A trip down factorial lane - see what Javascript functions and some functional programming goodies can do

factorial.js

 
// the most basic factorial
function fact1(n){
	if(n < 2) return 1;
	return n*fact1(n-1);
}
 
// let's add some precondition verifications
function fact2(n){
	if(typeof n !== 'number' || isNaN(n) || n < 0)
		return -1;
	if(n<2) return 1;
	return n*fact2(n-1);
}
 
// no need to make the verification on each recursive call, let's place the logic in an inner function
function fact3(){
	if(typeof n !== 'number' || isNaN(n) || n < 0)
		return -1;
	var innerFactorial1 = function(n){
		if(n<2) return 1;
		return n*innerFactorial1(n-1);
	};
	return innerFactorial1(n);
}
 
 
// let's place the recursive call in a tail position, and hope that ES6 will bring with it tail call optimization
function fact4(n){
	if(typeof n !== 'number' || isNaN(n) || n < 0)
		return -1;
	var innerFactorial2 = function(n, accum){
		if(n<2) return accum;
		return innerFactorial2(n-1, n*accum);
	};
	return innerFactorial2(n,1);
}
 
// well, we don't actually have to wait for ES6 to bring the tail call optimization, we can use trampoline
function fact5(n){
	if(typeof n !== 'number' || isNaN(n) || n < 0)
		return -1;
	var trampoline = function (func){
		var res = func;
		while(typeof res === "function")
			res = res();
		return res;
	};
	var innerFactorial3 = function (n, accum){
		if(n < 2){
			return accum;
		}
		return function(){
			return innerFactorial3(n-1, accum*n);
		};
	};
	
	return trampoline(
			function(){
				return innerFactorial3(n,1);
			}
		);
}
 
// Now let's cache the results within the function so we can use it in an identical future call, and we have our
// final version of factorial
 
function fact6(n){
	if(typeof n !== 'number' || isNaN(n) || n < 0)
		return -1;
	fact6._cache = fact6._cache || [];
	if(fact6._cache[n])
		return fact6._cache[n];
	var trampoline = function (func){
		var res = func;
		while(typeof res === "function")
			res = res();
		return res;
	};
	var innerFactorial4 = function (n, accum){
		if(n < 2){
			return accum;
		}
		return function(){
			return innerFactorial4(n-1, accum*n);
		};
	};
	
	var result = trampoline(
			function(){
				return innerFactorial4(n,1);
			}
		);
	fact6._cache[n] = result;
	return result;
}