On Oct 13, 2008, at 2:40 PM, Brendan Eich wrote:
> If we don't require let initializers then users will sometimes forget
> them, and use-before-set errors
s/errors/mistakes/
> will happen -- unless we require
> analysis or a read barrier to make use-before-set an error. If we do
> not mandate analysis, then test coverage gaps will leave some such
> errors to be found in the field.
I hope this was clear enough the first time, but in case not, my
argument is that let should not require an initializer because var
does not, and we wish to migrate code using var to use let, and
encourage let instead of var in new code. Requiring an initializer is
unnecessary, and it taxes all programers whether trying to migrate old
code they didn't write, use let as they're accustomed to using var in
new code, or something in between.
Anyway, that's all about let. For const, which requires an
initializer, everyone agrees that use before initialization is an
error. Whether it must be a static error is not yet decided by the
committee, and the latest ES3.1 draft does not say more than "Any
attempts to access the value of a constant before it is assigned a
value throws a ReferenceError exception." (Ugly wording there, I'll
help get it fixed.)
And ES3.1 specifies const hoisting to top of block, so you cannot use
an outer x in the first ... in { ... const x = 42; ... }.
/be
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