Update factorial.js

updated the factorial program to work with Negative Numbers also.

src/algorithms/math/factorial/factorial.js

@@ -3,6 +3,10 @@
  * @return {number}
  */
 export default function factorial(number) {
+  //resulting to a negative number
+  if(number<0)
+    return NaN;
+  
   let result = 1;
 
   for (let i = 2; i <= number; i += 1) {