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Comment by anderskaseorg | Hacker News Reader

MDN goes into great detail, but the important point is that JavaScript fixed this with let and const.

    > a = []; for (i = 0; i < 3; i++) a.push(() => i); a.map(f => f())
    [ 3, 3, 3 ]
    > a = []; for (let i = 0; i < 3; i++) a.push(() => i); a.map(f => f())
    [ 0, 1, 2 ]
    > a = []; for (i of "abc") a.push(() => i); a.map(f => f())
    [ 'c', 'c', 'c' ]
    > a = []; for (const i of "abc") a.push(() => i); a.map(f => f())
    [ 'a', 'b', 'c' ]

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But isn't it completely wild that the `for (let i =`... version works like that? What does the loop 'desugar' into if written as a while-loop?

Before let/const, scopes could only be introduced at function level, so @babel/plugin-transform-block-scoping transpiles it using an extra function:

    var _loop = function (i) {
      a.push(() => i);
    };
    for (var i = 0; i < 3; i++) {
      _loop(i);
    }

The key is that the scoping happens for each iteration, not around the entire loop. That detail is nonobvious, given how many other languages have gotten it wrong, but I wouldn’t say it’s wild.

(If you’re curious how Babel deals with the more complicated cases of break/continue, labelled break/continue, and return, try it out at https://babeljs.io/repl.)

Right, the wild thing for me is when you mutate `i` in the loop body. So at the same time `i` is scoped to the iteration so that you can capture it, but also mutating it affects the loop-scoped `i` that is incremented between iterations and checked for the termination condition. The iteration-scoped `i` is assigned back to the loop-scoped `i` at the end of the loop body. So if you have a closure close over `i` in the loop body and mutate it, whether that mutation affects the actual loop variable depends on whether the closure is called during the iteration it was created in or during a later iteration. Kinda spooky, but sure, less of a footgun than the original behavior.

In contrast, Java would have a compile error so long as the variable was not declared final.

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