adding an imaginary "return undefined" can sometimes help

function f() {
  if (foo) return x();
  if (!bar) return y();
}

==>

function f() {
  return foo ? x() : bar ? void 0 : y();
}

lib/output.js

@@ -889,6 +889,10 @@ function OutputStream(options) {
         var def = self.definition();
         output.print_name(def ? def.mangled_name || def.name : self.name);
     });
+    DEFPRINT(AST_Undefined, function(self, output){
+        // XXX: should add more options for this
+        output.print("void 0");
+    });
     DEFPRINT(AST_This, function(self, output){
         output.print("this");
     });