How to set radio button status with JavaScript

Very simple

radiobtn = document.getElementById("theid");
radiobtn.checked = true;

the form

<form name="teenageMutant">
  <input value="aa" type="radio" name="ninjaTurtles"/>
  <input value="bb" type="radio" name="ninjaTurtles"/>
  <input value="cc" type="radio" name="ninjaTurtles" checked/>
</form>

value="cc" will be checked by default, if you remove the "checked" non of the boxes will be checked when the form is first loaded.

document.teenageMutant.ninjaTurtles[0].checked=true;

now value="aa" is checked. The other radio check boxes are unchecked.

see it in action: http://jsfiddle.net/yaArr/

You may do the same using the form id and the radio button id. Here is a form with id's.

<form id="lizardPeople" name="teenageMutant">
  <input id="dinosaurs" value="aa" type="radio" name="ninjaTurtles"/>
  <input id="elephant" value="bb" type="radio" name="ninjaTurtles"/>
  <input id="dodoBird" value="cc" type="radio" name="ninjaTurtles" checked/>
</form>

value="cc" is checked by default.

document.forms["lizardPeople"]["dinosaurs"].checked=true;

now value="aa" with id="dinosaurs" is checked, just like before.

See it in action: http://jsfiddle.net/jPfXS/

Vanilla Javascript:

yourRadioButton.checked = true;

jQuery:

$('input[name=foo]').prop('checked', true);

or

$("input:checkbox").val() == "true"

You can also explicitly set value of radio button:

<form name="gendersForm">
  <input type="radio" name="gender" value="M" /> Man
  <input type="radio" name="gender" value="F" /> Woman
</form>

with the following script:

document.gendersForm.gender.value="F";

and corresponding radio button will be checked automatically.

Look at the example on JSFiddle.