Ecma/TC39 Meeting – Nov 19 2013

link Nov 19 Meeting Notes

John Neumann (JN), Allen Wirfs-Brock (AWB), Yehuda Katz (YK), Eric Ferraiuolo (EF), Erik Arvidsson (EA), Rick Hudson (RH), Matt Sweeney (MS), Rafael Weinstein (RWS), Alex Russell (AR), Rick Waldron (RW), Dmitry Soshnikov (DS), Jeff Morrison (JM), Sebastian Markbage (SM), Ben Newman (BN), Reid Burke (RB), Waldemar Horwat (WH), Doug Crockford (DC), Tom Van Custem (TVC), Mark Miller (MM)

link Welcome

JN: (Introductions)

DC: (Logistics)

JN: ...updating status of RFTG ...Adopt agenda?

Unanimous approval

JN: Approval of Sept Minutes?

Unanimous approval

link ES6 Status

AWB: (presenting Luke's spreadsheet with remaining features that need attention)!59602&app=Excel&authkey=!AAMixsO0TuyPYwc

...Function.prototype.toString still needs attention

BE: Mark is to write spec, in this case under-specify


  • Refutable pattern matching is deferred
  • still need specification for enumerate / getOwnPropertyKeys/Symbols in various places
  • Proxy handlers, cut
  • C-style for-let

BE: We agreed on semantics

AWB: We agreed on outer capture, but won't try to update per iteration. Need per iteration binding.

YK: This is the consensus I recall


  • Modules, static semantics complete
  • No loader/runtime semantics yet.
  • Dave will have a complete spec this meeting

YK: (confirms that it's complete)

AWB: Yes, but not yet in the spec.

  • Standard Modules deferred.

(TVC dials in)

link 4.4 Finalizing the Proxy API for ES6

(Presented by Tom Van Cutsem)

TVC: First three relate to es-discuss threads, re: simplifying Proxy. Jason Orendorff expressed concerns.

  • hasOwn()
  • invoke()


AWB: Looked for traps for call

BE: Total traps?

AWB: Now 14 traps.

BE: Cool. Not including hasOwn()?

AWB: Not including

TVC: The next is invoke() trap. Leave out for now to avoid inconsistencies with get()?

YK: How do you implement virtual objects? ie. an object whose this object is always the proxy. Can't do it reliably without invoke.

WH: Still not reliable, even with invoke.

YK: So what are the cases?

AWB: Can still implement a virtual object or full membrane, or thin wrapper.

YK: Not the use case. Want to make an object where this is the proxy and not the target.

TVC: Are you arguing that this should remain bound to the proxy object upon forwarding? If yes, this is the default.

YK: As long as maintaining equivalence between and

AWB: yes.

TVC: Regarding Handler API: not enough motivating use cases for proxy handlers without community use. Propose to defer.

AWB: Let's publish the library code TVC: It's already on github, as part of my shim. I will publish it as a separate project to make it more accessible.

TVC: Proxy as a constructor? Currently, no new throws TypeError

AWB: Not really a "class"

AR: You create new ones?

AWB: No prototype

AR: Gives an instance, why not "new"

AWB: Would need an @@create

YK: Then shouldn't be capitalized

BE: We can do "proxy"

AWB: (to Tom) the concern is: if it's not "new-able", should it be little-p proxy?

TVC: Given choice between little-p proxy and requiring new, I prefer new

RW: Agree with Alex, new Proxy() creates new Proxies, so allow new

AR: Let's not duck the charge on @@create.

WH: Proxy is not a class.

YK: ...But has allocation

WH: I would hate to specify what happens when subclassing from Proxy

AWB: Must create a special constructor

TVC: @@create doesn't make sense here

BE: Is Proxy a class?

(General: no)


DS: What is typeof and instanceof

AWB/BE: object

BE: Capital P

AWB: Ca???

DS: Whatever Proxy creates?

BE: That depends on what is created.

DS: By default?

BE: typeof depends if there is a call trap. instanceof depends on the prototype chain. All in the spec, so can create any object (apart from private state issues)

AWB: Ok, into the future... would value objects allow new?

BE: (int64 example)

...back to why should new throw on Proxy constructor.

