Explanation of `let` and block scoping with for loops

Question

I understand that let prevents duplicate declarations which is nice.

let x;
let x; // error!

Variables declared with let

Answer 1

let introduces block scoping and equivalent binding, much like functions create a scope with closure. I believe the relevant section of the spec is 13.2.1, where the note mentions that let declarations are part of a LexicalBinding and both live within a Lexical Environment. Section 13.2.2 states that var declarations are attached to a VariableEnvironment, rather than a LexicalBinding.

The MDN explanation supports this as well, stating that:

It works by binding zero or more variables in the lexical scope of a single block of code

suggesting that the variables are bound to the block, which varies each iteration requiring a new LexicalBinding (I believe, not 100% on that point), rather than the surrounding Lexical Environment or VariableEnvironment which would be constant for the duration of the call.

In short, when using let, the closure is at the loop body and the variable is different each time, so it must be captured again. When using var, the variable is at the surrounding function, so there is no requirement to reclose and the same reference is passed to each iteration.

Adapting your example to run in the browser:

// prints '10' 10 times
for (var i = 0; i < 10; i++) {
  setTimeout(_ => console.log('var', i), 0);
}

// prints '0' through '9'
for (let i = 0; i < 10; i++) {
  setTimeout(_ => console.log('let', i), 0);
}

certainly shows the latter printing each value. If you look at how Babel transpiles this, it produces:

for (var i = 0; i < 10; i++) {
  setTimeout(function(_) {
    return console.log(i);
  }, 0);
}

var _loop = function(_i) {
  setTimeout(function(_) {
    return console.log(_i);
  }, 0);
};

// prints '0' through '9'
for (var _i = 0; _i < 10; _i++) {
  _loop(_i);
}

Assuming that Babel is fairly conformant, that matches up with my interpretation of the spec.