group by for arrays

Array.prototype.groupBy.js

/**
 * @method Array.prototype.groupBy(group)
 * Creates a dictionary grouping by an attribute or if `group` is a function, what it returns.
 * @param {function|string} group - attribute name or function(item, index, array): string
 * @returns {object.<array>}
 */
if (!Array.prototype.groupBy) {
	Object.defineProperty(Array.prototype, 'groupBy', {
		enumerable: false,
		value: function groupBy(group) {
			return this.reduce(function (hash, item, index, array) {
				var key = typeof group === 'function' ? group(item, index, array) : String(item[group]);

				if (!hash[key]) {
					hash[key] = [];
				}

				hash[key].push(item);

				return hash;
			}, Object.create(null));
		}
	});
}