BE: Seems counter intuitive: Proxy constructs objects. int64 creates a value. callables construct objects

[ inaudible ]

BE: these are object constructor functions, which is what people want to do with "new"

AWB: this is somewhere in the middle between newing a class and a random function that returns an object

BE: in either case, it returns a new object and proxies are factories for object

AWB: yeah, I agree...spec currently calls them "proxy factory functions"

BE: pretty weird not to have "new" on this.. feels natural

YK: Intuitively, the difference between returning an object and not a value

AWB: we can do it...need to make it an exotic object with a special [[Construct]]

AR: agree. Making it exotic is good.

TVC: what's the summary?

AWB: we allow new, we do it by making it exotic.

EA: Do we REQUIRE "new"?

WH: what does Map do without new?

EA: Throws

BE: Why are we requiring new again?

RW: Needed for subclassing

AWB: my objection is that we're trying to tell a story about objects, classes, and @@create... this confuses the story

YK: there's already a weird split...saying you have to use new avoids the confusion

AR/RW: agree

AWB: we could go either way, and it's subtle, but most people won't see the subtlety

RW/BE: the conservative thing to do is to throw now and we can walk it back later

AWB: agree

JM: Removing the throw might change control flows?

BE: non-issue, rare.

link Consensus/Resolution

  • Drop hasOwn()
  • Drop invoke()
  • Postpone proxy handlers API
  • REQUIRE new Proxy by making Proxy an exotic function that has it's own internal construct. (this is only "action" item)

link ES6 Status (cont)


- Symbols in place
- Binary data deferred
- RegExp look behind deferred

WH: No comprehensive specification of which variant of RegExp lookbehind was wanted. Followed up on es-discuss and got no good answer to questions.

AWB: No one has stepped up, deferred to ES7

  • Completion reform has bugs, but that's just work for me
  • Global scope, Dave has some possible changes he wants to discuss at this meeting
  • Eval scope, now simplified because can't eval a module
  • function scope, needs spec some design issues still open
  • Promises, Domenic has a complete delta, just needs to be added
  • Internal Method Invariants. If we're going to keep this section, someone needs to provide spec ready text.

MM: If Tom will collaborate with me on this, I will commit to producing this.

AWB: I can live without this section

MM: ES5 had that section, I will talk to Tom about creating this for ES6


AWB: For the end of this year, we need a feature complete spec. Essentially a "candidate spec". This means all the features there are sufficiently specified to allow an implementor to implement. I think we have a shot at it. There is work to do. Questions remain in modules, but Dave can update us. Dave and Jason (Orendorff) have been working like crazy to finish modules, including a reference implementation.

MM: The spec we want to be feature complete includes modules?

AWB: Yes.

MM: The loader stuff as well?

AWB: Yes. ... Over the next 6-9 months, we need implementors to work and provide feedback. We should push forward with what he have now (this includes modules)

link 4.1 Review Latest Specification Draft

(Allen Wirfs-Brock)

(request slides)

Discussion re: arguments.caller, arguments.callee in non-strict and strict mode.

Discussion re: default param aliasing

BE: (tries aliasing on SM)

YK: Issues with arguments.caller, arguments.callee in framework uses.

BE: Do we want three types of arguments?

YK: It's not really three types


BE: Fair to say there are three observably different kinds of arguments -


MM: Adopt some semantics where aliasing

BE: Deep aliasing of destructured parameters is bad

AR: Walk back poisoning entirely?

MM: No.

MM: Any simple parameter is aliased and any new style parameter is not ... on defaults I could go either way.

AWB: Back to scheduling

BE: If there is destructuring, no deep aliasing.

WH: Easier to explain if the rules are compositional.

link Consensus/Resolution

  • Parameter issues
  • default params, alias is independent
  • destructuring has no name for top level object, no aliasing
  • rest has no aliasing
  • non-strict mode arguments have unpoisoned caller and callee

... Continue discussion re: do Arrow Functions have an arguments object?

link 4.3 Should 'arguments' in an arrow body be from nearest outer defining scope, or early error?

(Brendan Eich)

AWB: Arrows don't have an arguments object, but arguments is a keyword inside arrow functions.

WH: let arguments = ...?

not allowed, recapping ES5 strict rules that were applied to new forms in ES6.

questions about conditionally reserved words

AWB: what if the outer function does say let arguments = ...;, what is it in the arrow function?

