runs before the component is removed. The variable tracker shows the component state changing from not mounted, to mounted, then back to not mounted after unmount. Remember, onBeforeMount runs before the component is visible, and onBeforeUnmount runs before it is gone. These hooks help manage side effects tied to the component's presence on the page.