Angular Constructor vs ngOnInit

If you are confused about Constructor vs ngOnInit, then I will explain in a very easy way.

What is a Constructor in Angular?

The constructor is a feature of a TypeScript class. it’s not a part of Angular.

It is used to:

1. Initialize class-level variables.
2. Inject dependencies (Services).
3. Set up basic values before Angular starts rendering the component.

Note: Angular only calls the constructor once when the component is created.

Example (Angular 17+ version):

export class AppComponent {
  constructor(private userService: UserService) {
    console.log('Constructor called!');
  }
}

Note: It is best for dependency injection.

What is ngOnInit() in Angular?

ngOnInit() is a lifecycle hook provided by Angular.

It runs after the constructor.

When we use ngOnInit()?

1. You want to call APIs
2. You need to access @Input() values
3. You need lifecycle-based logic
4. You want to run code once after component initialization

Example (Angular 17+)

export class AppComponent implements OnInit {
  ngOnInit() {
    console.log('ngOnInit called!');
  }
}

Best use of ngOnInit

1. Best for API calls
2. Best for initialization logic
3. Best for using @Input() values

Constructor vs ngOnInit – Key Differences

Angular Constructor vs ngOnInit – Interview Questions

Angular Constructor vs ngOnInit – Objective Questions (MCQs)