May 21, 22, 23 TC39 Meeting Notes

Rick Waldron waldron.rick at
Sun Jun 2 09:46:08 PDT 2013

Technical Notes (by Erik Arvidsson):
Tuesday May 21
John Neumann (JN), Allen Wirfs-Brock (AWB), Eric Ferraiuolo (EF), Erik
Arvidsson (EA), Luke Hoban (LH), Doug Crockford (DC), Yehuda Katz (YK),
Brendan Eich (BE), Sam Tobin-Hochstadt (STH), Alex Russell (AR), Dave
Herman (DH) (calling in), Bernd Mathiske (BM), Andreas Rossberg (ARB), Mark
Miller (MM), Tom Van Cutsem (TVC), Jasvir Naga (JNA), Istvan Sebestyen (IS)

JN: Going through the agenda
Adding __proto__
Unifying iterator/generator APIs
Talking about getting user stats for test-262...
YK: Prioritize ES6 items. So that we don’t get do ES7+ items before

Minutes approved unanimously

4.1 Object.freeze

DC: Today Object.freeze throws when primitives are passed in. Suggesting
not throwing when a value type is passed in.
MM: Object.isExtensible would return false for primitives
EA: This would give an inconstint view for primitives.
AWB/YK: (In strict mode) numbers and strings lazily box so the assignment
never fails.
MM: Proxies are allowed to be non extensible and throw away.
ARB: Is the suggestion to lazily wrap primitives?
MM: No, then isExtensible(7) would return true because the wrapper is
AWB: In most of the new changes we are not doing unnecessary coercion.
YK: The Chrome dev tools, console.dir(7), says “no properties” which
supports treating these as empty objects.
MM: The only observable wrapper is the `this` wrapper in non strict mode.
AWB: In the new spec, Object.setPrototypeOf(7) throws.
MM: Agrees violently!

Conclusion: DC+AWB to work out the details

4.2 WeakSet

Do we need them?
MM: Trivial shim around WeakMap.
YK: Often wanted it
AWB: Adds no new capabilities.
AR: We should not limit ourselves to what is a new primitive capabilities
AI(AWB): add to spec

Consensus to add WeakSet.

4.4 Proxies

TVC’s presentation on Notification Proxies:

Arguments against:
- shifts the burden from spec writers/implementors to users (need to use
shadow target even for non-frozen objects)
- implementors will deal with spec bugs related to invariant violations as
they come up

Conclusion: Notification proxies are not approved. MM & TVC are still happy
with direct proxies.

Proxy Invoke Trap and wrong |this|-binding on built-in methods

AWB: with current default behavior of “get”, “Caretaker” will break on
built-ins such as Date, because the |this| binding is by default set to the
proxy, so the Date built-in method will not find the correct private state.
ARB: Same issue with binary methods
STH: We should add invoke trap but not change the object model
MM: Pleasant to have. Separate from private state.
AWB: used to think this was an issue with proxies, but convinced that it’s
an API issue: we need to provide default handlers that do the right thing,
and which users can subclass. In particular, want a handler that, on
forwarding, rebinds |this| to the target.
STH: If you want to proxy a Date method the underlying `this` needs to be a
non wrapped Date object.
TVC: previously proposed a Handler API that defines derived traps and
fundamental traps, allows you to subclass and inherit correct behavior for
derived traps. Can be used as the basis.

AWB/TVC: invoke trap would make it easier to control |this|-binding
DH: Never liked breaking the semantics of [[Get]] + [[Call]]
TVC: there already exist invoke-only properties on platforms with
AWB: For a [[Call]] it might be important to control `this` but by the time
the [[Call]] is happening you do not know what `this` to use.
DH: ActionScript has a proxy and they do have an invoke trap.
BM: The most common action is to invoke a method.
? : we already gave up on the |this| invariant for accessors: in ES5, if
obj.x is a getter, |this| will always be bound to obj in the getter. With
proxies this is no longer true.

AI(AWB, TVC): Add spec for invoke. Tom and Allen to work out details of a
Handler API that accommodates both “caretaker” (aka forwarding) and
“virtual object” use cases.

Consensus: Add invoke trap.

