>>> ''.tr

tr.js

function (a, b) {
  a = a || "aeioubcdfghjklmnpqrstvwxyz";
  return this.replace(RegExp("([" + a + "])", "ig"), function (c,d,e) {
    e = c.toLowerCase();
    d = (b || "iouaenpqrstvwxyzbcdfghjklm")[a.indexOf(e)];
    return c==e ? d : d.toUpperCase();
  });
}

function(a,b){a=a||"aeioubcdfghjklmnpqrstvwxyz";return this.replace(RegExp("(["+a+"])","ig"),function(c,d,e){e=c.toLowerCase();d=(b||"iouaenpqrstvwxyzbcdfghjklm")[a.indexOf(e)];return c==e?d:d.toUpperCase();});}