Possible elaboration on errnoException() for node 0.5.x

gistfile1.js

//XXX Maybe someday there will be a nice way for user-land modules to
//      generate pseudo-errno errors?

function errnoException(errno, syscall, message, path) {

  var e = new Error();

  var msg = '';
  if ( syscall !== undefined ) {
    msg += syscall + ' ';
    e.syscall = syscall;
  }

  if ( errno !== undefined ) {
    // Try to resolve errno to both a name and a value
    if ( typeof errno === 'string'  &&  isOwn.call(constants, errno) ) {
      e.code  = errno;
      e.errno = constants[errno];
      msg += errno;
    } else {
      e.code = e.errno = errno;
      msg += errno.toString();
    }
  }

  if ( message !== undefined ) {
    if ( msg !== '' ) { msg += ', '; }
    msg += message;
  }

  if ( path !== undefined ) {
    if ( msg !== '' ) { msg += ' '; }
    msg += '\'' + path + '\'';
    e.path = path;
  }

  e.message = msg;

  return e;
}



  Saw real exception     object true  (instanceof Error)
{ stack: [Getter/Setter],
  arguments: undefined,
  type: undefined,
  message: 'ELOOP, Too many levels of symbolic links \'/home/Tom/study/javascript/node.js/play/test/tmp/testrpx/realpath-3a\'',
  errno: 40,
  code: 'ELOOP',
  path: '/home/Tom/study/javascript/node.js/play/test/tmp/testrpx/realpath-3a' }

  Saw fake exception     object true  (instanceof Error)
{ stack: [Getter/Setter],
  arguments: undefined,
  type: undefined,
  code: 'ELOOP',
  errno: 40,
  path: '/home/Tom/study/javascript/node.js/play/test/tmp/testrpx/realpath-3a',
  message: 'ELOOP, Too many levels of symbolic links \'/home/Tom/study/javascript/node.js/play/test/tmp/testrpx/realpath-3a\'' }


Throwing the 'real' exception

  node.js:134
          throw e; // process.nextTick error, or 'error' event on first tick
          ^
  Error: ELOOP, Too many levels of symbolic links '/home/Tom/study/javascript/node.js/play/test/tmp/testrpx/realpath-3a'
      at Object.statSync (fs.js:400:18)
      at realpathX (/home/Tom/study/javascript/node.js/play/pathroot.js:408:18)
      at test_cyclic_link

Throwing the 'fake' exception

  node.js:134
          throw e; // process.nextTick error, or 'error' event on first tick
          ^
  Error: ELOOP, Too many levels of symbolic links '/home/Tom/study/javascript/node.js/play/test/tmp/testrpx/realpath-3a'
      at errnoException (/home/Tom/study/javascript/node.js/play/pathroot.js:51:11)
      at realpathX (/home/Tom/study/javascript/node.js/play/pathroot.js:395:21)
      at test_cyclic_link_protection (/home/Tom/study/javascript/node.js/play/test/test_realpathx02.js:281:12)



  lt('  Saw real exception    ', typeof ex, ex instanceof Error);
  lt(ex);

  var exx = errnoException('ELOOP', undefined,
              'Too many levels of symbolic links', base);
  lt('  Saw fake exception    ', typeof exx, exx instanceof Error);
  lt(exx);
  throw exx;

Whoops, isOwn() is

var isOwn = Object.prototype.hasOwnProperty;