"scope & closures": Block Scoping Revisited - clarification of scope created by let (question)

[bl4de]

Hi,

I've got a question about scope created by let.

We've got an example (similar as in YDKJS):

"use strict";

for (let a = 1; a < 10; a++) {
    setTimeout( function() {
        console.log(a);
    }, 0);
}

Works perfect, closure in setTimeout() has access to valid a in each iteration as is defined inside scope created by let.

But let's do something like this:

"use strict";
function foo() {
    {
        let b = 10;
        for (let a = 1; a < 10; a++) 
        {
            console.log("in loop: ", a);
            fn();
        }
    }
}

foo();

console.log(a);  // undefined
console.log(b);  // Uncaught ReferenceError: b is not defined

function fn() {
    console.log("in fn(): ", a);
}

I define fn() outside of scope created by let (in for statement). Also there's one more block scope created by outside brackets. a should not be visible, right? And everything is enclosed in foo() which is created yet another scope.

And the result is:

in loop: 1 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 2 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 3 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 4 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 5 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 6 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 7 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 8 closure2.js:20 in fn(): 10 closure2.js:7 in loop: 9 closure2.js:20 in fn(): 10 closure2.js:15 10 closure2.js:16 Uncaught ReferenceError: b is not defined

fn() has an access to a as is called inside the scope, where a is defined (right?). But how a might be accessible outside of all scopes which surround it? (this line with console.log(a), where there's undefined - as I was expecting Reference Error, the same as for b)

And why a in each iteration, printed by fn(), behaves as if fn() was declared as closure, similar as in your example from "Loops + Closure" chapter?

Is there's something I didn't I understand from scopes?

[getify]

IIUC the only explanation for this is that in whatever environment you're trying this in, there's already an a in the outer scope, diff from the a in the loop. Or whatever implementation you're running in has a massive bug. a most definitely should not be accessible in that scope.

[bl4de]

This is totally insane, but you're correct as usual.

I've tried code above in my Chrome Dev Tools (created some small snippet for this). Yeah, and there was an a in Chrome Dev Tools console defined an hour or two earlier today :/

In fresh environment, the code collapsed right after first call on fn(), which was of course expected.

I am sorry for this issue, I should check this before I've created one. Shame on me.

[getify]

No worries. :) Happens all the time, actually.