ngOnDestroy for cleanup in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when ngOnDestroy is called in an Angular component?

Consider an Angular component that subscribes to a data stream. What is the main purpose of implementing the ngOnDestroy lifecycle hook in this component?

A To update the component's view after data changes

B To initialize the component's data when it is created

C To unsubscribe from the data stream and release resources before the component is destroyed

D To handle user input events during the component's lifetime

💡 Hint

Think about what happens when a component is removed from the screen and how to avoid memory leaks.

The ngOnDestroy hook is called just before Angular destroys the component. It is used to clean up resources like subscriptions to avoid memory leaks.

📝 Syntax

intermediate

Which code snippet correctly implements ngOnDestroy to clean up a subscription?

You have a subscription stored in this.sub. Which option correctly unsubscribes in ngOnDestroy?

Angular

export class MyComponent implements OnDestroy {
  sub = this.dataService.getData().subscribe();

  ngOnDestroy() {
    // What goes here?
  }
}

A this.sub.unsubscribe();

B this.sub.close();

C this.sub.stop();

D this.sub.cancel();

💡 Hint

Subscriptions have a method to stop receiving data.

The unsubscribe() method stops the subscription and frees resources. Other methods like close(), stop(), or cancel() do not exist on subscriptions.

🔧 Debug

advanced

Why does this Angular component cause a memory leak?

Look at this component code. Why might it cause a memory leak when the component is destroyed?

Angular

export class LeakComponent implements OnDestroy {
  sub = this.dataService.getData().subscribe(data => {
    console.log(data);
  });

  ngOnDestroy() {
    // No cleanup here
  }
}

A Because the subscription is never unsubscribed in ngOnDestroy

B Because ngOnDestroy is not implemented correctly with a return type

C Because the subscription callback logs data to the console

D Because the component does not implement OnInit

💡 Hint

Think about what happens to active subscriptions when a component is removed.

Not unsubscribing from subscriptions causes them to stay active, holding memory and causing leaks. Logging data or missing OnInit does not cause leaks.

🧠 Conceptual

advanced

What is the best practice for cleaning up multiple subscriptions in ngOnDestroy?

If a component has multiple subscriptions, what is the recommended way to clean them up in ngOnDestroy?

A Call unsubscribe() on each subscription individually in ngOnDestroy

B Ignore unsubscribing because Angular cleans up automatically

C Set each subscription variable to null in ngOnDestroy

D Store all subscriptions in a Subscription object and call unsubscribe() once on it

💡 Hint

Think about grouping subscriptions for easier cleanup.

Using a Subscription container (like new Subscription()) to add all subscriptions and then calling unsubscribe once is cleaner and less error-prone.

❓ state_output

expert

What is the console output after the component is destroyed?

Given this Angular component, what will be logged to the console after the component is destroyed?

Angular

export class TimerComponent implements OnDestroy {
  count = 0;
  intervalId = setInterval(() => {
    this.count++;
    console.log('Count:', this.count);
  }, 1000);

  ngOnDestroy() {
    clearInterval(this.intervalId);
    console.log('Cleanup done');
  }
}

// Assume component is destroyed after 3500ms

A "Cleanup done", "Count: 1", "Count: 2", "Count: 3"

B "Count: 1", "Count: 2", "Count: 3", "Cleanup done"

C "Count: 1", "Count: 2", "Cleanup done"

D "Count: 1", "Count: 2", "Count: 3"

💡 Hint

Consider how many intervals run before clearInterval is called at 3500ms.

The interval runs every 1000ms. After 3500ms, three counts have logged (at 1000, 2000, 3000 ms). Then ngOnDestroy clears the interval and logs "Cleanup done".

Practice

1. What is the main purpose of the ngOnDestroy lifecycle hook in Angular?

easy

A. To handle user input events

B. To initialize component data when it is created

C. To run cleanup code when a component or directive is removed

D. To render the component template

Solution

1. Step 1: Understand the role of ngOnDestroy

   The ngOnDestroy hook is called when Angular removes a component or directive from the view.

2. Step 2: Identify its main use

   It is used to clean up resources like timers, subscriptions, or event listeners to avoid memory leaks.

3. Final Answer:

   To run cleanup code when a component or directive is removed -> Option C

4. Quick Check:

   ngOnDestroy = cleanup [OK]

Hint: Remember: ngOnDestroy cleans up before component removal [OK]

Common Mistakes:

• Confusing ngOnDestroy with ngOnInit
• Thinking it initializes data
• Assuming it handles user events

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

easy

A. destroy() { console.log('Component destroyed'); }

B. ngOnDestroy = () => { console.log('Component destroyed'); }

C. function ngOnDestroy() { console.log('Component destroyed'); }

D. ngOnDestroy() { console.log('Component destroyed'); }

Solution

1. Step 1: Recall Angular lifecycle hook syntax

   Angular lifecycle hooks are methods inside the component class with exact names like ngOnDestroy().

