SOURCE

function factorial(num){
  if(num <= 1){
    return 1
  }else {
    // return num * factorial(num - 1);
    return num * arguments.callee(num - 1)
  }
}
console.log(factorial(5));

var factorial = (function f(num){
  if(num <= 1){
    return 1;
  }else {
    return num * f(num -1)
  }
})