First

Emits only the first value (or the first value that meets some condition) emitted by the source Observable.

import { fromEvent } from 'rxjs';
import { first } from 'rxjs/operators';

const clicks = fromEvent(document, 'click');
const result = clicks.pipe(first());
result.subscribe(x => console.log(x));

Emits the first click that happens on a DIV.

import { fromEvent } from 'rxjs';
import { first } from 'rxjs/operators';

const clicks = fromEvent(document, 'click');
const result = clicks.pipe(first(ev => ev.target.tagName === 'DIV'));
result.subscribe(x => console.log(x));