Update 02/02/16 15:07:00 PST: Current state of the PR.
Update 02/01/16 14:10:00 PST: Current state of the PR.

Newcomers: As a heads up, GitHub has started clipping the thread down. I'm going to start cataloguing common questions tonight, and I'll include them under this notice so that you can quickly look to see if there's an answer to your issue without having to search through the entire thread. Thanks for taking a look & weighing in! Whether you're for this change or against it, your feedback is valuable and I'll respond to it as soon as I'm able.

FAQ / TL;DR for newcomers

What is the current proposal?

If landed, this will land behind a flag, moving out from behind the flag in the next major version as an unsupported API, finally becoming fully supported one major version after that. It does not replace or supersede the callback API, which will continue to be the canonical Node interface.

Promise-returning variants will be exposed for all "single-operation" Node methods — that is to say, Streams and EventEmitter-based APIs are not included in this discussion.

To use promises:

const fs = require('fs')

fs.readFile.promise(__filename).then(data => {
})

Using promises with destructuring assignment (where available):

const {readFile} = require('fs').promise

readFile(__filename).then(data => {
})

As a prerequisite to this PR landing, domains must work as expected with native Promises (they currently do not.) AsyncWrap is not blocked on this PR, neither is this PR blocked on AsyncWrap. Both PRs are blocked on getting adequate instrumentation of the microtask queue from V8 (we need a callback for "microtask enqueued" events so we can track their creation, and ideally a way to step the microtask queue forward ourselves, running code between invocations.)

Why use the promisify approach?

Response summed up here.

Why not fs.readFileAsync?

@phpnode notes that it could break existing users of bluebird.promisifyAll. Plus, the naming is contentious.

Why not return promises when callbacks are not given?

• @jasnell expresses deep discomfort with switching return types.
  • We already have to provide alternate APIs in some places due to

Should programmer errors throw, or reject?

Summed up here.

What is the value in adding them to core?

@rvagg has been massively helpful in driving this conversation forward:

• What are the killer arguments for including this, what is the value in adding this vs. leaving it userland?,
  • @domenic notes that async/await is spec'd and roadmapped.
  • My response.
  • @mikeal notes that promises are part of the platform already, and that adding support increases the base of folks willing to help fix bugs with in interoperation with them
• @rvagg asks for clarification on the value as a part of this comment.
  • My (wordy, sorry) response.

How do you address the need to keep core small while expanding the exposed API?

Where do we draw the line in supporting language-level constructs?

@yoshuawuyts, @rvagg, and @DonutEspresso have largely driven this conversation:

• @yoshuawuyts asks.
  • @mikeal responds.
  • @brianleroux responds.
  • My response.
• @DonutEspresso raises concerns.
  • My response.
• @rvagg asks as a part of this comment.
  • My (wordy, sorry) response.

Why are modules (like require('fs/promise')) not in scope for this discussion?

@phpnode, @RReverser, @mikeal, @ktrott, (and others, I'm sure!) have brought this up, and I've been pushing to have this conversation in a different, later issue. The conversation has mainly revolved around consensus, and the difficulty in attaining it when adding modules to the mix.

• @ktrott does an excellent job of summing up the concern.
  • My (again wordy, sorry!) response.

This is the original text of the PR. It does not reflect the current state of the conversation. See above for common themes!
This PR introduces Core support for Promises. Where possible, callbacks have
been made optional; when omitted, a Promise is returned. For some APIs (like
http.get and crypto.randomBytes) this approach is not feasible. In those
cases promise-returning *Async versions have been added
(crypto.randomBytesAsync). The naming there isn't set in stone, but it's
based on how bluebird's promisifyAll
method generates names.

This PR allows users to swap the implementation of promises used by core.
This helps in a few dimensions:

• Folks that prefer a particular flavor of Promises for sugar methods can set
  the promise implementation at app-level and forget about it.
• This should alleviate concerns about stifling ecosystem creativity — putting
  promises in Core doesn't mean Core needs to pick a winner.
• Folks who prefer a faster implementation of promises can swap in their
  preferred library; folks who want to use whatever native debugging is added
  at the V8 level for native promises can avoid swapping in promises.
• A world of benchmarks opens up for Promise library authors — any code in the
  ecosystem that uses promises generated by core and has a benchmarking suite
  is a potential benchmark for promise libraries.

