Hello Sukyoung, 3) Arguments Object
It is filed at https://bugs.ecmascript.org/show_bug.cgi?id=440 ;) https://mail.mozilla.org/pipermail/es-discuss/2012-July/023888.html Regards, Yusuke Suzuki On Thu, Sep 13, 2012 at 8:47 PM, Sukyoung Ryu <[email protected]>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 "15.2.3.3 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 functionâs > 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 > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss >
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