Create an object that has multiple keys pointing to the same value. It uses getters and setters to work its magic, but acts perfectly like a regular object, including enumeration, membership testing, and deleting properties.

multiKey.js

function multiKey(keyGroups) {
	let obj = {};
	let props = {};

	for (let keyGroup of keyGroups) {
		let masterKey = keyGroup[0];
		let prop = {
			configurable: true,
			enumerable: false,

			get() {
				return obj[masterKey];
			},

			set(value) {
				obj[masterKey] = value;
			}
		};

		obj[masterKey] = undefined;
		for (let i = 1; i < keyGroup.length; ++i) {
			if (keyGroup.hasOwnProperty(i)) {
				props[keyGroup[i]] = prop;
			}
		}
	}

	return Object.defineProperties(obj, props);
}

/* Example usage */
let test = multiKey([
	['north', 'up'],
	['south', 'down'],
	['east', 'left'],
	['west', 'right']
]);

test.north = 42;
test.down = 123;

test.up; // returns 42
test.south; // returns 123

'left' in test; // true
'west' in test; // true

let count = 0;
for (let key in test) {
	count += 1;
}

count === 4; // true; only unique (un-linked) properties are looped over