1 function factorial(num){
2      var result = 1;
3      while(num){
4          result *= num;
5          num--;    
6      }  
7       return result;
8 }
9    

2. 利用函数递归

1 function factorial(num){
2      if(num<=0){
3          return 1;
4     }else{
5          return num*arguments.callee(num-1);
6     }  
7 }