Allow redeclaration of loop variable in body

[openorclose]

Q	A
Fixed Issues?	Fixes #8913
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The following code

for (let i of 'foo') {
  let i = 'a';
  console.log(i);
}

now gives the error message

/repl.js: Duplicate declaration "i"
  1 | for (let i of 'foo') {
> 2 |   let i = 'a';
    |       ^
  3 |   console.log(i);
  4 | }

on 7.8.4 repl link

when this is valid js code.

Fix

We check to see if the immediate body of the for-of loop contains any bindings to variables declared in the left side of the for-of loop.

If so, we wrap the original for-of body in a block, and then prepend the necessary declarations/assignments during transformation:

So the above code would transform into:

for (let _i = 0, _foo = 'foo'; _i < _foo.length; _i++) {
  let i = _foo[_i]; /// Line A
  {
    let i = 'a';
    console.log(i);
  }
}

Of note is that this extra block is necessary, and we cannot simply omit Line A in the above code to avoid creating the extra block, because we need to account for cases like:

for ( const [first, ...rest] of array) {
  const first = 1;
  // no redeclaration of rest
}

[nicolo-ribaudo]

Thanks!

[openorclose]

Thanks!

Thanks for the review!

[existentialism]

Nice work @openorclose!