How to know what type of variable user enter in JavaScript?

Question

I tried to make a function that receive data from user and combine such data either by concatenation in case of string or by getting the sum as a result if the entered data was integer. My main problem is that I don't know what what condition in if statement I use to JavaScript act according to data entered by user.

That's my last invented code to solve such problem

function GetFullName() {
            var first = document.getElementById('FirstName').value;
            var last = document.getElementById('LastName').value;

            if (first == "string" || last == "string") {               
                document.getElementById('FullName').value = first + " " + last;

            } else {
                var first = parseInt(document.getElementById('FirstName').value);
                var last = parseInt(document.getElementById('LastName').value);

                document.getElementById('FullName').value = first + last;
               
            }
            document.getElementById('FirstName').focus();
        }

<form>
        First Name <input type="text" id="FirstName" />
        Last Name <input type="text" id="LastName" />
        <input type="button" value="submit" onclick="GetFullName()" />
        <input type="reset" value="reset" />
        <br />
        Full Name <input type="text" id="FullName" />
    </form>

Answer 1

when you get an element's value it will always be a string,

you can check of a variables type by typeof first

for your specific problem if you want to check if the user inputted integers then you will have to use isNaN

if(isNaN("123")) {

} else {
   //this executes
}

All in all the new code would be:

if (isNaN(first) || isNaN(last)) {
    document.getElementById('FullName').value = first + " " + last;
} else {
    document.getElementById('FullName').value = parseInt(first) + parseInt(last);
}