diff --git a/.DS_Store b/.DS_Store
index 9a874b5..feca861 100644
Binary files a/.DS_Store and b/.DS_Store differ
diff --git a/README.md b/README.md
index 63ad5f2..cf514c5 100644
--- a/README.md
+++ b/README.md
@@ -39,6 +39,13 @@ After page is loaded, you may also start a REPL connected to browser with:
 yarn shadow-cljs cljs-repl app
 ```
 
+Once the REPL is loaded, load the core namespace with:
+
+```
+(ns propeller.core)
+```
+Calling `(-main)` will run the default genetic programming problem.
+
 ## Description
 
 Propel is an implementation of the Push programming 
diff --git a/src/propeller/push/instructions/input_output.cljc b/src/propeller/push/instructions/input_output.cljc
index e5fccd4..55db2ee 100755
--- a/src/propeller/push/instructions/input_output.cljc
+++ b/src/propeller/push/instructions/input_output.cljc
@@ -19,7 +19,9 @@
   [state instruction]
   (if-let [input (instruction (:input state))]
     (state/push-to-stack state :exec input)
-    (throw (Exception. (str "Undefined input instruction " instruction)))))
+    (throw #?(:clj (Exception. (str "Undefined input instruction " instruction))
+              :cljs (js/Error
+                      (str "Undefined input instruction " instruction))))))
 
 ;; =============================================================================
 ;; OUTPUT Instructions
diff --git a/src/propeller/tools/character.cljc b/src/propeller/tools/character.cljc
index 32d64d3..6c8b846 100755
--- a/src/propeller/tools/character.cljc
+++ b/src/propeller/tools/character.cljc
@@ -1,18 +1,24 @@
 (ns propeller.tools.character)
 
+(defn get-ascii
+  "Gets the ASCII code of a char"
+  [c]
+  #?(:clj (int c)
+     :cljs (.charCodeAt c 0)))
+
 (defn is-letter
   "Returns true if the given character is a letter, A-Z or a-z."
   [c]
-  (<= (int \A) (int c) (int \z)))
+  (<= (get-ascii \A) (get-ascii c) (get-ascii \z)))
 
 
 (defn is-digit
   "Returns true if the given character is a digit, 0-9."
   [c]
-  (<= (int \0) (int c) (int \9)))
+  (<= (get-ascii \0) (get-ascii c) (get-ascii \9)))
 
 
 (defn is-whitespace
   "Returns true if the given character is whitespace (newline, space, tab)."
   [c]
-  (contains? #{(int \newline) (int \tab) (int \space)} (int c)))
+  (contains? #{(get-ascii \newline) (get-ascii \tab) (get-ascii \space)} (get-ascii c)))