把两个数字加在一起，得到5+10=510，而不是15

我刚开始学习JavaScript，我在摆弄一些代码，似乎找不到真正将两个变量相加并计算它们的和的方法。我声明3个变量，其中一个变量为答案，另外两个变量为两个数字。基本上是a，b，c，其中c=（a+b）。然而，每当我尝试运行代码时，结果都是"ab"而不是"a+b"，所以如果a=5，b=10，它会说"510"而不是"15"。

所有其他符号，如"-"、"/"answers"*"都按预期工作，唯一不工作的是"+"。

我想计算机认为我正在尝试打印出这两个字符串，但我想把它们加起来，就像你在提醒某个东西时所做的那样，例如：alert("Hello World" + a);

我是朝着正确的方向思考，还是问题出在其他地方？这是源代码：

function addTwoNumbers(firstNumber,secondNumber,numberAdded){
    if(numberAdded == '+'){
        numberAdded = (firstNumber + secondNumber);
        alert("The summ of the two numbers is equal to: " + numberAdded);
    }
    else if(numberAdded == '-'){
        numberAdded = (firstNumber - secondNumber);
        alert("The difference of the two numbers is equal to: " + numberAdded);
    }
    else if(numberAdded == '/')
    {
        numberAdded = (firstNumber / secondNumber);
        alert("The 'kvot' of the two numbers is equal to: " + numberAdded);
    }
    else if(numberAdded == '*'){
        numberAdded = (firstNumber * secondNumber);
        alert("The product of the two numbers is equal to: " + numberAdded);
    }
    else
    {
        alert("I told you to use '+, -, / and *' not anything else!");
    }
}
var checker = true;
while(checker == true){
    alert("You will now be prompted to enter two numbers.");
    var firstNumber = prompt("Please enter the first number.");
    var secondNumber = prompt("Please enter the second number.");
    var numberAdded = prompt("Would you like to use '+, -, /, or * ?'");
    checker = false;
    addTwoNumbers(firstNumber, secondNumber, numberAdded);
    if(numberAdded != '+' && numberAdded != '-' && numberAdded != '/' && numberAdded != '*'){
        checker = true;
    }
    option = 0;
    while(option != 'y' && option != 'n'){
        var option = prompt("Would you like to make a calculation again? (y/n)");
        if(option == 'y'){
            checker = true;
        }
        else if(option == 'n'){
            checker = false;
        }
        else{
            alert("I said (y/n), try again...");
        }
    }
}