for-of with const emits wrong JS code

[jrieken]

TypeScript Version: Playground

http://www.typescriptlang.org/play/#src=for%20(const%20a%20of%20%5B1%2C%202%2C%203%2C%204%5D)%20%7B%20console.log(a)%3B%20%7D

When using a for-of and a const loop variable the generated code still assigns the variable (a) which is counter const and different from nodejs running such code.

Code

// A *self-contained* demonstration of the problem follows...
for (const a of [1, 2, 3, 4]) { console.log(a); }

Expected behavior:

for (var _i = 0, _a = [1, 2, 3, 4]; _i < _a.length; _i++) {
    var a = _a[0];
    console.log(a);
}

Actual behavior:

for (var _i = 0, _a = [1, 2, 3, 4]; _i < _a.length; _i++) {
    var a = _a[_i];
    console.log(a);
}

[jrieken]

for reference run: node -e "for (const a of [1, 2, 3, 4]) { console.log(a); }" which prints

$ node -e "for (const a of [1, 2, 3, 4]) { console.log(a); }"
1
1
1
1

[weswigham]

@jrieken That would be an error in the js runtime. AFAIK, a const binding in a ForOf declaration is supposed to receive a new binding each iteration.

Run that same code in the latest chrome (and thus recent v8):

And you get the expected results. We generate the correct code here.

[vladima]

for reference: for-of with const variable

[jrieken]

wow - good to know