fix: empty destructuring with var should be converted to const

[pionxzh]

closes #350

[nene]

Thanks for bringing up this interesting corner-case. Does Babel actually generate code containing such empty destructuring cases or is it just an odd corner-case that the tests of Babel check?

[pionxzh]

It's more like corner cases that Babel is ensuring they are doing it right because they are... well... valid. You usually won't get this unless you do write one.

[nene]

Modifying the node.kind field here is definitely the easy way to fix this, but it doesn't align with how the rest of this transform is organized. Until now, there have been two distinct phases:

• first gather all the data of all the var-declarations to be modified.
• then go and convert the vars to let/const.

This change breaks this pattern. Now some var-declarations get modified during this first loop of gathering data, which breaks expectations of somebody reading this code in the future.

I'd rather leave these var-s unchanged than do it this way.

[pionxzh]

At first, I was trying to do it in the transformation phase, but nothing about empty restructuring will be collected in the "gathering data" stage. It's empty. 😢 That's why I ended up putting it here.

[nene]

Yeah, it would need to involve some more reorganization. Currently this conversion loop goes through all variables, and takes the var-declarations of them, but as there are no variables here such an empty declaration will never be reached.

It's definitely not an easy change to make. There's a currently a heavily beaked-in assumption that all variable declarations actually declare at least a single variable.

One possible middle-ground solution would be to separately gather these empty declaration nodes and then separately loop through them in the modification-phase. This way at least the modification and gathering phases will be kept separate, avoiding confusion in that area.

[pionxzh]

Good idea. I will try to separate them.