javascript phone formatting function (USA)

//Author:Parikshit Sehgal
function formatphone(num) //Need to call on onblur and keyup
{
var f1,f2,f3,orgnum;
orgnum = num.toString().replace(/\(\)\-/g,''); //Format the incoming phone number.
if(isNaN(orgnum))
return "";
else
{
f1 = orgnum.substring(0,3);
f2 = orgnum.substring(3,6)
f3 = orgnum.substring(6,10);
if (orgnum.length < 3)
{
return num;
}
if (orgnum.length == 3)
{
return "(" + f1 + ")"; // (XXX)
}
if (orgnum.length == 6)
{
return "(" + f1 + ")" + f2; // (XXX)XXX
}
if(orgnum.length >= 10)
{
return "(" + f1 + ")" + f2 + "-"+ f3;//(XXX)XXX-XXXX
}
return num;
}
}