MM: Everybody in this room wants classes and want to post pone private
state to after ES6
ARB: Disagrees.
ARB: Based on feedback, people do not want unique symbols, only private
MM: Private symbols do not work with proxies.
TVC: can still use WeakMap for private state.
DH: The most common cases where true information hiding is self hosting.
The stakes are too high for the browser engines.
YK: If “iterator” would be a private symbol, you cannot create a proxy that
will work with for-of loops.
ARB: Symbols (unique and private) and relations overlap.
BE: If we add symbols now we are stuck with them.
LH: Future users will be confused. They will not know what to use
BE: Unique symbol is very different from class private syntax.
AWB/MM: If we first did relationships we might not need symbols.
MM: Relationship published but not reflective.
MM: Difference between relationships and symbols: where is the mutability?
This forces us to have both relationships and unique symbols.

Conclusion: ?

Report from Geneva

IS: IPR, vote on june 11, 2 docs about policy. royalty free task group
should function. When approved, create royalty free group within tc39. must
transition to the royalty free task group. Collect before November. Hope to
make switch in 2 or 3 months.
IS: Intel submitted royalty free statement on ecma 262.
BE: So did Mozilla
IS: IPR ad hoc group, work out a solution for a software contribution from
non members. IPR ad hoc group has not finished their work.
AWB: Public RF solution? We never talked about that here.
IS: Additional channel from non members. Compromise; Fill in form on tc39
web site. Click through process. Agree RF TF. Can submit contribution
through the web site.
AWB: But not the software.
IS: It is still missing. It will be the next step.
AWB: This allows someone to write up a proposal and we are allowed to read
it and maybe even incorporate it.
IS: There is an ECMA recognition program. List contributors. Requests a
short list of nominees.
JN: Nominees before Thursday morning.
STH: And maybe a trophy?

Item 9.1.2
JN: RFTG mode. Keep things transparent. Unanimously approved
JN: Doc 24. Any objections? Unanimously approved
4.13 Endianness of Typed array
ARB: Remember it as if we should specify this.
BE: Endianness in Typed Arrays is unspecified.
DH: Keep it open for now... Same system to same system. Using data view,
which is explicit, there is no problem.
STH: We don’t know what WiiU will do?
AWB: Or they decide not to comply to the spec
DH: WebGL is endian agnostic.

Conclusion: Leaving it unspecified in ES6.

4.18 __proto__

STH: Recollection, first as data property, then as an accessor. Then
discussed the power of that setter. Set the [[Prototype]] in the [[Realm]].
Then Allen wrote the spec. Realized that there were some problems with that
design. Roughly the same power as Object.setPrototypeOf.
MM: Existence of a setter... as long as we have the extensibility
restriction, that is sufficient.
AWB: Why restrict __proto__ and not other
DH: Objects belonging to a realm is a bad idea.
MM: No more reason to restrict the setter.
STH: Bind __proto__ setter to the object upon extraction
MM: In SES objects that are non extensible. Not going to remove __proto__
going forward.
ARB: If Object.prototype.__proto__ is a data property, making it non
writable prevents other objects to use assign to set __proto__.
AWB: If Object.prototype.__proto__ is an accessor that just calls
AR: Best practice on the web is important even in the future.
TVC: If we have O.p.__proto__ do we want Object.setPrototypeOf or just
AWB: Makes sense to have Object.setPrototypeOf for consistency.
EA: Where do we draw the line (Object.x or Reflect.x)?
DH: People will need to be able to get this before we have a reflect module.
TVC: We need both because they have different return value (reflect
setPrototypeOf returns boolean success value).

Conclusion: __proto__ is an accessor on Object.prototype. The setter
mutates [[Prototype]]. There is no “poison pill”. We will provide both
Object.setPrototypeOf and std:reflect setPrototypeOf.

Naming of @@iterator
AWB: Suffix with $
STH: Opposed to special naming. People don’t do this kind of naming
convention. Why do we want to introduce this concept?

class Foo {
  *[iterator]() {
    yield ...

Conclusion: No special naming

Generators and iterators
AWB: send is gone in favor of next(arg) (only first arg is passed through
in yield*)
YK: Whether generators return a frozen object or not?
BE: close is gone

Wednesday May 22 2013
John Neumann (JN), Allen Wirfs-Brock (AWB), Eric Ferraiuolo (EF), Erik
Arvidsson (EA), Luke Hoban (LH), Doug Crockford (DC), Yehuda Katz (YK),
Brendan Eich (BE), Sam Tobin-Hochstadt (STH), Alex Russell (AR), Dave
Herman (DH) (calling in), Bernd Mathiske (BM), Andreas Rossberg (ARB), Mark
Miller (MM), Tom Van Cutsem (TVC), Istvan Sebestyen (IS), Jasvir Naga (JNA)

4.16 Spec update

YK: ToPositiveInteger is needed by JSIDL
AI(YK+AWB): Put an algorithm in the spec that DOM can use so that we get
the same behavior in JS and DOM.

