用javascript写一个数学表达式的程序

public String result(){
double num1 =(new Double(firstNum)).doubleValue();
double num2 =(new Double(secondNum)).doubleValue();
if(operation.equals("add")){
return (""+(num1+num2));
}
else if(operation.equals("subtract")){
return(""+(num1-num2));
}
else if(operation.equals("division")){
if(num2==0d){
return("Errer!");
}
else{
return(""+(num1/num2));
}
}
else if(operation.equals("multiply")) {
return(""+(num1*num2));
}
return("Errer!");
}
}