// RxJS v6+
import { interval } from 'rxjs';
import { take, ignoreElements } from 'rxjs/operators';
​
//emit value every 100ms
const source = interval(100);
//ignore everything but complete
const example = source.pipe(
  take(5),
  ignoreElements()
);
//output: "COMPLETE!"
const subscribe = example.subscribe(
  val => console.log(`NEXT: ${val}`),
  val => console.log(`ERROR: ${val}`),
  () => console.log('COMPLETE!')
);

// RxJS v6+
import { interval, throwError, of } from 'rxjs';
import { mergeMap, ignoreElements } from 'rxjs/operators';
​
//emit value every 100ms
const source = interval(100);
//ignore everything but error
const error = source.pipe(
  mergeMap(val => {
    if (val === 4) {
      return throwError(`ERROR AT ${val}`);
    }
    return of(val);
  }),
  ignoreElements()
);
//output: "ERROR: ERROR AT 4"
const subscribe = error.subscribe(
  val => console.log(`NEXT: ${val}`),
  val => console.log(`ERROR: ${val}`),
  () => console.log('SECOND COMPLETE!')
);