Comment by pcthrowaway

OK so for one the title of that section is off:

> JS loops pretend their variables are captured by value

This has to do with how for loops work with iterators, but also what `let` means in variable declaration. You talk about 'unrolling a for loop' but what you're doing is 'attempting to express the same loop with while'. Unrolling would look like this;

    // original:
    for (let i = 0; i < 3; i ++) { setTimeout(()=>console.log(i)) }
    // unrolled:
    { let i = 0; setTimeout(()=>console.log(i)) };
    { let i = 1; setTimeout(()=>console.log(i)) };
    { let i = 2; setTimeout(()=>console.log(i)) };

    // original:
    let i = 0;
    for (i = 0; i < 3; i++) { setTimeout(()=>console.log(i)) };
    // unrolled:
    let i = 0;
    { i = 0; setTimeout(()=>console.log(i)); };
    { i = 1; setTimeout(()=>console.log(i)); };
    { i = 2; setTimeout(()=>console.log(i)); };

Now you can begin to explain what's going wrong in the second example; 'i' is declared with 'let' outside of the block, and this means the callback passed to the setTimeout is placed in the next stack frame, but references i from the outer scope, which is modified by the time the next stack frame is running.

In the original example, a different 'i' is declared inside each block and the callback passed to setTimeout references the 'i' from its scope, which isn't modified in adjacent blocks. It's confusing that you're making this about how loops work when understanding what the loop is doing is only one part of it; understanding scoping and the event loop are 2 other important pieces here.

And then if you're going to compare a while loop to a for loop, I think a critical piece is that 'while' loops (as well as 'do .. while') take only expressions in their condition, and loop until the expression is false.

'for' loops take three-part statements, the first part of which is an initialization assignment (for which 'var' and 'let' work differently), and the second of which is an expression used as the condition. So you can declare a variable with 'let' in the initialization and modify it in the 'afterthought' (the third part of the statement), but it will be treated as if each iteration of the loop is declaring it within the block created for that iteration.

So yes, there are some 'for' loop semantics that are specific to 'for' loops, but rather than explain that, you appear to be trying to make a point about loops in general that I'm not following.

I'm not saying the examples won't help people avoid pitfalls with for and while loops, but I do think they'll be unable to generalize any lessons they take away to other situations in JS, since you're not actually explaining the principles of JS at play.