code: function factorialOfNumber(number) {
          if (isInteger(number)) {
              var arrayOfNumbers = [];
              while (number >= 1) {
                  arrayOfNumbers.push(number);
                  number--;
              }
              return this.mul(arrayOfNumbers);
          }
          else {
              return 0;
          }
      }
docs: Example: number = 3; return = 6;