es6/operators/elementAt.js

import { Subscriber } from '../Subscriber';
import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
/**
 * 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>
 *
 * <img src="./img/elementAt.png" width="100%">
 *
 * `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 <caption>Emit only the third click event</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var result = clicks.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(index, defaultValue) {
    return (source) => source.lift(new ElementAtOperator(index, defaultValue));
}
class ElementAtOperator {
    constructor(index, defaultValue) {
        this.index = index;
        this.defaultValue = defaultValue;
        if (index < 0) {
            throw new ArgumentOutOfRangeError;
        }
    }
    call(subscriber, source) {
        return source.subscribe(new ElementAtSubscriber(subscriber, this.index, this.defaultValue));
    }
}
/**
 * We need this JSDoc comment for affecting ESDoc.
 * @ignore
 * @extends {Ignored}
 */
class ElementAtSubscriber extends Subscriber {
    constructor(destination, index, defaultValue) {
        super(destination);
        this.index = index;
        this.defaultValue = defaultValue;
    }
    _next(x) {
        if (this.index-- === 0) {
            this.destination.next(x);
            this.destination.complete();
        }
    }
    _complete() {
        const destination = this.destination;
        if (this.index >= 0) {
            if (typeof this.defaultValue !== 'undefined') {
                destination.next(this.defaultValue);
            }
            else {
                destination.error(new ArgumentOutOfRangeError);
            }
        }
        destination.complete();
    }
}
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