MM: (whiteboard)

function f() {
var arguments = 1;
[3, 2, 1].forEach(v => v + arguments);

YK: this, super, arguments should refer to its outer.

WH: and are inconsistent. The former omits keywords such as "arguments", while claiming to be based on the latter.

link Consensus/Resolution

  • arguments is lexically captured by arrow functions
  • and are inconsistent (file a bug)
  • yield cannot mean "yield to the outer"

AWB/BE: yield just follows the rules for yield

MM: (whiteboard)

function foo() {
var yield = 8;
return function * bar() {
yield baz();
function baz() {
return yield;
var f = foo();
// 8

AWB: ...Computed property keys: No dynamic checking for duplicate computed property name in object literals or classes

[expr1]: 1,
[expr2]: 2
// Does not check if expr1 === expr2

MM: Cannot statically repeat properties in object literals

AWB: This isn't a static object literal

MM: ...

YK: The most common usage will by for symbols, eg. @@iterator

AWB: We don't have a mechanism for runtime checks like this and shouldn't be adding such checks

MM: The create and overwrite paths are very different (define v. assign)

WH: Pick one or the other. If we're going to do static checking, do it consistently. Either always check or never check

AWB: The complications I ran into were in gets and sets, it's not just "does this property exist"...

BE: We do want computed property gets and sets ...We should do checks.

AWB: It's not that non-strict doesn't check...

MM: given that we do dynamic checks in strict mode, you prefer NOT doing them in sloppy mode?

AWB: I don't like the dynamic checks either way

MM: that wasn't the question

AWB: the precedent in ES5 for object literals is that strict mode has "more" static checks.

AWB: "from a language design perspective", we should have them the same in both cases

BE: I'm w/ WH and ARB, we want the check in strict mode

AWB: so you don't want the check in sloppy mode?

BE: I don't think there is only one consistent position, and I'm ok with no dynamic check in non-strict mode

... discussion of the current static checks in non-strict mode ...

link Consensus/Resolution

  • No change, this is the semantics:
var name = "x";
var o = {x:42, x:45}, // static strict error
o2 = {x:42, get x()}, // static error
o3 = {x:42, [name]:45 }, // dynamic strict error
o4 = {x:42, get [name]() {}}; // dynamic error

AWB: the consensus is that for dynamically computed property names we will dynamically check the things we would have statically checked.

WH: Pop quiz. What does the following parse as?

baz = ...
function foo() {
var yield = 8;
return function * bar() {
* baz
yield (baz) => yield * baz

[Almost nobody guessed all of these right.]

Current answer, with all optional semicolons inserted:

baz = ...
function foo() {
var yield = 8;
return function * bar() {
yield; // The yield expression rule has a [no LineTerminator here]
yield // The yield * expression rule doesn't. No optional semicolon here!
* baz;
yield (baz) => yield * baz; // The * here is a product of two variables

link Consensus/Resolution

  • Update yield * [Lexical goal InputElementRegexp] => yield [no LineTerminator here] * [Lexical goal InputElementRegexp]

link Class/optional yield arg ambiguity

function *g() {
class foo extends yield {} // is that an object or the class body?

Proposed solutions:

  • Disallow trailing yield in extends clause
  • Requires an extra parameter to Expression and AssignmentExpression
  • extends LeftHandSideExpression
  • would be only place an expression isnt explicitly Expression or AssignmentExpression

BE: (whiteboard)

// Don't want to require these parens:
class C extends (A + B) {

BE: Should have no trailing yield, as a static error

... lost track here...

WH: (whiteboard) Counterexample to claim about never requiring parentheses in expressions that would be unambiguous without them:

a = b * (yield c)

WH: Think of "extends" as having the same precedence as assignment operators. That's why class C extends (A + B) would require parentheses.

link Consensus/Resolution

  • extends LeftHandSideExpression
  • would be only place an expression isnt explicitly Expression or AssignmentExpression

link Cross-Realm Symbol Registration

(need slide)


// key is a string
Symbol.for(key) => aSymbol // creates a new Symbol if key is not registered
Symbol.keyFor(aSymbol) => key
  • Where for all strings S: Symbol.keyFor(Symbol.for(S)) === S
  • the use case for Symbol.keyFor is serialization

Revisiting discussion from last meeting, re: Symbol.

Discussion regarding the value, or lack of, registered Symbols over Strings.

