A new ES6 draft is available

Andreas Rossberg rossberg at google.com
Tue Oct 1 03:10:56 PDT 2013

On 30 September 2013 18:20, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:
> The lastest draft throws when doing an implicitly ToString conversion  [1]
> of a Symbol primitive value. This means that  (aSymbol + "suffix") or
> ("prefix" + aSymbol) [2] will throw, which was the case that I believe
> people were most concerned about.
> ToNumber conversion of a Symbol value is currently specified [3] as
> producing a NaN for implicit numeric conversions of Symbols resulting in NaN
> values that will propagate through numeric computations.  We could make
> ToNumber throw for Symbol values but NaN seems more consistent with the ES
> handling of invalid numeric conversions.
> ToBoolean of a String value is defined[3] to return true.  This seems
> consistent with the rest of ES, although it would be easy enough to define
> this conversion to throw.
> This isn't current in the spec, but we should probably also make implicit
> string or number conversions of Symbol wrappers throw.  We can to this
> simply by defining a @@ToPrimitive method [4] on Symbol.prototype that
> throws .

OK, thanks for the summary, that makes sense. The one worry I have,
though, is that not tainting S.p.toString itself will miss cases where
code tries to convert some value x to a property name by explicitly
calling x.toString(). Currently, that works for everything but null
and undefined, so I assume that this pattern is used quite a bit.

> Because, all of the appropriate implicit conversions produce error results,
> I didn't see any real reason why an explicit call to
> String.prototype.toString should throw.  This certainly seems like a
> reasonable way for a program to explicitly convert a Symbol value to a
> string value for debugging, logging, or other proposes.  This method is
> currently defined [5] to produce a string of the form "Symbol(<descriptive
> string provided when created>)". (modulo a small spec. bug)

There always is the generic Object.prototype.toString that can be made
to handle symbols. That is what V8 currently does (producing a string
just like you describe above).


> As discussed at the meeting, (new Symbol) will throw.  This is accomplished
> by making Symbol's @@create method [6] throw.
> Object(aSymbol) is currently spec'ed to return a wrapper for the Symbol via
> a call to ToObject [7]. While idea of throwing was mentioned at the meeting,
> I'm not sure why it should be special cased to throw.  This is how a
> programmer would generically convert primitive values to objects in
> situations where this is necessary.  For example,
>      "toString" in Object(expr)
> Note that a call to ToObject is also how the implicit conversions used to
> deference Reference values with primitive value bases (eg, aSymbol.foo)
> works.
> I'm not sure why my change log says (new Object(Symbol())) throws because
> that isn't actually in the draft [9].  Perhaps I was about to make that
> change, based upon the meeting minutes, and got interrupted or something.
> Regardless, it does need to do something.  The most consistent thing would
> be to treat it, like all other primitive values, as a request to create a
> Symbol wrapper object.
> Allen
> [1] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-7.1.8
> [2] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-
> [3] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-7.1.2
> [4] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-7.1.1
> [5] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-
> [6] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-
> [7] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-7.1.9
> [8] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-
> [9] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-

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