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Actually that code example should throw an exception when the first `a` is evaluated.

It doesn't in v8 -- what do you mean by `should`?

It's an implementation error. In Firefox, it correctly throws:

> ReferenceError: can't access lexical declaration `a' before initialization

No, legacy const is not spec'd by ES5. It behaves differently in FireFox and V8 because it was added at different times with different semantics in the live range hole case. In strict mode, it will throw on V8 as well.

No it shouldn't. `a` at the first console.log invocation IS undefined, so it is entirely correct.

Not according to the spec[1]:

> The variables are created when their containing Lexical Environment is instantiated but may not be accessed in any way until the variable’s LexicalBinding is evaluated.

I.e. given a variable declared with `const`, you may not access it before it is set.

[1] http://people.mozilla.org/~jorendorff/es6-draft.html#sec-let...

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