Auto let Fix

[IamIpanda]

Fix missing let decs inner declaration.
I'm not sure if it's needed to fix that breaking return let which won't happen on auto Var as autovar move all variables to top.

[edemaine]

If we want autoLet to be a viable option, I think we should fix the return let...

[IamIpanda]

If we want autoLet to be a viable option, I think we should fix the return let...

Return let can be fixed, but it would be only the start as let don’t return value, that means any assignment as an expression should be checked, and hoist to a proper position (Should be nearest position where can put a let, or maybe just block top to make it possible.) I’m not sure if now AST support to detect the AssignmentExpression’s return value is “used”

I’m checking the PR similar to that for coffeescript: jashkenas/coffeescript#5395

[STRd6]

Here's an idea:

When a declaration is in return position prepend it with a simple let ref, then assign the final declaration to the ref, then return the ref.

->
  let a = 1, b = 2
---
function() {
  let ref
  let a, b = ref = 2
  return ref
}

This may be useful for Civet in general to have declarations behave more like expressions.

[edemaine]

Nice, yes, maybe we can use this to allow let declarations within if..else expressions (along with return). But it doesn't work with const... An alternative is to wrap in IFFEs, like for and do. This seems more universal. Hmm, except then it's hard to return out of the block (e.g. if x then return let y = 5).

[IamIpanda]

I think it would be quite normal for following code before how it interact with implicit return

a = b = 3
c = (d = 3)
export default MyReactFC = () => <></>

[edemaine]

Anyway, this PR is probably worth merging already, as it fixes some bugs in autoLet (right? or am I misunderstanding?). Perhaps should add a comment to the return let example like // TODO This is currently invalid. And we can continue to improve inner declarations in future PRs.

[IamIpanda]

Comment added.

[edemaine]

c = (d = 3)

Right, this currently compiles to let c = (let d = 3) which is invalid. So it's an issue in many different contexts, though more of an issue for autoLet. For now it could be a "beware" for autoLet users, but it would be nice to fix it in the way Daniel suggests. @IamIpanda are you willing to try working on this? Presumably whenever the added let isn't immediately inside a code block, the let needs to get hoisted to that code block.

[STRd6]

autoVar does hoisting to the enclosing function blocks so a similar mechanism could be used here as well.