how to use the defer function from the rxjs library in typescript

To use the defer function from the rxjs library in TypeScript, you can follow the example below:

index.ts
import { defer, Observable } from 'rxjs';

// create an observable that emits values from 1 to 3
const myObservable: Observable<number> = defer(() => {
  console.log('creating observable');
  return Observable.of(1, 2, 3);
});

// subscribe to the observable
myObservable.subscribe((value: number) => console.log(value));
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The defer function takes a factory function that returns an observable. The factory function is not executed until an observer subscribes to the observable created by defer. This allows you to delay creating the observable until it is actually needed.

In the example above, the observable returned by defer emits the values 1, 2, and 3. The subscribe method is called on the myObservable instance to receive the emitted values. The output of this program will be:

index.ts
creating observable
1
2
3
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Note that the console.log statement inside the factory function is only called once, when the observable is first subscribed to. This demonstrates the lazy behavior of defer.