Lexical declarations in top-level lexical scope are omitted

[bakkot]

const f = 0; is considered by Prepack to be totally dead. But it's not: the top-level lexical scope is shared across scripts, just below the global scope. So while that may not be a global declaration, in that it does not create a property of the global object, it is available to other code on the page just as if it had been var instead of const; other scripts can write console.log(f) to print 0. In that sense it is not safe to eliminate.

This may or may not be a bug, depending on your perspective. I'm just not sure what things prepack considers dead: { const f = 0; } definitely is, var f = 0; definitely isn't, but this is kind of in-between.

[NTillmann]

This is a valid bug that should get fixed.

Prepack's main focus is on the JavaScript ES5 feature set, and thing tend to be brittle for ES6 features.

[bomsy]

I would love to work on this!

[mdaj06]

@NTillmann if this is till open could direct me on where to get started