This works with any explorer.

document.querySelector('input[name="genderS"]:checked').value;

This is a simple way to get the value of any input type. You also do not need to include jQuery path.

document.forms.your-form-name.elements.radio-button-name.value

Since jQuery 1.8, the correct syntax for the query is

$('input[name="genderS"]:checked').val();

Not $('input[@name="genderS"]:checked').val(); anymore, which was working in jQuery 1.7 (with the @).

ECMAScript 6 version

let genderS = Array.from(document.getElementsByName("genderS")).find(r => r.checked).value;

Here's a nice way to get the checked radio button's value with plain JavaScript:

const form = document.forms.demo;
const checked = form.querySelector('input[name=characters]:checked');

// log out the value from the :checked radio
console.log(checked.value);

Source:

Using this HTML:

<form name="demo">
  <label>
    Mario
    <input type="radio" value="mario" name="characters" checked>
  </label>
  <label>
    Luigi
    <input type="radio" value="luigi" name="characters">
  </label>
  <label>
    Toad
    <input type="radio" value="toad" name="characters">
  </label>
</form>

You could also use Array Find the checked property to find the checked item:

Array.from(form.elements.characters).find(radio => radio.checked);

In case someone was looking for an answer and landed here like me, from Chrome 34 and Firefox 33 you can do the following:

var form = document.theForm;
var radios = form.elements['genderS'];
alert(radios.value);

or simpler:

alert(document.theForm.genderS.value);

refrence:

Edit: As said by Chips_100 you should use :

var sizes = document.theForm[field];

directly without using the test variable.

Old answer:

Shouldn't you eval like this ?

var sizes = eval(test);

I don't know how that works, but to me you're only copying a string.

Try this

function findSelection(field) {
    var test = document.getElementsByName(field);
    var sizes = test.length;
    alert(sizes);
    for (i=0; i < sizes; i++) {
            if (test[i].checked==true) {
            alert(test[i].value + ' you got a value');     
            return test[i].value;
        }
    }
}


function submitForm() {

    var genderS =  findSelection("genderS");
    alert(genderS);
    return false;
}

A fiddle here.

This is pure JavaScript, based on the answer by @Fontas but with safety code to return an empty string (and avoid a TypeError) if there isn't a selected radio button:

var genderSRadio = document.querySelector("input[name=genderS]:checked");
var genderSValue = genderSRadio ? genderSRadio.value : "";

The code breaks down like this:

• Line 1: get a reference to the control that (a) is an <input> type, (b) has a name attribute of genderS, and (c) is checked.
• Line 2: If there is such a control, return its value. If there isn't, return an empty string. The genderSRadio variable is truthy if Line 1 finds the control and null/falsey if it doesn't.

For JQuery, use @jbabey's answer, and note that if there isn't a selected radio button it will return undefined.

First, shoutout to ashraf aaref, who's answer I would like to expand a little.

As MDN Web Docs suggest, using RadioNodeList is the preferred way to go:

// Get the form
const form = document.forms[0];

// Get the form's radio buttons
const radios = form.elements['color'];

// You can also easily get the selected value
console.log(radios.value);

// Set the "red" option as the value, i.e. select it
radios.value = 'red';

One might however also select the form via querySelector, which works fine too:

const form = document.querySelector('form[name="somename"]')

However, selecting the radios directly will not work, because it returns a simple NodeList.

document.querySelectorAll('input[name="color"]')
// Returns: NodeList [ input, input ]

While selecting the form first returns a RadioNodeList

document.forms[0].elements['color']
// document.forms[0].color # Shortcut variant
// document.forms[0].elements['complex[naming]'] # Note: shortcuts do not work well with complex field names, thus `elements` for a more programmatic aproach
// Returns: RadioNodeList { 0: input, 1: input, value: "red", length: 2 }

This is why you have to select the form first and then call the elements Method. Aside from all the input Nodes, the RadioNodeList also includes a property value, which enables this simple manipulation.

Reference:

Here is an Example for Radios where no Checked="checked" attribute is used

function test() {
var radios = document.getElementsByName("radiotest");
var found = 1;
for (var i = 0; i < radios.length; i++) {       
    if (radios[i].checked) {
        alert(radios[i].value);
        found = 0;
        break;
    }
}
   if(found == 1)
   {
     alert("Please Select Radio");
   }    
}

DEMO : [Click Find without selecting any Radio]

Source (from my blog):

Putting Ed Gibbs' answer into a general function:

function findSelection(rad_name) {
    let rad_val = document.querySelector('input[name=' + rad_name + ']:checked');
    return (rad_val ? rad_val.value : "");
}

Then you can do findSelection("genderS");

lets suppose you need to place different rows of radio buttons in a form, each with separate attribute names ('option1','option2' etc) but the same class name. Perhaps you need them in multiple rows where they will each submit a value based on a scale of 1 to 5 pertaining to a question. you can write your javascript like so:

<script type="text/javascript">

    var ratings = document.getElementsByClassName('ratings'); // we access all our radio buttons elements by class name     
    var radios="";

    var i;
    for(i=0;i<ratings.length;i++){
        ratings[i].onclick=function(){
            var result = 0;
            radios = document.querySelectorAll("input[class=ratings]:checked");
            for(j=0;j<radios.length;j++){
                result =  result + + radios[j].value;
            }
            console.log(result);
            document.getElementById('overall-average-rating').innerHTML = result; // this row displays your total rating
        }
    }
</script>

I would also insert the final output into a hidden form element to be submitted together with the form.

I prefer to use a formdata object as it represents the value that should be send if the form was submitted.

Demo:

let formData = new FormData(document.querySelector("form"));
console.log(`The value is: ${formData.get("choice")}`);

<form>
    <p>Pizza crust:</p>
    <p>
        <input type="radio" name="choice" value="regular" >
        <label for="choice1id">Regular crust</label>
    </p>
    <p>
        <input type="radio" name="choice" value="deep" checked >
        <label for="choice2id">Deep dish</label>
    </p>
</form>

Using a pure javascript, you can handle the reference to the object that dispatched the event.

function (event) {
    console.log(event.target.value);
}

If it is possible for you to assign a Id for your form element(), this way can be considered as a safe alternative way (specially when radio group element name is not unique in document):

function findSelection(field) {
    var formInputElements = document.getElementById("yourFormId").getElementsByTagName("input");
    alert(formInputElements);
        for (i=0; i < formInputElements.length; i++) {
        if ((formInputElements[i].type == "radio") && (formInputElements[i].name == field) && (formInputElements[i].checked)) {
            alert(formInputElements[i].value + ' you got a value');     
            return formInputElements[i].value;
        }
    }
}

HTML:

<form action="#n" name="theForm" id="yourFormId">

 document.querySelector('input[name=genderS]:checked').value

I like to use brackets to get value from input, its way more clear than using dots.

document.forms['form_name']['input_name'].value;