why does this happen?

cljs.user=> (ns foo.bar (:require [clojure.string :as str]))
foo.bar=> str/upper-case
#object[Function]
foo.bar=> (ns foo.bar)
foo.bar=> str/upper-case
          ⬆
WARNING: No such namespace: str, could not locate str.cljs, str.cljc, or JavaScript source providing "str" at line 1 
          ⬆
WARNING: Use of undeclared Var str/upper-case at line 1 
str is not defined