Buffer.byteLength(string, [encoding])

str = '/u00bd + /u00bc = /u00be';
console.log(str + ": " + str.length + " characters, " +
  Buffer.byteLength(str, 'utf8') + " bytes");
// ½ + ¼ = ¾: 9 characters, 12 bytes

Buffer.byteLength = function(str, enc) {
  var ret;
  str = str + '';
  switch (enc) {
    case 'ascii':
    case 'binary':
    case 'raw':
      ret = str.length;
      break;
    case 'ucs2':
    case 'ucs-2':
    case 'utf16le':
    case 'utf-16le':
      ret = str.length * 2;
      break;
    case 'hex':
      ret = str.length >>> 1;
      break;
    default:
      ret = internal.byteLength(str, enc);
  }
  return ret;
};