6 General implementation experiences

ARB: We started implementing generators but things are pretty smooth.
BE: Doing modules at the moment.
AWB: Bunch of bug fixes in the spec related to classes.

4.9 String templates

MM: Suggests status quo.
AR: Objects.
MM: Controversy related to tag-less templates. Alternatives include making
tag-less templates an error, delayed evaluation (contextually provided)
AR: Econics: naked interpolation is too attractive. Should always have a
tag to encourage users to think about which behavior is correct.
YK: I cannot support Alex’s proposal.
STH: What would the name of this tag be?
AR: Something that is imported from a module.
STH: Concerned about short names and conflicts.
YK: People will just use ‘s’ without thinking.
YK: People should use HTML templating engines.
DC: Alex’s testimony about application developer feedback is relevant.
LH: it sounded like Google engineers were using a template system
EA: Correct.
MM: Does anyone prefer taking out TS if they don’t get tag-less TS?
Everyone: Agrees that it is better to require tag to remove TS from ES6.
AR: Strings are always used later in some context. Communicating the intent
AWB: String concat vs string interpolation have the same issue.
LH: Assumes that maybe only 20% of the uses ot TS are susceptible to XSS
MM: Removing tag-less does not reduce XSS because people will just use s`.
TS helps people transition to a better world. Once they have have a TS it
will be easy to add  an html tag at the front as needed.
ST: It will be painful to import String raw and alias that to s.
MM: Maybe put tag-less in appendix?  Withdrawn idea because no one likes it.
YK: You should not have use string based APIs.
AR: Willing to abstain but “Y’all are making a big mess”
BM: Half convinced by Alex.
LH: Different code bases will use different tags for normal string
interpolation so moving between code bases will be hard to.
AR: That is a good thing. Forces people to think.
MM: Template strings in E.
STH: Lots of contexts where XSS is not an issue.
BM: More ways to XSS is a bad thing.
BE: if people have to import s then the economics change and people will
stick to +

Conclusion: AR and BM sustains. We continue with the status quo (tag-less
ts is supported)


DC: IETF wants to change JSON
MM: The 2 documents should have exactly the same text except for
IS: Should it be done in TC39?
DC: Most of the work will be on the mailing lists
AWB: Who will be the editor?
DC: Hopes they (IETF) will provide an editor.
JN: Should this be fast tracked to ISO?
DC: That makes sense.
JN: How long do you expect this to take?
DC: Has taken a long time to coordinate and get started.
DC: 5.1 specs the 2 functions that uses the JSON format.

4.10 Modules

STH: Progress since last meeting. Discuss “module naming”, “naming standard
STH: Wiki is up to date with the current proposal. Spec is “wiki complete”.
Jason Orendorff of Mozilla has worked on flushing out semantic issues. Moz
is implementinb parsing of modules.
STH: Syntax: Made a couple of changes.
A. To support anonymous exports

  export default expr;

  import $ from ‘jquery’;  // imports default anonymous export

If there is no default then the above is an error

  import {ajax} from ‘jquery’;

  import {ajax as A} from ‘query’;

to reduce confusion and to make it clear that this is not destructuring.

  module fs from ‘js/fs’

fs is a module instance object

The following is not valid:

  import {...} from fs;  // SyntaxError

Renaming on export:

  let foo = 13;
  export {foo as bar};
  export {foo};

The following is not valid:

  export foo;

STH: The only evaluation here is “13”. The rest are just bindings that are
shared with the outside/module importer.
MM: Bad idea to allow external modules to assign to imports.
DH: Imported bindings are read only to the importer.
AWB: This is new semantics to the language. Is there a list of these new
semantics modules introduce?
AWB: Is there a way to get the default export from the instance module
STH: There will be a well known symbol name to get to it.
AWB: Does module instance objects inherit from Object.prototype.
DH: No. Because we do not want any pollution.
JNA: Is it an error to assign to an imported binding?

  import {ajax} from ‘jquery’;
  ajax = 14;  // Error

AR: What is the reason for not extending Object.prototype or some other
YK: To prevent people from expecting toString to be there (???)
DH: fs.readFile We don’t want to statically check this deeply inside an


