Domenic Denicola domenic at
Tue Nov 6 10:59:36 PST 2012

I guess now is a good a time as any to pre-announce Promises/A+:

It’s an attempt to improve significantly on the minimal-but-perhaps-too-minimal Promises/A of CommonJS, making the language more rigorous and speccing several important things Promises/A missed. Here’s a short summary of the differences:

The two most important ones, in my opinion, are:

 Cover the case of handlers returning a promise (chaining)
Require asynchronous resolution

Promises/A+ is a collaborative effort led by Brian Cavalier (when.js) with help from myself and Kris Kowal (Q), as well as Yehuda Katz (rsvp.js, TC39) and others with whom I am less personally familiar with but have also been very helpful.


I say “pre-announce” because there are a number of bookkeeping issues we want to take care of before saying it’s truly done:

But everything important is already in the repo. We also have a (again, preliminary) conformance test suite at


I hope this is helpful to TC39 or others considering promise standardization. My ideal vision is that the community experiments with promises + generators (a la taskjs) in the ES6 timeframe, then in ES7 we standardize on something like the concurrency strawman or a C#-like async/await pattern based on promises.

We welcome feedback from TC39 and others, preferably as GitHub issues in the repo (and thus we can avoid derailing this thread). We’d especially love the eyes of spec-experienced folks such as those that frequent this list.

From: David Bruant
Sent: ‎November‎ ‎6‎, ‎2012 ‎13‎:‎47
To: EcmaScript
Subject: Promises


In a post to public-script-coord yesterday, Alex Russel wrote the
following [1]:
"[Web]IDL *is handy. *More to the point, it's the language of the specs
we have now, and the default mode for writing new ones is "copy/paste
some IDL from another spec that looks close to what I need and then hack
away until it's close". This M.O. is exacerbated by the reality that
most of the folks writing these specs are C++ hackers, not JS
developers. For many, WebIDL becomes a safety blanket that keeps them
from having to ever think about the operational JS semantics or be
confronted with the mismatches."

I wasn't aware of this and then read through about a dozen WebAPIs [2]
between yesterday and today and... discovered it's the case. In my
opinion, one of the most absurd example is the DOMRequest thing which
looks like:
    readonly attribute DOMString readyState; // "processing" or "done"
    readonly attribute DOMError? error;
    attribute EventListener      onsuccess;
    attribute EventListener      onerror;
    attribute readonly any?      result;

Read it carefully and you'll realize this is actually a promise... but
it has this absurd thing that it has to have both an error and result
field while only one is actually field at any given point.
Also, these APIs and JavaScript as it they are won't support promise
chainability and the excellent error forwarding that comes with it
off-the-shelf. Also, the lack of a built-in Q.all really doesn't promote
good code when it comes to event synchronization.
Oh yes, of course, you can always build a promise library on top of the
current APIs, blablabla... and waste battery with these libraries [3].

I'm coming with the following plan:
1) get promises in ECMAScript
2) get WebIDL to support ECMAScript promises
3) get browser APIs to use WebIDL promises

About the first step, there is a strawman [4] that contains promises and
it requires to define the event loop, so that's probably too much for
ES6. Yet, it doesn't prevent to agree on a promise API that will be
adopted in ES7.
Besides the strawman, promises have run a long way from CommonJS [5] to
jQuery [6] to Q [7] to Irakli's excellent post [8] to Domenic's recent
rant [9] and I've missed a lot of other examples probably. The JS
community is ready for promises. The idea has been used a lot.
Different libraries have different APIs and I have no preference. The
only things I really care about is chaining (with error forwarding) and
a promise-joining function à la Q.all. I'll let people who care about
naming fight.

I'm sure TC39 can come to an agreement *before* ES7 standardization,
agreement that can be used by WebIDL and browser APIs (why not
implemented long before ES7 work has even started).
If you're a JS dev and care about promises, please show some support to
this proposal :-)


es-discuss mailing list
es-discuss at

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the es-discuss mailing list