DH: Is it necessary to be Symbol.keyFor()? What aboutSymbol.prototype.key`

DS: Is there direct correspondance between Symbol and toString?

DH: If the symbol is registered, toString does the same as Symbol.keyFor

AWB: is this good?

DH: What does Symbol.keyFor return if the symbol is unregistered?


link Consensus/Resolution

  • Agree on proposed API.
  • If the symbol is unregistered: Symbol.keyFor(unregistered symbol) returns undefined
  • Symbol.keyFor(not a symbol) throws

AWB: An issue about Symbols not being usable as WeakMap keys... ...which is ok...

This lead to discussion about (re)naming of WeakMap. It's possible that WeakMap may be renamed SideTable

link Introduce a Prototype object that contains sloppy arguments object @@iterator function?

Discussion re:

  • arguments prototype
  • arguments.prototype.constructor

AWB/MM: instanceof is generally misused

AWB: Current spec: all built-in iterators. Self hostable

MM: Issues are with consistency.

BE: (whiteboard)

Array.prototype[Symbol.iterator] !== (function() {
return arguments[Symbol.iterator];

MM: ...future JS programmers learning classes without understanding prototypes. Let's not unnecessarily introduce new abstractions that can't be understood within these semantics.

(meta discussion)

AWB: (describing a spec that used class inheritance for all new inherited objects, eg. Array.Iterator)

BE: Return to the question... should arguments be iterable, and how?

AWB/MM: (discussion re:

link Consensus/Resolution


link Conventions for ignore over-ride of @@iterator

MM: Why is this a spec issue?

AWB: @@iterator is bad example, @@toStringTag is better

BE: Use undefined, not "falsey"

AWB: re: new Map(?, "is")

BE/RW: new Map(undefined, "is") defaults to empty list

AWB: Happy with undefined.

link Consensus/Resolution

  • Just use undefined

link (function Foo() {}).bind(x).name?


AWB: What about anonymous function expressions?

RW: No name is made

AWB: What about bound anonymous function expressions?

RW: Same, no name.

DS: Could bound functions delegate their name? Do we want to do that?

BE: Might be interesting to find out, can chat with Brandon about this.

link Consensus/Resolution

  • "bind" wins over "bound"
  • Brandon needs to review the spec.

link 9 ECMA 404 W3C TAG / TPAC report

(Alex Russell)

AR: (recapping JSON specification leading to 404) ... There are people at the IETF that want more restrictions, eg. a number types, character encoding.

WH/MM: what does that mean?

AWB: Let's avoid doing what they're doing.

AR: They want to restrict all the things, that we're simply not willing to do. ... The draft revision includes non-normative "advice". They've also diverged on what is "valid JSON".

RW: Specifically, they've changed JSON so that JSONValue is not the top level grammar production, using JSONText instead. This means only "{}" and "[]", which is incompatible with Ecma-404.

AR: (proposal to work together)

STH: 3 levels.

  1. Bytes on the wire. For example reject UTF-32
  2. Sequence of unicode code points that make up a valid JSON sequence. This is covered by ECMA 404.
  3. Semantics of this structure. For example, reject numbers with a thousand digits.

link Consensus/Resolution

  • Alex will draft statement to send to IETF last call, due Thursday

link 4.9 Reconsidering

(Dave Herman)

DH: might be a mistake

AWB: Could be trying to fill one of two purposes:

  • the finest distinction possible
  • what you really wish triple equal was

For general use, you want -0 and +0 to be equated, NaN to equal NaN

MM, WH: Multiple NaNs are not visible in JavaScript

AWB, DH: Different NaNs are distinguishable if aliased via TypedArrays

AWB: Hence does not discriminate as finely as possible

WH: That would be a mistake. ES1-5 clearly state that the NaN values are indistinguishable, and some implementations rely on that for NaN-boxing. should consider all NaNs to be the same.

[ discussion ]

AWB: NaNs are technically still not distinguishable in ECMAScript because an implementation has the freedom to write any quiet NaN bit pattern into a TypedArray, not necessarily the one that generated the NaN; in fact, writing the same NaN twice might generate different bit patterns.

Discussion about equating NaN values (

MM: The spec allows 0/0 to be written to a TypedArray twice with different actual bit pattern.

More discussion re: IEEE NaN

link Consensus/Resolution

  • Status Quo