Lifecycle execution order mental model in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the lifecycle hook that runs after Angular sets input properties.

Angular

import { Component, [1] } from '@angular/core';

Drag options to blanks, or click blank then click option'

AOnInit

BOnChanges

CDoCheck

DAfterViewInit

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing OnInit instead of OnChanges

Confusing AfterViewInit with input changes

The OnChanges hook runs after Angular sets or resets input properties.

2fill in blank

medium

Complete the code to implement the lifecycle hook that runs once after the first ngOnChanges.

Angular

export class MyComponent implements [1] { ngOnInit() { console.log('Init'); } }

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AOnDestroy

BAfterContentInit

CDoCheck

DOnInit

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnDestroy instead of OnInit

Confusing DoCheck with OnInit

The OnInit hook runs once after the first ngOnChanges and is used for component initialization.

3fill in blank

hard

Fix the error in the lifecycle hook name that runs after Angular projects external content into the component.

Angular

export class MyComponent implements AfterContentInit { ngAfterContent[1]() { console.log('Content projected'); } }

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ADestroy

BCheck

CInit

DChanges

Attempts:

3 left

💡 Hint

Common Mistakes

Using ngAfterContentCheck instead of ngAfterContentInit

Misspelling the method name

The correct method name is ngAfterContentInit, which runs after content projection.

4fill in blank

hard

Fill both blanks to complete the lifecycle hook that runs after Angular initializes the component's views and child views.

Angular

export class MyComponent implements [1] { ng[2]() { console.log('View initialized'); } }

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AAfterViewInit

BOnInit

DOnDestroy

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnInit instead of AfterViewInit

Confusing AfterContentInit with AfterViewInit

The interface and method are both AfterViewInit and ngAfterViewInit, which run after view initialization.

5fill in blank

hard

Fill both blanks to complete the lifecycle hook that runs just before Angular destroys the component.

Angular

export class MyComponent implements [1] { ng[2]() { console.log('Cleanup before destroy'); } }

Drag options to blanks, or click blank then click option'

AOnDestroy

BOnInit

CDestroy

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnInit instead of OnDestroy

Misspelling ngOnDestroy

The interface is OnDestroy, the method is ngOnDestroy, which runs before component destruction.

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