The API for swapping implementations is process.setPromiseImplementation().
It may be used many times, however after the first time deprecation warnings
are logged with the origin of the first call. This way if a package misbehaves
and sets the implementation for an application, it's easy to track down.

Example:

process.setPromiseImplementation(require('bluebird'));

const fs = require('fs');

fs.readFile('/usr/share/dict/words', 'utf8')
  .then(words => console.log(`there are ${words.split('\n').length} words`))
  .catch(err => console.error(`there are no words`));

Streams are notably missing from this PR – promisifying streams is more
involved since the ecosystem already relies on a common (and more importantly,
often pinned) definition of how that type works. Changing streams will take
effort and attention from the @nodejs/streams WG. All other callback-exposing
modules and methods have been promisified (or an alternative made available),
though.

Three new internal modules have been added:

• lib/internal/promises — tracks the original builtin Promise object, as
  well as the currently selected Promise implementation. All promises
  created by Node are initially native, and then passed to the selected
  implementation's .resolve function to cast them.
• lib/internal/promisify — turn a callback-accepting function into one that
  accepts callbacks or generates promises. Errors thrown on the same tick
  are still thrown, not returned as rejected promises — in other words,
  programmer errors, such as invalid encodings, are still eagerly thrown.
• lib/internal/callbackify — turn a synchronous or promise-returning function
  into a callback-accepting function. This is only used in lib/readline for
  the completer functionality. This could probably fall off this PR, but it
  would be useful for subsequent changes to streams.

Breaking Changes

• In general: any API that would have thrown with no callback most likely does
  not throw now.
• fs APIs called without a callback will no longer crash the process with an
  exception.
• ChildProcess#send with no callback no longer returns Boolean, instead
  returns Promise that resolves when .send has completed.
• Wrapped APIs:
  • child_process.ChildProcess#send
  • cluster.disconnect
  • dgram.Socket#bind
  • dgram.Socket#close
  • dgram.Socket#send
  • dgram.Socket#sendto
  • dns.lookupService
  • dns.lookup
  • dns.resolve
  • fs.access
  • fs.appendFile
  • fs.chmod
  • fs.chown
  • fs.close
  • fs.exists
  • fs.fchmod
  • fs.fchown
  • fs.fdatasync
  • fs.fstat
  • fs.fsync
  • fs.ftruncate
  • fs.futimes
  • fs.lchmod
  • fs.lchown
  • fs.link
  • fs.lstat
  • fs.mkdir
  • fs.open
  • fs.readFile
  • fs.readdir
  • fs.readlink
  • fs.realpath
  • fs.rename
  • fs.rmdir
  • fs.stat
  • fs.symlink
  • fs.unlink
  • fs.utimes
  • fs.writeFile
  • net.Socket#setTimeout
  • readline.Interface#question
  • repl.REPLServer#complete
  • zlib.deflateRaw
  • zlib.deflate
  • zlib.gunzip
  • zlib.gzip
  • zlib.inflateRaw
  • zlib.inflate
  • zlib.unzip

New APIs

• process.setPromiseImplementation
• net.connectAsync
• tls.connectAsync
• crypto.randomBytesAsync
• crypto.pbkdf2Async
• crypto.pseudoRandomBytesAsync
• crypto.rngAsync
• crypto.prngAsync
• http.getAsync
• https.getAsync
• http.requestAsync
• https.requestAsync
• child_process.execAsync
• child_process.execFileAsync

Next steps

I haven't finished this PR yet: the primary missing piece is tests for the
promise-based APIs, to ensure that they resolve to the correct values. Docs
also need updated. I'll be working on this in my free time over the next week.

Here are the bits that I'd like folks reading this to keep in mind:

• The code changes here are fairly cheap, time-wise — one evening's worth of work.
  • Technical possibility has never been the primary blocker.
• We know async/await is coming down the pipe, and many devs are interested in
  it.
  • With ChakraCore potentially coming in, this may happen sooner than anyone
    previously imagined.
  • Even sans the ChakraCore PR, it's my understanding that V8 will be supporting
    async/await by EOY (Correct me if I'm wrong, here!)
• Promises may not be your cup of tea. This is okay. This is not an attempt to
  replace callbacks or streams with promises. They can co-exist. While Promises
  are complicated, much of that complication falls out of the problem space that
  both callbacks and promises abstract over. Give this a fair shake.