What is ngOnInit? How is it defined?

In Angular, ngOnInit is a lifecycle hook that is defined as part of the OnInit interface. It is a method that is automatically invoked by Angular once after the component has been initialized and its inputs have been bound.

The purpose of ngOnInit is to provide a place for the component to perform any initialization tasks or setup logic that is required before the component is displayed or used. It is commonly used for tasks such as retrieving data from a server, initializing component properties, subscribing to observables, or setting up third-party libraries.

To define and use ngOnInit in an Angular component, follow these steps:

1.Import the OnInit interface from the @angular/core module:

typescript
import { Component, OnInit } from '@angular/core';

2.Implement the OnInit interface in your component class:

typescript
export class MyComponent implements OnInit {
  ngOnInit() {
    // Initialization logic goes here
  }
}

3.Implement the logic inside the ngOnInit method to perform any required initialization tasks:

typescript
export class MyComponent implements OnInit {
  ngOnInit() {
    // Initialization logic
    console.log('Component initialized');
  }
}

4. Angular will automatically invoke the ngOnInit method once, after the component has been instantiated and its inputs have been bound:

typescript
@Component({
  selector: 'app-my-component',
  template: '...'
})
export class MyComponent implements OnInit {
  ngOnInit() {
    console.log('Component initialized');
  }
}

When the component is initialized, the ngOnInit method will be called, and the message 'Component initialized' will be logged to the console.

It's important to note that ngOnInit is called only once during the component's lifecycle, after the inputs have been initialized. If you need to perform logic when inputs change, you can use the ngOnChanges lifecycle hook instead.

By using ngOnInit, you can ensure that your component is properly initialized and ready for use before it is displayed or interacts with other components or services in your Angular application.