Add the environment variables support.

src/main/blog/core.cljs

@@ -1,8 +1,9 @@
 (ns blog.core
-  (:require ["express" :as express]))
+  (:require ["express" :as express]
+            [blog.env :as env]))
 
 (def app (express))
-(def port 3000)
+(def port (env/get-port))
 
 (defn main []
   (. app get "/"

src/main/blog/env.cljs

@@ -0,0 +1,9 @@
+(ns blog.env)
+
+(defn get-port
+  "Gets the environment variable PORT. If no variable, then the default port is 3000. Server will listen requests on this port."
+  []
+  (or (-> (.-env js/process)
+          (.-PORT))
+      "3000"))
+