Written Proposal for "package" Keyword for ES20XX Imports

Greg McLeod cleod9 at gmail.com
Tue Feb 21 19:48:49 UTC 2017

@Logan: Oh I see what you mean, I suppose I was thinking about it in a
different way. I do know in ES6 the import keywords are always hoisted to
the top, so it should be a given that the modules will be "resolved"
already as a consequence of that. But there is no guarantee that all of the
modules are linked successfully, namely in the edge cases of inheritance.

Consider the following

// app.js
import A from "./a.js";
import B from "./b.js";

var a = new A();
var b = new B();

// a.js
import B from "./b.js";

console.log("A's reference to B", B);

export default class A {
  constructor() {

// b.js
import A from "./a.js";

console.log("B's reference to A", A);

export default class B extends A {
  constructor() {

In the latest Babel the above code fails to run on the first line of app.js
(at the time it attempts to execute `_inherits(B, _A);`.). This is because
"A" is undefined in module b.js due to the circular dependency. In this
case Babel basically executed code in the incorrect order for this to be
able to work. The package keyword avoids this by creating a hoist-like
behavior for classes that are inherited, which is safe to do given the
proposed limitations in the top-level scope

I should note that circular dependencies like this are (hopefully) not too
common, but one coming from another language might expect this to work
based on how ES6 imports are described. I think the behavior of the package
keyword could be much more transparent about what it's capable of.

On Tue, Feb 21, 2017 at 1:56 PM, Logan Smyth <loganfsmyth at gmail.com> wrote:

> I don't have tons of comments on the proposal as a whole, but I do think
> there may be confusion about ES6 imports. Imports are resolved and
> processed and modules linked before any execution of the module body takes
> place. I can't tell from your description if you are reading that
> differently.
> On Tue, Feb 21, 2017 at 10:48 AM, Greg McLeod <cleod9 at gmail.com> wrote:
>> Thanks Ryan. I think there would be a bit of an issue with introducing
>> dictionary support alone in that it does not provide a new way for the
>> language to define where modules actually live in the application itself as
>> opposed to the file system. This misses out on the benefits the package
>> keyword could provide. "package" should impose some limitations on the
>> top-most scope of a module to facilitate the ability to define the packages
>> before actually executing a module's body.
>> To elaborate, one of the biggest benefits I'd like to emphasize is the
>> complete isolation of dependency resolution and actual module body
>> execution.
>> The package keyword in my proposal would allow you to arbitrarily say
>> "here's the complete structure of my app, all dependencies already
>> resolved" and subsequently take action on it after it has been fully loaded
>> into memory in its entirety. Augmenting existing import syntax cannot allow
>> this since the core body of each module is being executed amidst the
>> dependency graph being resolved as each "import" is encountered by the
>> interpreter.
>> On Tue, Feb 21, 2017 at 12:52 PM, Ryan Birmingham <rainventions at gmail.com
>> > wrote:
>>> I like the idea you're working from here, it seems to partially resemble
>>> header files. I'm not sure if a package keyword is needed though; couldn't
>>> you accomplish the same thing with just adding dictionary support and an
>>> optional alias parameter to import/export?
>>> -Ryan Birmingham
>>> On 21 February 2017 at 12:27, Greg McLeod <cleod9 at gmail.com> wrote:
>>>> Hi all,
>>>> Back in 2015 I was working on a proposal for the "package" keyword, but
>>>> stopped prematurely as ES6/ES2015 had already been shipped. Since then I've
>>>> been occasionally iterating on it anyway in order to flesh out my ideas
>>>> with how it could fit with the new import syntax. I wanted to share with
>>>> the mailing list to get some feedback on it, as well as help facilitate
>>>> some of the discussion regarding future import syntax.
>>>> Proposal can be found here:
>>>> https://gist.github.com/Cleod9/7f73017905d124210ec8
>>>> I should note that a lot of this is derived from an ActionScript to
>>>> JavaScript proof-of-concept transpiler I wrote a few years back. (Live
>>>> demo: https://as3js.org/demo/ ). I believe this type of package
>>>> syntax/behavior has already been possible since ES5, although I have yet to
>>>> find any proposals for the keyword that resemble that of battle-tested
>>>> languages like Java/C#.
>>>> I think there are a lot of time-saving qualities that packages could
>>>> provide for bundled JS applications we have yet to explore, and it could
>>>> even open the door to a JS standard library. Even if this doesn't go
>>>> anywhere I certainly hope by making this proposal public it will be a
>>>> useful for reference for what is possible in the current state of affairs.
>>>> Any feedback would be greatly appreciated!
>>>> Greg
>>>> P.S. Screen-cap of a potential IDE future with packages:
>>>> http://i.imgur.com/D5EGNUN.gifv
>>>> _______________________________________________
>>>> es-discuss mailing list
>>>> es-discuss at mozilla.org
>>>> https://mail.mozilla.org/listinfo/es-discuss
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