Javascript Number Integer factorial

function factorial(n){

  if(n < 0 ) return 'does not exist';

  if(n === 0 || n === 1) return 1;

  var x = 1;/*w w w.j  a  v a 2 s  . c o m*/

  for(var i = 1; i <= n; i++){
      x *= i
  }

    return x;

}

console.log(factorial(-6));
console.log(factorial(0));
console.log(factorial(1));
console.log(factorial(2));
console.log(factorial(3));
console.log(factorial(5));
console.log(factorial(9));