es6/operators/pluck.js

import { map } from './map';
/**
 * Maps each source value (an object) to its specified nested property.
 *
 * <span class="informal">Like {@link map}, but meant only for picking one of
 * the nested properties of every emitted object.</span>
 *
 * <img src="./img/pluck.png" width="100%">
 *
 * Given a list of strings describing a path to an object property, retrieves
 * the value of a specified nested property from all values in the source
 * Observable. If a property can't be resolved, it will return `undefined` for
 * that value.
 *
 * @example <caption>Map every click to the tagName of the clicked target element</caption>
 * var clicks = Rx.Observable.fromEvent(document, 'click');
 * var tagNames = clicks.pluck('target', 'tagName');
 * tagNames.subscribe(x => console.log(x));
 *
 * @see {@link map}
 *
 * @param {...string} properties The nested properties to pluck from each source
 * value (an object).
 * @return {Observable} A new Observable of property values from the source values.
 * @method pluck
 * @owner Observable
 */
export function pluck(...properties) {
    const length = properties.length;
    if (length === 0) {
        throw new Error('list of properties cannot be empty.');
    }
    return (source) => map(plucker(properties, length))(source);
}
function plucker(props, length) {
    const mapper = (x) => {
        let currentProp = x;
        for (let i = 0; i < length; i++) {
            const p = currentProp[props[i]];
            if (typeof p !== 'undefined') {
                currentProp = p;
            }
            else {
                return undefined;
            }
        }
        return currentProp;
    };
    return mapper;
}
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