Possible inconsistencies in the 5th edition

Yusuke Suzuki utatane.tea at gmail.com
Thu Sep 13 05:16:28 PDT 2012

Hello Sukyoung,

3) Arguments Object

It is filed at https://bugs.ecmascript.org/show_bug.cgi?id=440 ;)

Yusuke Suzuki

On Thu, Sep 13, 2012 at 8:47 PM, Sukyoung Ryu <sukyoung.ryu at gmail.com>wrote:

> Hi,
> We found possible inconsistencies in the ECMAScript specification and we'd
> like to double check what we've found with experts. :-)
> 1) absent/present
> As "8.6.1 Property Attributes" says, a named accessor property may have
> four attributes: [[Get]], [[Set]], [[Enumerable]], and [[Configurable]].
> As "8.10 The Property Descriptor and Property Identifier Specification
> Types" says, "Values of the Property Descriptor type are records composed
> of named fields where each field's name is an attribute name and its value
> is a corresponding attribute value as specified in 8.6.1.  In addition, any
> field may be present or absent."
> However, in "8.10.4 FromPropertyDescriptor(Desc)", the steps 4-a and 4-b
> do not check whether [[Get]] and [[Set]] are "present" in "Desc" but just
> access them.  Is it an oversight of the specification or are we missing
> something here?
> We tried the following code:
>    var o = { get abc() {} }
>    x = Object.getOwnPropertyDescriptor(o, "abc")
>    for (y in x) { document.writeln(y); }
> where " Object.getOwnPropertyDescriptor" calls
> FromPropertyDescriptor, and an online JavaScript interpreter prints the
> following:
>    get set enumerable configurable
> which suggests that [[Set]] is "present" even though we didn't define a
> setter.  Calling the setter as follows:
>    x.abc(3)
> results in the following error:
>    TypeError at line NaN: 'undefined' is not a function
> which suggests that [[Set]] is "present" and its value is "undefined".
> So, we're confused.  When 8.10 says that "any field may be present or
> absent", does the word "absent" mean that the field exists but its value is
> "undefined"?  A similar question is when the step 12 of "8.12.9
> [[DefineOwnProperty]](P, Desc, Throw)" says "For each attribute field of
> Desc that is present, ..." does the word "present" mean that the field
> exists and its value is not "undefined"? Yet another similar question is
> the step 2 of "8.10.1 IsAccessorDescriptor(Desc)": "If both Desc.[[Get]]
> and Desc.[[Set]] are absent, ..."
> Finally, when the step 2 of "8.10.2 IsDataDescriptor(Desc)" says "If both
> Desc.[[Value]] and Desc.[[Writable]] are absent, then return false." we
> might be able to say that Desc.[[Value]] is absent if its value is
> undefined, but how about Desc.[[Writable]]?  Its type is Boolean and its
> default value is false.  Does the specification say that Does.[[Writable]]
> is absent if its value is false?
> 2) LabelledStatement
> The first paragraph of "12 Statements" says "A LabelledStatement has no
> semantic meaning other than the introduction of a label to a label set."
>  However, "12.12 Labelled Statements" says that "If the result of
> evaluating Statement is (break, V, L) where L is equal to Identifier, the
> production results in (normal, V, empty)."  This might be a picky comment
> but I think because of the sentence in 12.12, a LabelledStatement has some
> semantic meaning. :-)
> 3) Arguments Object
> "10.6 Arguments Object" says "When control enters an execution context for
> function code, an arguments object is created unless (as specified in 10.5)
> the identifier arguments occurs as an Identifier in the function‘s
> FormalParameterList or occurs as the Identifier of a VariableDeclaration or
> FunctionDeclaration contained in the function code." which suggests that if
> there is a variable named "arguments" in a function body then it does not
> create the name "arguments".  However, in 10.5, variable bindings in a
> function body is handled in the step 8 and checking the name "arguments" is
> in the step 6, so even though the function body declares a variable of name
> "arguments" the name "arguments" is created.  We found them inconsistent.
>  What do you think?
> Thank you for your comments in advance!
> Best,
> --
> Sukyoung
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