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That's still a check that var is not subject to. Hence, it's a little slower (in theory).

In theory, the opposite, because a compiler knows that a constant binding never changes so the value it references can be inlined everywhere it is used without having to do runtime checks. But I think engines are not doing this yet.

I haven't written enough ES6 to answer this for myself, but doesn't the dynamic nature of the language mean that constants can be at risk of invalid LHS operations at run-time? (At least, those positioned high enough in the scope to be subject to the necessary operations, which could be a lot.) I'm thinking of eval specifically, but perhaps there are other ways of doing this.

I think if you use eval, all bets are off anyway.

But I think that in ES6 without eval you can't refer to the current scope in a dynamic way; which means that the engine can resolve bindings at compile time without worrying about people fiddling with scopes behind the scenes, which should simplify optimisation a lot.

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