Consider this ClojureScript code:

(defn foo []
  (let [location 12345] (prn js/location.hash)))

This won't work. js/ isn't a real namespace. This code will throw an exception at runtime.

The reason is that the code expands to this JS snippet:

cljs.user.foo = (function cljs$user$foo(){
  var location = (12345);
  return cljs.core.prn.call(null,location.hash);
});

As you can see, the location let binding is shadowing the js/location var.

The moral of the story is: be careful when using global vars like js/window and js/location. Don't use them in the context of let bindings or function arguments with common names like window or location.