This is because the return from prompt() is always a string . Then the addition will do the concatenation of both. If you want to add the numbers you need to convert the variable to a numeric type, this happens with parseInt() , for example.

num1 = prompt("Digite um número ","");
num2 = prompt("Digite outro número para somar ","");

function soma(){
    var res = parseInt(num1) + parseInt(num2);
    alert("Reultado da Operação "+ res);
}

soma();

Or if you prefer that the variables are already numeric:

    num1 = parseInt(prompt("Digite um número ",""));
    num2 = parseInt(prompt("Digite outro número para somar ",""));

    function soma(){
        var res = num1 + num2;
        alert("Reultado da Operação "+ res);
    }

    soma();

You can also use ParseFloat() if you want the number allow numbers with decimal part.

Note that if the conversion is not possible, after all the person can type a text, it may not give the expected result since the value will probably be 0, or something that it can convert, but not well what was typed.

The problem is that the return of prompt is a string ( take a look here ).

So, if you convert these strings into numbers your code will already work.

For example, using Number :

var res = Number(num1) + Number(num2);

jsFiddle: link

Note:

• different ways to convert strings to Numbers

In JavaScript there is parseInt that converts a string in an integer, removing the decimal part without rounding. You can also use parseFloat , or Number . All convert strings into numbers.