diff --git a/.jshintrc b/.jshintrc
index 587d66f..1b15e51 100644
--- a/.jshintrc
+++ b/.jshintrc
@@ -2,6 +2,7 @@
   "curly":    true,
   "eqeqeq":   true,
   "evil":     true,
+  "forin":    true,
   "laxbreak": true,
   "noempty":  true,
   "nonew":    true,
diff --git a/bin/pegjs b/bin/pegjs
index 3275d22..4c7eb8f 100755
--- a/bin/pegjs
+++ b/bin/pegjs
@@ -70,7 +70,9 @@ function addExtraOptions(options, json) {
   }
 
   for (var key in extraOptions) {
-    options[key] = extraOptions[key];
+    if (extraOptions.hasOwnProperty(key)) {
+      options[key] = extraOptions[key];
+    }
   }
 }
 
diff --git a/lib/compiler.js b/lib/compiler.js
index ab03a37..ffdff9b 100644
--- a/lib/compiler.js
+++ b/lib/compiler.js
@@ -48,7 +48,9 @@ module.exports = {
     });
 
     for (stage in passes) {
-      utils.each(passes[stage], runPass);
+      if (passes.hasOwnProperty(stage)) {
+        utils.each(passes[stage], runPass);
+      }
     }
 
     switch (options.output) {
diff --git a/lib/peg.js b/lib/peg.js
index 55c6095..aea3022 100644
--- a/lib/peg.js
+++ b/lib/peg.js
@@ -24,7 +24,9 @@ module.exports = {
       var converted = {}, stage;
 
       for (stage in passes) {
-        converted[stage] = utils.values(passes[stage]);
+        if (passes.hasOwnProperty(stage)) {
+          converted[stage] = utils.values(passes[stage]);
+        }
       }
 
       return converted;
diff --git a/lib/utils.js b/lib/utils.js
index 8ca3ce1..3cdeca4 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -69,7 +69,9 @@ var utils = {
   keys: function(object) {
     var result = [];
     for (var key in object) {
-      result.push(key);
+      if (object.hasOwnProperty(key)) {
+        result.push(key);
+      }
     }
     return result;
   },
@@ -77,7 +79,9 @@ var utils = {
   values: function(object) {
     var result = [];
     for (var key in object) {
-      result.push(object[key]);
+      if (object.hasOwnProperty(key)) {
+        result.push(object[key]);
+      }
     }
     return result;
   },
@@ -85,15 +89,19 @@ var utils = {
   clone: function(object) {
     var result = {};
     for (var key in object) {
-      result[key] = object[key];
+      if (object.hasOwnProperty(key)) {
+        result[key] = object[key];
+      }
     }
     return result;
   },
 
   defaults: function(object, defaults) {
     for (var key in defaults) {
-      if (!(key in object)) {
-        object[key] = defaults[key];
+      if (defaults.hasOwnProperty(key)) {
+        if (!(key in object)) {
+          object[key] = defaults[key];
+        }
       }
     }
   },
diff --git a/spec/generated-parser.spec.js b/spec/generated-parser.spec.js
index 61a3a4c..4695474 100644
--- a/spec/generated-parser.spec.js
+++ b/spec/generated-parser.spec.js
@@ -15,7 +15,9 @@ describe("generated parser", function() {
          * changed later.
          */
         for (key in options) {
-          clonedOptions[key] = options[key];
+          if (options.hasOwnProperty(key)) {
+            clonedOptions[key] = options[key];
+          }
         }
 
         describe(
@@ -123,10 +125,12 @@ describe("generated parser", function() {
           } else {
             if (details) {
               for (key in details) {
-                if (!this.env.equals_(e[key], details[key])) {
-                  this.message = buildKeyMessage(key, e[key]);
+                if (details.hasOwnProperty(key)) {
+                  if (!this.env.equals_(e[key], details[key])) {
+                    this.message = buildKeyMessage(key, e[key]);
 
-                  return false;
+                    return false;
+                  }
                 }
               }
             }
diff --git a/spec/helpers.js b/spec/helpers.js
index 64970ce..41b7954 100644
--- a/spec/helpers.js
+++ b/spec/helpers.js
@@ -29,9 +29,11 @@ beforeEach(function() {
           if (!isObject(value)) { return false; }
 
           for (key in details) {
-            if (!(key in value)) { return false; }
+            if (details.hasOwnProperty(key)) {
+              if (!(key in value)) { return false; }
 
-            if (!matchDetails(value[key], details[key])) { return false; }
+              if (!matchDetails(value[key], details[key])) { return false; }
+            }
           }
 
           return true;
diff --git a/spec/parser.spec.js b/spec/parser.spec.js
index cb4322c..cd30efe 100644
--- a/spec/parser.spec.js
+++ b/spec/parser.spec.js
@@ -145,10 +145,12 @@ describe("PEG.js grammar parser", function() {
           } else {
             if (details) {
               for (key in details) {
-                if (!this.env.equals_(e[key], details[key])) {
-                  this.message = buildKeyMessage(key, e[key]);
+                if (details.hasOwnProperty(key)) {
+                  if (!this.env.equals_(e[key], details[key])) {
+                    this.message = buildKeyMessage(key, e[key]);
 
-                  return false;
+                    return false;
+                  }
                 }
               }
             }