| import { Operator } from '../Operator'; |
| import { Subscriber } from '../Subscriber'; |
| import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError'; |
| import { Observable } from '../Observable'; |
| import { MonoTypeOperatorFunction, TeardownLogic } 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.png) |
| * |
| * `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 } from 'rxjs'; |
| * import { elementAt } from 'rxjs/operators'; |
| * |
| * 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 |
| * ArgumentOutOrRangeError 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 {Observable} An Observable that emits a single item, if it is found. |
| * Otherwise, will emit the default value if given. If not, then emits an error. |
| * @method elementAt |
| * @owner Observable |
| */ |
| export function elementAt<T>(index: number, defaultValue?: T): MonoTypeOperatorFunction<T> { |
| 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()), |
| ); |
| } |