THS: The plan is to allow the above to be a static error in the future.
DH: To keep things clean.
AWB: Concerned about the dot operator
ARB: Don’t want less checking if you do not use import.
DH: Do not want refactoring hazards.
ARB: This only affect the static semantics.
AWB: Can you use square bracket?
STH: Square bracket is dynamic.
AR: This is only a static check that is lost. At runtime there will still
be errors.
LH: Concerned about default export. Now people will have to decide which
approach to use.
STH: This is already the case in Node.js today.
LH: Today you might get any object, it might be callable with properties.

  var fs = require(‘fs’);  // module instance
  var glob = require(‘glob’);  // function with properties
  var parse = require(‘parse’);  // function

  module fs from ‘fs’;
  import glob from ‘glob’;
  import {sync} from ‘glob’;
  import parse from ‘parse’;

Lots of discussion...

  import {sync} from ‘glob’;

  import glob from ‘glob’;
  var {sync} = glob;
  import {ajax} from ‘jquery’;

LH: Prefers “export =” and lose static checking when people opt in to
single anonymous export.
STH/YK: We already agreed that we want static checking.
LH: Even for new things being built, this is causing a confusion.
AWB: It is unclear when and what you want to export as the default export.
BM: Wants

  import default $ from ‘jquery’

to ensure that people have to be explicit about what they import.
DH: This is just syntax and we are wasting time “bikeshedding”
AWB: What is the best practice? Is there a single module containing Map,
Set & WeakMap or...
YK: WeakMap should be its own import:

  import WeakMap from ‘collections/WeakMap’;

BE: We have to pay attention to what Node/AMD do today.
YK: AMD tries to make modules small to reduced byte size of the

STH: And now to semantics
STH: Major things that changed. Use options object more consistently.
STH: The wiki page is up to date.
STH: Need to decide whether the browser loader is in the appendix or if it
is in some w3c spec. Want core language semantics to treat the names as
strings, not the semantics of these strings.
STH: Bulk loading. One HTTP request to load multiple modules. Possible to
implement. Create fecth hook. Stores module notations in a side table. In
the xhr response, split the result and call the different fulfill hooks.
EF: Sounds like what we do today in YUI loaders.
EF: How would you write the HTML?
DH: Initial script tag with configuration. Second script tag as usual. Alt
2 is to have configuration and dynamic module load in the same script block.

  <script src=”main.js” async></script>

alt 2

  System.require(“main.js”, function() { …. });
DH: script[async] today have to use an external src.

STH: Naming and declarations of modules.
ARB: Presenting slides...
AWB: The rate that internal vs external names changes is very different.

  module ‘m’ { … }
  module ‘n’ {
    import x from ‘m’;
    …  // this part is not executed.
  import x from ‘m’;

STH: Configuration step is mostly about other people’s code.

  module ‘m’ { … }
  module ‘n’ {
    import m from ‘m’;
    function f() {
      Loader.eval(“import m from ‘m’”);

m is fixed at compile time

ARB: Not opposed to logical modules. Wants both lexical and logical
DH: Not opposed to lexical modules.
YK: Too late to work out lexical modules for ES6.
ARB: If we wait we will have redundancy.
YK: Want declarative form to be able to prefetch etc.
BE: I want lexical modules (in the future) but logical modules are easier
to use.
ARB: Since I don’t seem to be able to convince anyone I’m going to drop this
ARB: For the record. Major concern about the global registry becoming the
new global object.

Conclusion: Move along with Dave and Sam’s proposal. Work on lexical
modules for ES7

Promises vs Monads

MM: Presenting...

Thursday May 23 2013
John Neumann (JN), Allen Wirfs-Brock (AWB), Eric Ferraiuolo (EF), Erik
Arvidsson (EA), Luke Hoban (LH), Doug Crockford (DC), Yehuda Katz (YK), Sam
Tobin-Hochstadt (STH), Alex Russell (AR), Dave Herman (DH) (calling in),
Bernd Mathiske (BM), Andreas Rossberg (ARB), Mark Miller (MM), Tom Van
Cutsem (TVC), Istvan Sebestyen (IS)

