When printing, check to see if value is undefined. If so, print nothing.

lib/repl.js

@@ -165,6 +165,10 @@ function printValue (value) {
     return;
   }
   
+  if (value === undefined) {
+    return;
+  }
+  
   try {
     puts(JSON.stringify(value));
   } catch (e) {