Lifecycle execution order mental model in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the first lifecycle hook called when an Angular component is created?

The first lifecycle hook called is ngOnChanges if the component has input properties. Otherwise, ngOnInit is called next.

intermediate

Order these Angular lifecycle hooks in the sequence they are called during component creation: ngOnInit, ngOnChanges, ngDoCheck.

The order is:
1. ngOnChanges (called when input properties change)
2. ngOnInit (called once after first ngOnChanges)
3. ngDoCheck (called after ngOnChanges and ngOnInit for custom change detection)

beginner

What lifecycle hook is called just before Angular destroys a component?

The ngOnDestroy hook is called just before Angular removes the component from the DOM. It is used to clean up resources like subscriptions.

intermediate

Explain the purpose of ngAfterViewInit in Angular lifecycle.

ngAfterViewInit runs after Angular initializes the component's views and child views. It is useful to access or manipulate the DOM elements after they are rendered.

advanced

Which lifecycle hooks run every time Angular runs change detection?

The hooks ngOnChanges (if inputs changed) and ngDoCheck run every change detection cycle. They help detect and respond to data changes.

Which lifecycle hook is called only once after the first ngOnChanges?

A ngDoCheck

B ngOnInit

C ngAfterViewInit

D ngOnDestroy

ngOnInit runs once after the first ngOnChanges to initialize the component.

When is ngOnDestroy called in Angular?

A Before component destruction

B After every change detection

C After component initialization

D Before ngOnInit

ngOnDestroy is called just before Angular removes the component to clean up.

Which hook allows you to act after the component's child views are initialized?

A ngOnDestroy

B ngOnChanges

C ngDoCheck

D ngAfterViewInit

ngAfterViewInit runs after the component's views and child views are fully initialized.

What triggers ngOnChanges to run?

A When input properties change

B Any change detection cycle

C When the component is destroyed

D After view initialization

ngOnChanges runs only when input properties receive new values.

Which lifecycle hook is best for custom change detection logic?

A ngOnInit

B ngOnChanges

C ngDoCheck

D ngAfterViewInit

ngDoCheck allows you to implement custom change detection beyond Angular's default.

Describe the order of Angular lifecycle hooks from component creation to destruction.

Think about what happens when a component starts, updates, and ends.

Explain when and why you would use ngOnDestroy in an Angular component.

Consider what happens when a component is removed from the page.

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