Global lexical tier

Brendan Eich brendan at mozilla.org
Thu Sep 3 22:31:28 UTC 2015


Andreas Rossberg wrote:
> What I mean is that in a model with mutable scope (or worse, the 
> global object) _declarations_ themselves become global effects. Later 
> conflicts can affect all code that has been loaded _before_, e.g. by 
> smashing over an existing binding.

I get it, but this is just "the way JS works" for var and function 
declarations at top level. For example, people have figured out how to 
use var for object detection without violating strict mode by creating a 
global variable by assignment.

Adding let/const/class at top level either:

1. Adds a "second way JS works" (either your favored counter-proposal, 
the ML-like nested scopes; or the one Saam favored that Allen likes, 
where each script has an implicit {} around its body as far as those 
declarations -- but not function -- are concerned; or something else).

or:

2. Works by analogy to var (and doesn't bother with TDZ and other 
peculiarities of bindings in ES6).

My point is (2) doesn't add to the top level model-set, ignoring common 
complexities of const and class. It just equates let and var. This has a 
cost too, I agree, but there's no cost-free solution.

Perhaps Allen's point about

<script>
   var Widget = class {...}
</script>

is better from a certain point of view (Crockford advocates var f = 
function(){} over function f(){}, IIRC, in JS:TGP), but I would bet lots 
of real money that ES6 users would be bummed to learn that they cannot 
declare classes (or consts) in one script and use them in the next.

Your nesting counter-proposal allows that, cleverly, but at the cost of 
another model. The only proposal that doesn't require thinking in a new 
"second" way for the common cases of class, let without TDZ actually 
firing, const without TDZ or non-initialiser assignemnt, is the one 
Jason favored in the o.p.

/be


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