Return undefined from functions by default.

Author: Neil Fraser

LICENSE

@@ -186,7 +186,7 @@ Apache License
       same "printed page" as the copyright notice for easier
       identification within third-party archives.
 
-   Copyright 2014 Neil Fraser
+   Copyright 2013 Google Inc.
 
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
@@ -199,4 +199,3 @@ Apache License
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
-

acorn_interpreter.js

@@ -1570,6 +1570,7 @@ Interpreter.prototype['stepCallExpression'] = function() {
           thisExpression: state.funcThis_
         };
         this.stateStack.unshift(funcState);
+        state.value = this.UNDEFINED;  // Default value if no explicit return.
       } else if (state.func_.nativeFunc) {
         state.value = state.func_.nativeFunc.apply(state.funcThis_,
                                                    state.arguments);
@@ -1894,7 +1895,7 @@ Interpreter.prototype['stepReturnStatement'] = function() {
     state.done = true;
     this.stateStack.unshift({node: node.argument});
   } else {
-    var value = state.value;  // Possibly undefined.
+    var value = state.value || this.UNDEFINED;
     do {
       this.stateStack.shift();
       if (this.stateStack.length == 0) {