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.
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