arr=[1,2,3];     function modi(str){         switch(str){     case 1:          str = "a";          break;     case 2:          str = "b";          break;     case 3:          str = "c";          break;         }     }       modi(arr[0]);      modi(arr[1]);      modi(arr[2]);      console.log(arr);//返回 1,2,3                  要怎样才能返回 a,b,c