Component lifecycle overview in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the Angular component lifecycle?

It manages the different stages a component goes through from creation to destruction, allowing you to run code at specific moments.

beginner

Name the lifecycle hook that runs once after the component's data-bound properties are initialized.

ngOnInit() runs once after input properties are set, perfect for initialization tasks.

beginner

Which lifecycle hook is called before Angular destroys a component?

ngOnDestroy() is called just before the component is removed, useful for cleanup like unsubscribing.

intermediate

What does the ngOnChanges() hook do?

It runs whenever input properties change, letting you react to those changes.

intermediate

Why is ngAfterViewInit() important?

It runs after Angular fully initializes the component's view, so you can safely access child elements or components.

Which lifecycle hook is called only once after component inputs are set?

A ngAfterViewInit()

B ngOnDestroy()

C ngOnChanges()

D ngOnInit()

ngOnInit() runs once after inputs are initialized.

When is ngOnDestroy() called?

A Before component removal

B After component view initialization

C Before component creation

D When input properties change

ngOnDestroy() runs just before the component is removed.

What does ngOnChanges() detect?

A Component destruction

B Changes in input properties

C View initialization

D Component creation

ngOnChanges() runs when input properties change.

Which hook lets you access child components after the view is ready?

A ngAfterViewInit()

B ngOnChanges()

C ngOnDestroy()

D ngOnInit()

ngAfterViewInit() runs after the component's view is fully initialized.

Why use lifecycle hooks in Angular?

A To style components

B To write HTML templates

C To manage component stages and run code at key moments

D To create new components

Lifecycle hooks help manage component stages and run code at important times.

Explain the main Angular component lifecycle hooks and when they run.

Think about the component's journey from creation to destruction.

Describe a real-life scenario where you would use ngOnDestroy() in an Angular component.

Consider what happens when a component is no longer needed.

Practice

1. Which Angular lifecycle hook is called once after the component's data-bound properties have been initialized?

easy

A. ngOnInit()

B. ngOnDestroy()

C. ngAfterViewInit()

D. ngDoCheck()

Solution

1. Step 1: Understand lifecycle hook purpose

   ngOnInit() runs once after Angular sets component input properties.

2. Step 2: Compare other hooks

   ngOnDestroy() runs before component removal, ngAfterViewInit() after view initialization, ngDoCheck() on every change detection.

3. Final Answer:

   ngOnInit() -> Option A

4. Quick Check:

   Initialization hook = ngOnInit() [OK]

Hint: Initialization after inputs = ngOnInit() [OK]

Common Mistakes:

• Confusing ngOnInit with ngAfterViewInit
• Thinking ngOnDestroy runs on init
• Mixing ngDoCheck with ngOnInit

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

easy

A. class MyComponent implements OnDestroy { ngDestroy() { } }

B. class MyComponent implements OnDestroy { ngOnDestroy() { } }

C. class MyComponent { onDestroy() { } }

D. class MyComponent implements OnDestroy { destroy() { } }

Solution

1. Step 1: Recall correct method name

   The lifecycle hook method for destruction is ngOnDestroy().

2. Step 2: Check interface implementation

   Implementing OnDestroy requires defining ngOnDestroy() exactly.

3. Final Answer:

   class MyComponent implements OnDestroy { ngOnDestroy() { } } -> Option B

4. Quick Check:

   OnDestroy method = ngOnDestroy() [OK]

Hint: Method name must be ngOnDestroy() exactly [OK]

Common Mistakes:

• Using wrong method name like ngDestroy or destroy
• Not implementing the interface
• Omitting parentheses in method

3. Given this Angular component code snippet, what will be logged to the console when the component is initialized and then destroyed?

export class SampleComponent implements OnInit, OnDestroy {
  ngOnInit() {
    console.log('Init');
  }
  ngOnDestroy() {
    console.log('Destroy');
  }
}

medium

A. Only Destroy

B. Only Init

C. Init followed by Destroy

D. No output

Solution

1. Step 1: Understand ngOnInit behavior

   ngOnInit() runs once when component initializes, logging 'Init'.

2. Step 2: Understand ngOnDestroy behavior

   ngOnDestroy() runs once before component is removed, logging 'Destroy'.

3. Final Answer:

   Init followed by Destroy -> Option C

4. Quick Check:

   Init then Destroy logs = Init followed by Destroy [OK]

Hint: Init logs on start, Destroy logs on removal [OK]

Common Mistakes:

• Assuming ngOnDestroy runs on init
• Expecting logs in reverse order
• Thinking no logs appear

4. Identify the error in this Angular component lifecycle hook implementation:

export class TestComponent implements OnInit {
  ngOnInit() {
    console.log('Component initialized');
  }
  ngOnDestroy() {
    console.log('Component destroyed');
  }
}

medium

A. Missing OnDestroy interface in implements clause

B. ngOnInit method name is incorrect

C. ngOnDestroy should not be used here

D. Console.log statements are invalid

Solution

1. Step 1: Check implemented interfaces

   The class implements OnInit but defines ngOnDestroy() without implementing OnDestroy.

2. Step 2: Understand interface contract

   To use ngOnDestroy(), the class should implement OnDestroy interface for clarity and correctness.

3. Final Answer:

   Missing OnDestroy interface in implements clause -> Option A

4. Quick Check:

   Implement interface for each lifecycle hook used [OK]

Hint: Implement interfaces for all lifecycle hooks used [OK]

Common Mistakes:

• Ignoring interface implementation for ngOnDestroy
• Thinking method names are wrong
• Believing console.log is invalid

5. You want to run code after Angular fully initializes the component's view and its child views. Which lifecycle hook should you use, and why? Choose the correct hook and reason.

hard

A. ngAfterContentInit() because it runs after projected content is initialized.

B. ngOnInit() because it runs after input properties are set.

C. ngDoCheck() because it runs on every change detection cycle.

D. ngAfterViewInit() because it runs after the component's view and child views are initialized.

Solution

1. Step 1: Identify hook for view initialization

   ngAfterViewInit() runs after Angular initializes the component's view and child views.

2. Step 2: Compare other hooks

   ngOnInit() runs earlier, ngAfterContentInit() runs after projected content, and ngDoCheck() runs repeatedly.

3. Final Answer:

   ngAfterViewInit() because it runs after the component's view and child views are initialized. -> Option D

4. Quick Check:

   View init completion = ngAfterViewInit() [OK]

Hint: Use ngAfterViewInit() for post-view initialization code [OK]

Common Mistakes:

• Confusing ngAfterContentInit with ngAfterViewInit
• Using ngOnInit for view-dependent code
• Expecting ngDoCheck to run once