Send Form fields to jquery

I'm trying to pass all my form fields, to a ajax function where i will insert the user into the database.

But for some reason, my alert (in my JS file) isn't showing anything.

Any ideas what i'm doing wrong?

My HTML:

        <form id="signupForm">
            <input id="signupFormEmail" type="text" name="email" placeholder=" E-mail"><br />
            <input id="signupFormPassword" type="text" name="password" placeholder=" Password"><br />
            <input id="signupFormUsername" type="text" name="userName" placeholder=" User Name"><br />
            <input id="submitSignup" type="button" value="SIGN UP" onclick="signUp(this);">
        </form>

My javascript file:

function signUp(elem)
{   
var postData = $(this).serializeArray();

//$('#myResults').html(postData);
alert($.param($(elem).serializeArray()));

if($(elem).parent().children('#signupFormEmail').val() != ''){      
    // verifica se o email já existe
    $.ajax(
    {
          url: "/newsletter/check-email/",
          type: "POST",
          data: {type:'check',email:$(elem).parent().children('#signupFormEmail').val()}
    }).done(function(response)
    {   
        if(response == -1) {
            $.ajax(
            {
              url: "/newsignup/registare/",
              type: "POST",
              data: postData
            }).done(function(userCreated) {
                if(userCreated == 1) {
                alert('user created');
                    /*
                    $(elem).parent().children('#signupForm').val('');
                    $('#signUpCompleted').show();
                    */
                } 
                else
                {
                    /*$('#signUpError').show();*/
                    alert('user not created');
                }
            })

            //testing
            //$('#signUpCompleted').show();
        }
        else //testing
        {
            $('.emailError').show(); //testing
        }

    }
    );
}
}