import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
|
import { Observable } from '../Observable';
|
import { OperatorFunction } from '../types';
|
import { filter } from './filter';
|
import { throwIfEmpty } from './throwIfEmpty';
|
import { defaultIfEmpty } from './defaultIfEmpty';
|
import { take } from './take';
|
|
/**
|
* Emits the single value at the specified `index` in a sequence of emissions
|
* from the source Observable.
|
*
|
* <span class="informal">Emits only the i-th value, then completes.</span>
|
*
|
* 
|
*
|
* `elementAt` returns an Observable that emits the item at the specified
|
* `index` in the source Observable, or a default value if that `index` is out
|
* of range and the `default` argument is provided. If the `default` argument is
|
* not given and the `index` is out of range, the output Observable will emit an
|
* `ArgumentOutOfRangeError` error.
|
*
|
* ## Example
|
*
|
* Emit only the third click event
|
*
|
* ```ts
|
* import { fromEvent, elementAt } from 'rxjs';
|
*
|
* const clicks = fromEvent(document, 'click');
|
* const result = clicks.pipe(elementAt(2));
|
* result.subscribe(x => console.log(x));
|
*
|
* // Results in:
|
* // click 1 = nothing
|
* // click 2 = nothing
|
* // click 3 = MouseEvent object logged to console
|
* ```
|
*
|
* @see {@link first}
|
* @see {@link last}
|
* @see {@link skip}
|
* @see {@link single}
|
* @see {@link take}
|
*
|
* @throws {ArgumentOutOfRangeError} When using `elementAt(i)`, it delivers an
|
* ArgumentOutOfRangeError to the Observer's `error` callback if `i < 0` or the
|
* Observable has completed before emitting the i-th `next` notification.
|
*
|
* @param {number} index Is the number `i` for the i-th source emission that has
|
* happened since the subscription, starting from the number `0`.
|
* @param {T} [defaultValue] The default value returned for missing indices.
|
* @return A function that returns an Observable that emits a single item, if
|
* it is found. Otherwise, it will emit the default value if given. If not, it
|
* emits an error.
|
*/
|
export function elementAt<T, D = T>(index: number, defaultValue?: D): OperatorFunction<T, T | D> {
|
if (index < 0) {
|
throw new ArgumentOutOfRangeError();
|
}
|
const hasDefaultValue = arguments.length >= 2;
|
return (source: Observable<T>) =>
|
source.pipe(
|
filter((v, i) => i === index),
|
take(1),
|
hasDefaultValue ? defaultIfEmpty(defaultValue!) : throwIfEmpty(() => new ArgumentOutOfRangeError())
|
);
|
}
|
