[build-optimizer] docs about `sideEffects: false` and maybe bug

[ghetolay]

Bug Report or Feature Request

- [ ] bug report (maybe)
- [x] feature request (docs I guess)

Area

- [x] devkit
- [ ] schematics

Versions

@angular-devkit/build-optimizer@0.6.8

Issue

After adding sideEffects: false to a library doing webgl stuff, I found out some code was removed because the library contains unpure functions (any function doing webgl stuff).

Turns out build-optimizer is also using that flag in his process to prefix functions with /*@__PURE__*/ and that is the cause of code removal.
I have build up a repro with one of the function :
https://repl.it/@GhetGhetolay/buildoptimizerwebgl
I'm not sure if

1. this is the expected behavior and we should set isSideEffectFree to false
2. it's a bug and webgl functions shouldn't be prefixed with pure

If it's not a bug this means we're using same flag for different purposes, webpack sideEffects and build-optimizer isSideEffectFree options are not about the same kind of side effects (module vs functions).

Anyway I think we're missing a bit of docs, neither the Angular Package Format nor the build-optimizer docs mentionned the use of sideEffects flag by build-optimizer.

[clydin]

The underlying issue is that the build optimizer does not currently understand the usage of the object literal function definition syntax used in the example. Changing the function definition to the following results in proper behavior:

buildTexture: (gl, image) => {

[ghetolay]

Thanks !

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