Wait, so is there no variable shadowing allowed then?

13.1.1<http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors> Static Semantics: Early Errors
Block : { StatementList }

*         It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries.

*         It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.
StatementList can contain Blocks whose LexicallyDeclaredNames and VarDeclaredNames algorithms return the values for their StatementLists, so it recursively collects the names from all nested lexical and var declarations.

It looks like VarDeclaredNames is missing a definition for VariableStatement because I can't see any way for the bound names of a VariableStatement to get added to VarDeclaredNames lists.

But barring for the moment that I cannot find an algorithm definition that adds BoundNames of a VariableStatement to VarDeclaredNames, this second early error bullet implies that shadowing of bound names is not allowed at all.

Is this correct?

Ian


From: Allen Wirfs-Brock [mailto:allen at wirfs-brock.com]
Sent: Friday, November 8, 2013 4:16 PM
To: Ian Halliday
Cc: es-discuss at mozilla.org
Subject: Re: Specification of use before declaration errors


On Nov 8, 2013, at 3:35 PM, Ian Halliday wrote:


Hello es-discuss,

I'm having difficulty figuring out where the ES6 draft spec specifies that a "use before declaration" error should be thrown.  My last understanding of the temporal dead zone was that ECMAScript would always issue a "use before declaration" error at runtime, regardless whether it can be statically determined or not.  However it seems like the evaluation semantics of var declarations, for example, do not lead to any line that throws a ReferenceError.



That is, consider this code:

function f() {
    {
        var x = 5;
        let x;
declaring the same name using a let and a var is an early error: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors
or for the function level http://people.mozilla.org/~jorendorff/es6-draft.html#sec-function-definitions-static-semantics-early-errors



    }
}

f();
I think the var declaration creates a binding for x in the function's lexical environment, but then binds to the x in the block's environment for the initialization.  As such, the initialization should throw a "use before declaration" error.  But this is what I cannot find in the spec.  Maybe I am wrong about the semantics here?

because an early error exists, the surround script or module is never evaluated.

Allen
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Wait, so is there no variable shadowing allowed then?

> ### [13.1.1 Static Semantics: Early Errors](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors)
>
> Block : { StatementList }
>
> *         It is a Syntax Error if the LexicallyDeclaredNames of StatementList contains any duplicate entries.
>
> *         It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.

StatementList can contain Blocks whose LexicallyDeclaredNames and VarDeclaredNames algorithms return the values for their StatementLists, so it recursively collects the names from all nested lexical and var declarations.

It looks like VarDeclaredNames is missing a definition for VariableStatement because I can't see any way for the bound names of a VariableStatement to get added to VarDeclaredNames lists.

But barring for the moment that I cannot find an algorithm definition that adds BoundNames of a VariableStatement to VarDeclaredNames, this second early error bullet implies that shadowing of bound names is not allowed at all.

Is this correct?