I'd wonder too if this would be subject to sideEffects when tree shaking is utilized? - perhaps the global.cssParser option in webpack configs may be a nice way to remove these blocks for those not used.

It can't webpack has no way of knowing we use one or the other. So it will add the 3. That s why I talked about global var for cssParder so that webpack can remove what we don't use

Instead of adding yet another global, we can add "magic" strings

In this case, I would do

switch(__CSS_PARSER__) {

and just make ts happy with

declare readonly var __CSS_PARSER__: string

in global-types.d.ts

We would inject the variable with the DefinePlugin in webpack.

I dont think it would work Igor. Dont know if the tree shaker will clean the switch . I think we have to something like if (__CSS_PARSER__ === 'rework')...

Yep, we can convert to if/else if's

This rollup repl shows it working!

I expect it to work the same through webpack as well!

Various React ecosystem packages use if(__DEV__){} blocks explicitly because Webpack does treeshake paths based on the value of the constant defined by WebpackDefinePlugin, if it’s any help.