2. Step 2: Check method declaration style

   The correct syntax is a method, not an arrow function or standalone function, and must be named exactly ngOnDestroy.

3. Final Answer:

   ngOnDestroy() { console.log('Component destroyed'); } -> Option D

4. Quick Check:

   ngOnDestroy method syntax = ngOnDestroy() { console.log('Component destroyed'); } [OK]

Hint: Use method syntax named exactly ngOnDestroy [OK]

Common Mistakes:

• Using arrow functions for lifecycle hooks
• Naming method incorrectly
• Defining lifecycle hook outside class

3. Consider this Angular component code snippet:

export class TimerComponent implements OnDestroy {
  intervalId = setInterval(() => console.log('tick'), 1000);

  ngOnDestroy() {
    clearInterval(this.intervalId);
    console.log('Timer stopped');
  }
}

What will happen when this component is removed from the view?

medium

A. The interval stops and logs 'Timer stopped' once

B. An error occurs because clearInterval is not valid here

C. Nothing happens because ngOnDestroy is not called automatically

D. The interval keeps running and logs 'tick' every second

Solution

1. Step 1: Understand what ngOnDestroy does here

   When the component is removed, Angular calls ngOnDestroy, which clears the interval timer.

2. Step 2: Effect of clearing the interval

   Clearing the interval stops the repeated 'tick' logs, and the message 'Timer stopped' is logged once.

3. Final Answer:

   The interval stops and logs 'Timer stopped' once -> Option A

4. Quick Check:

   ngOnDestroy clears timer = The interval stops and logs 'Timer stopped' once [OK]

Hint: ngOnDestroy stops timers to prevent leaks [OK]

Common Mistakes:

• Thinking interval keeps running after destroy
• Assuming ngOnDestroy is not called
• Confusing clearInterval usage

4. You have this Angular component:

export class DataComponent implements OnDestroy {
  subscription = this.dataService.getData().subscribe();

  ngOnDestroy() {
    this.subscription.unsubscribe;
  }
}

What is the problem with this code?

medium

A. The unsubscribe method is not called correctly

B. The subscription is never created

C. ngOnDestroy is missing the return type

D. There is no problem; code works fine

Solution

1. Step 1: Check how unsubscribe is used

   The code uses this.subscription.unsubscribe; without parentheses, so the method is not actually called.

2. Step 2: Correct usage of unsubscribe

   It should be this.subscription.unsubscribe(); to properly stop the subscription and avoid memory leaks.

3. Final Answer:

   The unsubscribe method is not called correctly -> Option A

4. Quick Check:

   Call unsubscribe() with parentheses [OK]

Hint: Always call unsubscribe() as a function [OK]

Common Mistakes:

• Forgetting parentheses on unsubscribe
• Assuming unsubscribe is a property
• Ignoring ngOnDestroy lifecycle

5. You have a component that subscribes to multiple observables and sets up an event listener:

export class MultiCleanupComponent implements OnDestroy {
  sub1 = this.service1.getData().subscribe();
  sub2 = this.service2.getData().subscribe();

  constructor() {
    window.addEventListener('resize', this.onResize);
  }

  onResize() {
    console.log('Window resized');
  }

  ngOnDestroy() {
    // What should be done here?
  }
}

Which is the best way to clean up all resources in ngOnDestroy?

hard

A. Only call this.sub1.unsubscribe(); this.sub2.unsubscribe();

B. Call this.sub1.unsubscribe(); this.sub2.unsubscribe(); window.removeEventListener('resize', this.onResize);

C. Only call window.removeEventListener('resize', this.onResize);

D. No cleanup needed because Angular handles it automatically

Solution

1. Step 1: Identify all resources to clean

   The component has two subscriptions and an event listener on the window.

2. Step 2: Proper cleanup in ngOnDestroy

   To avoid memory leaks, unsubscribe both subscriptions and remove the event listener using removeEventListener.

3. Final Answer:

   Call this.sub1.unsubscribe(); this.sub2.unsubscribe(); window.removeEventListener('resize', this.onResize); -> Option B

4. Quick Check:

   Unsubscribe and remove listeners in ngOnDestroy [OK]

Hint: Clean all subscriptions and listeners in ngOnDestroy [OK]

Common Mistakes:

• Forgetting to remove event listeners
• Only unsubscribing some observables
• Assuming Angular cleans up automatically