AJAX: Calling a php script and modifiying javascript variable

Question

I am trying to understand how AJAX works. I have a problem in my code and I think it comes from the fact that AJAX is asynchronous (It might not be reason of the problem though). I am asking for your help to figure out what is wrong.

This is my javascript function. It calls a php page that check if the email is already in the database.

function validateEmail() {
    var email = $("#email").val();
    var isValid = false;

    if (email == "" || email.length == 0) {
        $("#emailresult").html("You must enter your email");
        isValid = false;
    } else if (email.length > 50) {
        $("#emailresult").html("Your email cannot have more than 75 characters");
       isValid = false;
    } else {
        $.post("checkemail.php", { email: email }, function(result) {
            if (result == 1) {
                alert("Inside result==1");
                $("#emailresult").html("Valid");
                isValid = true;
                alert(isValid);
            } else {
                alert("Inside result != 1");
                $("#emailresult").html("Already in use");
                isValid = false;
            }
        });
    }
    alert(isValid);
    return isValid;
}

Assume that checkemail.php returns 1. The alerts I have are:

1. Inside result==1
2. true
3. false

So, in the end, the value returned is always false even if the email should be valid. Anyone can help me fix this?

Thank you!