求和

function sum(...rest) {
   var sum=0;
   for(var i in rest){
       sum=sum+rest[i];
   }  
   return sum;
}

求面积

function area_of_circle(r, pi) {
    if(arguments.length<2){
      return 3.14*r*r;
    }else{
      return pi*r*r;
    }
}