The only code affected by this would be code that uses typeof to check for the existence of a variable the same code defines later, which sounds like dumb code to begin with. I don't think there's any valid uses of typeof on possibly undefined variables besides checking for browser built-ins.

Using typeof(foo) === "undefined" is actually something that comes from back when it was pretty common to pollute the global namespace. There were actual javascript plugins/libraries/snippets that defined undefined and thus broke code that compared to it.

It's not an issue since using a non-hoisted variable before it was declared is an error.

Your editor should immediately highlight this error with a squiggly line.

That's... interesting. I guess it makes a little sense -- `let` seems to be for developers who don't like `var` semantics. Usually that seems to be about wanting block instead of function scope, but maybe there's a contingent that dislikes `undefined` too.

`let` doesn't disappear `undefined` though.

    >> let a;
    >> a;
    undefined