JavaScript manMany, Projects each element of a sequence to an Array and flattens the resulting sequences into one sequence.

Javascript mapMany

// Projects each element of a sequence to an Array and flattens the resulting sequences into one sequence.
if (!Array.prototype.mapMany) {
	Object.defineProperty(Array.prototype, 'mapMany', {
		value: function (callback) {
			if (this === null) {
				throw new TypeError('Array.prototype.mapMany ' +
					'called on null or undefined');
			}
			if (typeof callback !== 'function') {
				throw new TypeError(callback +
					' is not a function');
			}

			// 1. Let O be ? ToObject(this value).
			var o = Object(this);

			// 2. Let len be ? ToLength(? Get(O, "length")).
			var len = o.length >>> 0;

			// Steps 3, 4, 5, 6, 7
			var k = 0;
			// Start with empty array
			var value = [];

			// 8. Repeat, while k < len
			while (k < len) {
				// if k exists in o
				if (k in o) {
					value = value.concat(callback(o[k], k, o))
				}

				// d. Increase k by 1.
				k++;
			}

			// 9. Return accumulator.
			return value;
		}
	});
}

Usage Example:

	var arr = [
		{"id": 1, "Contacts": [{"id": 1},{"id": 2},{"id": 3},{"id": 4}]}, 
		{"id": 2, "Contacts": [{"id": 1},{"id": 2},{"id": 3},{"id": 4}]},
		{"id": 3, "Contacts": [{"id": 1},{"id": 2},{"id": 3}]},
		{"id": 4, "Contacts": [{"id": 1},{"id": 2},{"id": 3}]},
		{"id": 5, "Contacts": [{"id": 1},{"id": 2},{"id": 3},{"id": 4}]}
	];

	arr.mapMany(m => m.Contacts.map(n => n.id));

Result:

	[1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 1, 2, 3, 1, 2, 3, 4]