First
Emits only the first value (or the first value that meets some condition) emitted by the source Observable.
first<T, D>(predicate?: (value: T, index: number, source: Observable<T>) => boolean, defaultValue?: D): OperatorFunction<T, T | D>
| Parameters | Description |
|---|---|
| predicate Optional | condition to match |
| defaultValue Optional | value to emit if no value matches the condition |
- Emits the first value if no predicate is present
- Emits the first matching value if the predicate is present
- Closes the stream after emitting a value
- If the source completes before emitting any matching value, then it raises the error notification.
OperatorFunction<T, T | D>: A function that returns an Observable that emits the first item that matches the condition.
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));