Submit form via AJAX using plain Javascript without Jquery get checked radio button value

I do have HTML form submitting via AJAX using plain Javascript:

function XMLhttp(){
    var formInputs = document.getElementById(formID).querySelectorAll("input, textarea");

    var selectFormGebdat = formID;

    var toCheckDatesFields = document.getElementsByClassName("checkDate");
    for (var i = 0; i < toCheckDatesFields.length; i++) {
        if(toCheckDatesFields.item(i).value != '') {
            var returnValue = validatedate(toCheckDatesFields.item(i));
            if(returnValue == false) {
                return false;
            }
        }
        
    }
                                                 
    var httpRequest = new XMLHttpRequest();
    var formData = new FormData();

    for( var i=0; i < formInputs.length; i++ ){
        formData.append(formInputs[i].name, formInputs[i].value);
    }

    httpRequest.onreadystatechange = function(){
        if ( this.readyState == 4 && this.status == 200 ) {
            resultData = JSON.parse(this.responseText);

            if(resultData.success == true) {
                document.getElementById('resultok').className += ' show';
            }
            else {
                document.getElementById('resulterror').className += ' show';
            };
           
        }
    };

    httpRequest.open(formMethod, formAction);
    httpRequest.send(formData);
}

selectButton.onclick = function(){
    XMLhttp();
}

selectForm.onsubmit = function(){
    return false;
}

My problem is, that I have radio buttons and select-box. But the selected values are not submitted. F.ex.

<input type="radio" name="typ" value="anfrage"><input type="radio" name="typ" value="reservierung">

If anfrage is checked, the script is not submitting this value - there is always reservierung submitting.

Thanks for help! Martin