On Jan 30, 2014, at 8:07 AM, Dean Landolt <d...@deanlandolt.com> wrote:
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>
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> On Thu, Jan 30, 2014 at 10:59 AM, John Barton <johnjbar...@google.com> wrote:
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> On Thu, Jan 30, 2014 at 7:54 AM, Brendan Eich <bren...@mozilla.com> wrote:
> John Lenz wrote:
> Generally, I've always thought of:
>
> "if (x) ..." as equivalent to "if (x) { ... }"
>
> let and const (and class) are block-scoped. {...} in your "if (x) {...}" is a
> block. An unbraced consequent is not a block, and you can't have a
> "conditional let binding".
>
> The restriction avoids nonsense such as
>
> let x = 0; { if (y) let x = 42; alert(x); }
>
> What pray tell is going on here, in your model?
>
> I'm with John: the alert should say 0 and I can't see why that is not obvious.
>
>
> It's not obvious at all -- what happens when you drop the initial `let x =
> 0;` and you just have `{ if (y) let x = 42; alert(x); }` -- now what happens?
> Is x declared or not?
>
> To my mind `if (y) let x = 42;` reads like it's own 1-line noop block -- at
> least, that's what I'd expect of the scope. So while it could be allowed in
> that sense, it'd only serve as a footgun when y is true.
This is exactly foot gun the language restriction is intended to avoid.
Most modern [Obj-]C[++] will warn on this (well, s/let/int/)
You might be getting confused because of the bizarro var hoisting semantics of
var
if (y) let x = “nope"; alert(x)
Results in an unusable binding of x, and so this would throw (the foot gun
occurs if you’re shadowing x, that’s another shadow that i think C compilers
will warn on), e.g..
y = true; let x = “whoops”; if (y) let x = “nope"; alert(x) // “whoops"
The var case
y = true; var x = “whoops”; if (y) var x = “nope"; alert(x); // “nope"
is actually interpreted as
var x; y= true; x = “whoops”; if (y) x = “nope”; alert(x); // “nope"
That craziness is the whole point of block scoping let. More interestingly
if (window.SomeCrazyDomFeature) var foo = true;
is a common web idiom as it brings foo into scope for everything, so makes
later "if (foo) “ statements safe. Anyone trying to do this with a |let| would
get incorrect (from their PoV) behaviour. Again that’s why we error out.
Give that this is the behaviour of every other block scoped language i don’t
see why this is confusing.
—Oliver
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