Church numerals

church.js

var addOne = function (x) { return x+1; };

var church = function(n) {
	if (n === 0) {
		return function(fn)  {
			return function(x) {
				return x;
			}
		}
	} else {
		return function(fn) {
			return function(x) {
				return fn(church(n-1)(fn)(x));
			}
		}
	}
};

var unchurch = function(fn) {
	return fn(addOne)(0);
};