How to disable the radio button using javascript ? (Not using any JS framework)

first of all your ids should be given in double quote. like this:

 <input type="radio" name="disableme" id="1"> Animal
 <input type="radio" name="disableme" id="2"> Mammal
 <input type="radio" name="disableme" id="3"> Human

and for disabling the button using javascript,use this:

document.getElementById("1").disabled=true;

document.getElementById("1").disabled=true;
document.getElementById("2").disabled=true;
document.getElementById("3").disabled=true;

Try this code

var radio=document.getElementsByName("disableme");
   var len=radio.length;
   for(var i=0;i<len;i++)
   {
       radio[i].disabled=true;
   }


<input type="radio" name="disableme" id=1> Animal
<input type="radio" name="disableme"  id=2> Mammal
<input type="radio" name="disableme"  disabled="disabled" id=3> Human

I tried like , document.formName.disableme.disabled = true; But it didn't worked..

Because if you have more than one form control with the same name, you will get back an HTML Form Controls Collection. So loop over the collection:

var radios = document.formName.disableme;

for (var i=0, iLen=radios.length; i<iLen; i++) {
  radios[i].disabled = true;
} 

There is no need to add an ID.