The question is vague. What do you mean by a "variable" here?

inside the app i want be able to branch:

(if (= mode :dev)
  )

js/goog.DEBUG by default captures that. it is true for watch/compile and false for release builds

@U05224H0W cool, thank you!

you can set your own via https://shadow-cljs.github.io/docs/UsersGuide.html#closure-defines