Is there no difference between let and const inside for of loop in Javascript in terms of declaration?

问题

I recently came across this code:

for (const temp of [1,2]) {
  // do something
}

I thought that it'd be better to use let declaration for temp because this way the variable would be declared only once. However, I also ran this example as well as the version with let through babel and this is what I see:

for (const p of [1,2]) {

}

for (let s of [1,2]) {

}

became:

for (var _i = 0, _arr = [1, 2]; _i < _arr.length; _i++) {
  var p = _arr[_i];
}

for (var _i2 = 0, _arr2 = [1, 2]; _i2 < _arr2.length; _i2++) {
  var s = _arr2[_i2];
}

So babel treats const and let identically. I'm wondering if Javascript runtime treats the 2 versions identically under the hood. Is the conclusion of this is that even if a variable is declared with let inside the loop it will still be redeclared on each iteration?

回答1:

I thought that it'd be better to use let declaration for temp because this way the variable would be declared only once.

There's a new version declared for each loop iteration either way; this is important for addressing the closures-in-loops problem:

const array = [1, 2, 3, 4];
for (const entry of array) {
    setTimeout(() => {
        console.log(entry);
    }, 0);
}

If there weren't a new variable created for each loop iteration, that would log the same value (probably 4) four times.

Choosing let or const is down to:

Do you want to be able to assign it a new value within the loop?
Your personal style preference (or your team's preference).

I'm wondering if Javascript runtime treats the 2 versions identically under the hood.

Yes, other than that you can assign the let variable a new value within the loop if you like.¹ For instance:

const strings = ["a", "b", "c"];
for (let str of strings) {
    str = str.toUpperCase();
    console.log(str);
}

E.g., the only difference is whether the variable is mutable or not.

¹ For the avoidance of doubt: All that assigning to it does is change that variable's value. It has no effect on the array/iterable that the value came from.

来源：https://stackoverflow.com/questions/58101855/is-there-no-difference-between-let-and-const-inside-for-of-loop-in-javascript-in

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javascript

loops

const

let

for-of-loop