Promises vs Monads

MM: Continuing from yesterday

STH: Don’t like resolve but not willing to die on this hill.
AR: DOM has a bunch of ad hocs APIs to do promise like things.
YK: Mozilla is also actively working on APIs using promises.
AR: A lot of methods today return void so we can change these to return a
promise. This is forward compatible.
AR: then does recursive unwrapping

Next Meetings

July 23 - 25 @ Microsoft, Redmond
Sept 17 - 19 @ Bocoup, Boston
Nov 19 - 21 @ PayPal, San Jose

ES6, ES7, ES8... Mark’s Strawman Roadmap

LH: The important part is not the features but the process.
AWB: Can things be decoupled?
LH: These kind of structural questions are the important part
MM: Suggests “currency” to be the main theme.
AWB: Thought about the event loop. All we need is a processing queue... put
things in the front and the back.
DH: Only need to add to the back.
STH: The callback is called at some later point.
AR: Don’t think we need to specify the order.
STH: If we are going to specify promises etc we need to be able to specify
things in detail. We can be loose in ES6 and then come back in ES7 and
provide a more tight spec.
DH: We could specify the pending events as a set or something.
DH: Not sure if there is a consensus that we want a fast small ES7. Not
opposed to a modularized approach.
AR: Are there any browsers that are not shipping stable ES6 features today.
YK: Yes. V8.
AWB: Where we have problem today is that there is a lot of interdependency.
MM: These (“concurrency”) are coupled together to the event loop
AWB: We can do it as a separate non 262 spec
DH: Opposed to a separate spec. Introduces versioning confusion.
AWB: Roll up
DH: Think of all the extra overhead.
STH: Big difference with 402 since it was run by different people.
LH: Lack of confidence in new features has been an issue for implementers.
Good exceptions were Object.observe and Proxies where the wiki contained a
mostly complete spec.
AWB: We need to have wiki proposals be deltas to the spec.
TVC: We could have “stable” wiki pages. These would have complete spec
DH: Very concerned about over modularizing.
AWB: We need to find a way to work faster and be less monolithic.
DH: Agree. ES6 process has blocked implementation work.
LH: We are not committed to our designs.
STH: We are not resolving issues until we start to spec. We are not getting
feedback until engines starts to implement.
EA: The problem is that we didn’t start to spec things until very late. We
had agreements on features long before there was any spec drafts for them.
YK: More from our champions before we get to concensus.
ARB: Lots of the proposals were very vague.
AWB: The more complete spec you bring to tc39 the better chance you have to
reach consensus.
ARB: Lack of early spec leads to lack of early implementations...
AWB: ...which leads to lack of feedback.
LH: Not more work, just doing the work earlier before things pile up too
DH: Need to look at the dependency graph. Hold of the work of later feature.
ARB: We need to higher bar before we accept proposals.
MM: What we agreed to 2 years ago was that the features are the one we want
to spend work on speccing.
LH: Less features to bite of.
DH: A lot of us have a hard time not getting too engage in too many
YK: if we focused more effort on managing the overall complexity instead of
getting stuck on a lot of technical discussions (and nit picking).
DH: Object.observe and Proxy moved fast but are fairly isolated features
TVC: Didn’t involve syntax.
AWB: With ES6 we had a long backlog.
DH: A language will have smaller and smaller complexity budgets as it grows.
AR: ES future needs events
DH: Since this is Mark’s wishlist people will throw in their pet features.
MM: This is the direction I am going to work.
LH: There is a page on the wiki outlining the goals.
LH: Looking for 2 things: Something that would allow earlier
implementations. Have not brought proposals (over the last 2 years) because
we have been blocked by ES6.
LH: When is the appropriate time to bring new proposals to TC39?
AWB: We are free to do what we want. We can issue 6.1, 6.2 etc or technical
reports which would serve as a recommendation.
DH: We cannot exclusively work on ES6.
YK: Time at f2f is the most important. Champions can go off and do what
they want.
DH: Suggests adding non ES6 items to the agenda. We will prioritize the non
es6 stuff we can get to given our limited time.
YK: We should reinstate the rule that agenda items needs links to wiki
YK: Spec language is good but examples at the top are a must.
ARB: Add step after proposal. For example “stable” or “spec” which a
proposal gets promoted to once there is a spec draft, good enough to start
DH: Strawman: Anything goes.
YK: Proposals used to mean approved.
DH: 3 sections: strawman, proposal, spec/candidate. Keep strawman. Work on
improving as a proposal, and when mature enough promoted to next level.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the es-discuss mailing list