Clojurians Log v2

Why am I able to use (Math/pow 2 3) in cljs (as js/Math exists) But I can't use (Reflect/get ...)? js/Reflect also exists!

cljs.user=> (Reflect/get #js{:a 42} "a")
WARNING: No such namespace: Reflect, could not locate Reflect.cljs, Reflect.cljc, or JavaScript source providing "Reflect" at line 1 
WARNING: Use of undeclared Var Reflect/get at line 1 
42 ;; actually works after many WARNINGS

Note: I know that I can use (js/Reflect.get ...). My question is "why some globals act like java classes and others do not"