12 or 16 Digit Aadhar Number Validation using JavaScript

I want to do adhar card validation i.e. values in textbox must be 12 digits or 16 digits.

I am trying below javascript code :

           adhar = document.getElementById("txtaadhar").value;
           var adharcard = /^\d{12}$/;
           var adharsixteendigit = /^\d{16}$/;
           if (adhar != '') {
                if (!adhar.match(adharcard))
                {
                    alert("Invalid Aadhar Number");
                    return false;
                }
            }
            if (adhar != '')
            {
                if(!adhar.match(adharsixteendigit))
                {
                    alert("Invalid Aadhar Number");
                    return false;
                }
            }

what changs i should do to get my output

Thanks