Extend Array with Proxies for fancy features

betterArray.js

const betterArray = xs => new Proxy(xs, {
	get (xs, key) {
		if (xs[key] || !xs.length) return xs[key];
		const idx = Number(key);
		if (!idx || idx > 0) return xs[key];
		return xs[xs.length + idx];
	},
});

const arr = betterArray([ 1, 2, 3, 4, 5 ]);
arr[-1]; // 5
arr[-2]; // 4

callableArray.js

const isNumber = x => typeof x === "number";
const isFunc = x => typeof x === "function";

const makeArrayCallable = xs => (...args) => {
	switch (args.length) {
		case 0: return callableArray([...xs]);
		case 1: {
			const [arg] = args;
			switch (typeof arg) {
				case "number": {
					if (arg >= 0) return xs[arg];
					else return xs[xs.length + arg];
				}
				case "function":
					return callableArray(xs.map(arg));
				default:
					throw new TypeError("Invalid call signature: " + typeof arg);
			}
		}
		case 2: {
			if (args.every(isNumber)) return callableArray(xs.slice(...args));

			const [arg1, arg2] = args;
			if (isFunc(arg1)) return callableArray(xs.reduce(arg1, arg2));
			else throw new TypeError(`Invalid call signature: ${typeof arg1} ${typeof arg2}`);
		}
	}
}

const callableArray = xs => new Proxy(makeArrayCallable(xs), { get (_, key) { return xs[key] } });

const arr = callableArray([ 1, 2, 3, 4, 5 ]);

arr(3); // 4
arr(1, 3); // Callable<[ 2, 3, 4 ]>
arr(x => x + 1); // Callable<[ 2, 3, 4, 5, 6 ]>;
arr((a, b) => a + b, 0); // 15