ngOnDestroy for cleanup in Angular - Mini Project: Build & Apply

Using ngOnDestroy for Cleanup in Angular

📖 Scenario: You are building a simple Angular component that starts a timer when it loads. To keep your app clean and avoid memory leaks, you need to stop the timer when the component is destroyed.

🎯 Goal: Create an Angular standalone component that starts a timer counting seconds on load and properly cleans up the timer using ngOnDestroy when the component is removed.

📋 What You'll Learn

Create a standalone Angular component named TimerComponent

Add a variable seconds initialized to 0

Start a timer using setInterval that increments seconds every second

Store the timer ID in a variable called timerId

Implement the ngOnDestroy lifecycle method to clear the timer using clearInterval(timerId)

💡 Why This Matters

🌍 Real World

Timers are common in apps for clocks, countdowns, or refreshing data. Cleaning up timers prevents app slowdowns and bugs.

💼 Career

Understanding Angular lifecycle hooks like ngOnDestroy is essential for building efficient, bug-free Angular applications.

1

Create the TimerComponent with seconds variable

Create a standalone Angular component named TimerComponent with a public variable seconds initialized to 0.

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Seconds: {{ seconds }}`
})
export class TimerComponent {
  // Your code here
}

Hint

Use public seconds = 0; inside the class to create the variable.

2

Add timerId variable to store the interval ID

Add a public variable called timerId of type any to store the ID returned by setInterval.

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Seconds: {{ seconds }}`
})
export class TimerComponent {
  public seconds = 0;
  // Add timerId variable here
}

Hint

Declare public timerId: any; inside the class.

3

Start the timer in the constructor to increment seconds

In the constructor of TimerComponent, start a timer using setInterval that increments seconds by 1 every 1000 milliseconds. Assign the returned ID to timerId.

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Seconds: {{ seconds }}`
})
export class TimerComponent {
  public seconds = 0;
  public timerId: any;

  // Start timer in constructor
}

Hint

Use setInterval(() => { this.seconds++ }, 1000) inside the constructor and assign it to this.timerId.

4

Implement ngOnDestroy to clear the timer

Implement the ngOnDestroy lifecycle method in TimerComponent that calls clearInterval(this.timerId) to stop the timer when the component is destroyed.

Angular

import { Component } from '@angular/core';

@Component({
  selector: 'app-timer',
  standalone: true,
  template: `Seconds: {{ seconds }}`
})
export class TimerComponent {
  public seconds = 0;
  public timerId: any;

  constructor() {
    this.timerId = setInterval(() => {
      this.seconds++;
    }, 1000);
  }

  // Add ngOnDestroy method here
}

Hint

Import OnDestroy and implement ngOnDestroy() to call clearInterval(this.timerId).

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