On 2010-02-01 14:51, Jan Sch tze wrote:

One issue with most of the suggestions I've seen for this
so far is that they require type testing to distinguish
success or failure.

If you're going to have a single callback, make it
unambiguous, so that the same pattern can be used regardless
of the range of values that might be returned.

In Felix's[1] example:

function loadConfig() {
  return posix.cat('config.json')
    (function(config, error) {
      return config || posix.cat('configuration.json')
    })
    (function(config, error) {
      return error || JSON.parse(config);
    })

[1]: https://gist.github.com/85c313e588a3fc9d8636

This works only until you need to return undefined, (or 0,
"", null, or Boolean false...) as a successful asynchronous
result.

I like to use values which are either [0,<error>] or [1,<data>],
which gives[2]:

function loadConfig() {
  return posix.cat('config.json')
    (function(either) {
      return either[0] ? either[1] : posix.cat('configuration.json')
    })
    (function(either) {
      return either[0] ? JSON.parse(either[1]) : "Could not read conf file: "+either[1]
    })

[2]: https://gist.github.com/67cbf71c79fd5b6bf3a1

Or if you prefer separate arguments, make the first indicate
success and the second be either the successful result or the
error:

function loadConfig() {
  return posix.cat('config.json')
    (function(succeeded,value) {
      return succeeded ? value : posix.cat('configuration.json')
    })
    (function(succeeded,value) {
      return succeeded ? JSON.parse(value) : "Could not read conf file: "+value
    })

--
http://inimino.org/~inimino/blog/