I've run into the same issue trying to share code between server and browser builds. A couple examples –

1. the Firebase API is fairly consistent across browsers and node.js, but you need the `"firebase-admin"` package on the server, and just `"firebase"` for the browser.
2. I have a namespace where I define a series of API endpoints. A subset of the API metadata should be visible to the browser build, while the node.js build should also include references node.js-specific resolver code, via vars to namespaces which do not exist at all in the browser.

I'm currently using a combination of workarounds, like (1) modifying how JS modules are resolved at compile time, eg. redirecting "firebase" to "firebase-admin" for node.js builds using shadow-cljs :js-options, (2) creating extra namespaces that are included/excluded from different builds which "inject" platform-specific functionality into the reusable namespaces by resetting vars / atoms, (3) using macros to read the current :target from the cljs compiler env and conditionally eliding/including code based on that, (4) using `goog-define`'s which are set per-build to conditionally evaluate code based on platform.

I like this proposal because reader conditionals would be a direct, inline expression of my intent, whereas these workarounds tend to add indirection, extraneous code, and more complicated build configurations.

I have another use case in my app:

Two different namespaces use the AWS and the Azure JavaScript SDKs for performing tasks. At the moment I have to require both and use a goog-define for picking the desired one, hoping unused parts gets trimmed away - using :simple in node I am not even sure it is possible to trim..

The proposal seems cleaner.

I was under the impression that JIRA is the place to have these discussions so they don't get lost in Slack Archives.

The Clojure Reader and tools.reader already support everything we need so no changes to either are necessary.

Only the :features option would become configurable, nothing more. Clojure itself does not need this feature since it only runs on the JVM, has its :clj option and already ignores every other option. I do think it is useful to have this for CLJS given the wide variety of host platforms.

https://github.com/clojure/clojurescript/blob/245bdee2c35e19a9685b7a0849f26fce8bdaf7ca/src/main/clojure/cljs/analyzer.cljc#L3652 and very few other places would need to change.

This is serious language level feature request. Such ideas need to be discussed around Clojure first.