Lifecycle execution order mental model in Angular

Introduction

Understanding the order of lifecycle events helps you control what happens when your Angular component starts, updates, or ends.

You want to run code right after a component appears on the screen.

You need to respond when input data to a component changes.

You want to clean up resources before a component disappears.

You want to track when Angular checks your component for changes.

You want to initialize child components or templates after they load.

Syntax

Angular

ngOnChanges(changes: SimpleChanges) {}
ngOnInit() {}
ngDoCheck() {}
ngAfterContentInit() {}
ngAfterContentChecked() {}
ngAfterViewInit() {}
ngAfterViewChecked() {}
ngOnDestroy() {}

Each method is called automatically by Angular at a specific time.

You only implement the methods you need in your component class.

Examples

This runs once after the component is created and inputs are set.

Angular

ngOnInit() {
  console.log('Component initialized');
}

This runs whenever input properties change, even before ngOnInit.

Angular

ngOnChanges(changes: SimpleChanges) {
  console.log('Input changed', changes);
}

This runs just before Angular removes the component, good for cleanup.

Angular

ngOnDestroy() {
  console.log('Component is about to be removed');
}

Sample Program

This component shows how Angular calls lifecycle methods in order. When the input data changes, ngOnChanges runs first and updates the message. Then ngOnInit runs once after the first change. When the component is removed, ngOnDestroy runs to clean up.

Angular

import { Component, Input, OnChanges, OnInit, OnDestroy, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-lifecycle-demo',
  template: `<p>{{message}}</p>`
})
export class LifecycleDemoComponent implements OnChanges, OnInit, OnDestroy {
  @Input() data = '';
  message = '';

  ngOnChanges(changes: SimpleChanges) {
    this.message = `Input changed to: ${this.data}`;
    console.log('ngOnChanges called');
  }

  ngOnInit() {
    console.log('ngOnInit called');
  }

  ngOnDestroy() {
    console.log('ngOnDestroy called');
  }
}

Important Notes

ngOnChanges runs before ngOnInit and every time input changes.

ngOnInit runs once after the first ngOnChanges.

ngOnDestroy is your chance to clean up like stopping timers or unsubscribing.

Summary

Angular calls lifecycle methods in a specific order to manage your component.

ngOnChanges runs first on input changes, then ngOnInit runs once.

Use ngOnDestroy to clean up before the component is removed.

Practice

1. Which Angular lifecycle hook runs first when a component receives new input values?
ngOnInit, ngOnChanges, ngAfterViewInit, or ngOnDestroy?

easy

A. ngOnChanges

B. ngOnInit

C. ngAfterViewInit

D. ngOnDestroy

Solution

1. Step 1: Understand lifecycle hooks on input change

   When input properties change, Angular calls ngOnChanges first to react to those changes.

2. Step 2: Identify the first hook called on input update

   ngOnInit runs only once after the first ngOnChanges, so it is not first.

3. Final Answer:

   ngOnChanges -> Option A

4. Quick Check:

   Input changes trigger ngOnChanges first [OK]

Hint: Remember: ngOnChanges always runs before ngOnInit on input change [OK]

Common Mistakes:

• Thinking ngOnInit runs before ngOnChanges
• Confusing ngAfterViewInit as first hook
• Assuming ngOnDestroy runs on input change

2. Which of the following is the correct syntax to implement the ngOnDestroy lifecycle hook in an Angular component?

easy

A. destroy() { console.log('Destroyed'); }

B. onDestroy() { console.log('Destroyed'); }

C. ngOnDestroy() { console.log('Destroyed'); }

D. ngDestroy() { console.log('Destroyed'); }

Solution

1. Step 1: Recall Angular lifecycle hook naming

   Angular lifecycle hooks have specific method names starting with 'ng', so ngOnDestroy is correct.

2. Step 2: Match method name exactly

   Only ngOnDestroy() { console.log('Destroyed'); } uses the exact method name ngOnDestroy required by Angular.

3. Final Answer:

   ngOnDestroy() { console.log('Destroyed'); } -> Option C

4. Quick Check:

   Lifecycle hooks start with 'ngOn' [OK]

Hint: Lifecycle hooks always start with 'ngOn' followed by event [OK]

Common Mistakes:

• Using incorrect method names like onDestroy or destroy
• Missing 'ngOn' prefix
• Confusing lifecycle hook names with custom methods

3. Given this Angular component code snippet:

export class MyComponent implements OnInit, OnChanges {
  @Input() data: string;

  ngOnChanges() { console.log('Changes detected'); }
  ngOnInit() { console.log('Init called'); }
}

What will be logged first when the component receives its first input value?

medium

A. Init called

B. Changes detected

C. No logs

D. Both logs at the same time

Solution

1. Step 1: Understand lifecycle order on first input

   When the component first receives input, Angular calls ngOnChanges before ngOnInit.

2. Step 2: Identify which log appears first

   The ngOnChanges method logs 'Changes detected' first, then ngOnInit logs 'Init called'.

3. Final Answer:

   Changes detected -> Option B

4. Quick Check:

   ngOnChanges runs before ngOnInit [OK]

Hint: ngOnChanges logs before ngOnInit on first input [OK]

Common Mistakes:

• Assuming ngOnInit runs before ngOnChanges
• Thinking logs happen simultaneously
• Ignoring ngOnChanges on first input

4. You notice your Angular component's ngOnDestroy method is not called when navigating away. What is the most likely cause?

medium

A. The component is never destroyed because it is cached or reused

B. You forgot to implement OnDestroy interface

C. You used ngOnDestroy instead of onDestroy

D. Angular does not call ngOnDestroy on navigation

Solution

1. Step 1: Understand when ngOnDestroy runs

   ngOnDestroy runs when Angular destroys the component instance.

2. Step 2: Identify why it might not run on navigation

   If the component is cached or reused (e.g., via route reuse strategy), it is not destroyed, so ngOnDestroy won't run.

3. Final Answer:

   The component is never destroyed because it is cached or reused -> Option A

4. Quick Check:

   ngOnDestroy runs only on actual destruction [OK]

Hint: ngOnDestroy runs only if component is truly destroyed [OK]

Common Mistakes:

• Confusing interface implementation with method call
• Using wrong method name
• Believing Angular never calls ngOnDestroy on navigation

5. Consider an Angular component with these lifecycle hooks:

ngOnChanges() { console.log('Changes'); }
ngOnInit() { console.log('Init'); }
ngAfterViewInit() { console.log('After View'); }
ngOnDestroy() { console.log('Destroy'); }

If the component receives new input twice, then is removed from the page, what is the correct order of console logs?

hard

A. "Changes", "Init", "Changes", "After View", "Destroy"

B. "Init", "Changes", "After View", "Changes", "Destroy"

C. "Changes", "Changes", "Init", "After View", "Destroy"

D. "Changes", "Init", "After View", "Changes", "Destroy"

Solution

1. Step 1: Order on first input and init

   First input triggers ngOnChanges, then ngOnInit runs once, then ngAfterViewInit after view setup.

2. Step 2: Order on second input and destroy

   Second input triggers ngOnChanges again. Finally, when component is removed, ngOnDestroy runs.

3. Final Answer:

   "Changes", "Init", "After View", "Changes", "Destroy" -> Option D

4. Quick Check:

   ngOnChanges before ngOnInit; ngAfterViewInit after init; ngOnDestroy last [OK]

Hint: Remember: ngOnChanges runs on every input change, ngOnInit once [OK]

Common Mistakes:

• Placing ngOnInit before first ngOnChanges
• Mixing order of ngAfterViewInit and ngOnChanges
• Forgetting ngOnDestroy runs last