Reconsider promisify treatment of multiple arguments

[medikoo]

Problem well discussed here: https://github.com/petkaantonov/bluebird/issues/307

In short: non-breaking change to external API may result in breaking change for module that relies on promisify, that certainly raises the issue.

[medikoo]

Proposal:

• By default always promisify first argument after error as resolved value, that's it
• Support options argument to promisify, with which we could alter how resolved value is prepared:
  • mulitpleResult: true When provided, each call will resolve with an array that would contain all arguments after error
  • resolveResult: function (arg1, arg2...) { ... } Custom arguments mapper, it will allow to map list of arguments to convinent form custom way, e.g. in case of Request we may make resolved result as { req: req, res: res }

[nejucomo]

I'm new to node, but familiar with Twisted's deferred (from python), and after a quick skim of this README this looks like the library I want. However, the very first node call I tried to promisify was child_process.execFile which is a multi-arg callback api: callback(error, stdout, stderr). I just wanted to point this out so that once a solution is implemented, the docs could use execFile as an example.

FWIW, I prefer the {multipleResult: true} API design, and dislike any kind of automatic coercion logic like:

if (arguments.length > 1) {
  /* use an array */
} else {
  /* use arguments[0] */
}

-because this introduces a type ambiguity: It prevents writing a generic callback that tries to distinguish these two cases, because arguments[0] may itself be an Array.

[medikoo]

@nejucomo thanks for comment, this one definitely will be addressed and will be published with v1