diff --git a/lib/XMLHttpRequest.js b/lib/XMLHttpRequest.js
index 4893913..4e25174 100644
--- a/lib/XMLHttpRequest.js
+++ b/lib/XMLHttpRequest.js
@@ -15,7 +15,7 @@ var Url = require("url");
 var spawn = require("child_process").spawn;
 var fs = require("fs");
 
-exports.XMLHttpRequest = function() {
+function XMLHttpRequest() {
   "use strict";
 
   /**
@@ -86,16 +86,6 @@ exports.XMLHttpRequest = function() {
   // Event listeners
   var listeners = {};
 
-  /**
-   * Constants
-   */
-
-  this.UNSENT = 0;
-  this.OPENED = 1;
-  this.HEADERS_RECEIVED = 2;
-  this.LOADING = 3;
-  this.DONE = 4;
-
   /**
    * Public vars
    */
@@ -618,3 +608,34 @@ exports.XMLHttpRequest = function() {
     }
   };
 };
+
+/**
+ * Constants
+ */
+
+[XMLHttpRequest, XMLHttpRequest.prototype].forEach(function(obj) {
+  Object.defineProperties(obj, {
+    UNSENT: {
+     value: 0
+    },
+
+    OPENED: {
+     value: 1
+    },
+
+    HEADERS_RECEIVED: {
+     value: 2
+    },
+
+    LOADING: {
+     value: 3
+    },
+
+    DONE: {
+     value: 4
+    }
+  });
+});
+
+exports.XMLHttpRequest = XMLHttpRequest;
+
diff --git a/tests/test-constants.js b/tests/test-constants.js
index 372e46c..20a9181 100644
--- a/tests/test-constants.js
+++ b/tests/test-constants.js
@@ -3,7 +3,14 @@ var sys = require("util")
   , XMLHttpRequest = require("../lib/XMLHttpRequest").XMLHttpRequest
   , xhr = new XMLHttpRequest();
 
-// Test constant values
+// Test constant values in class
+assert.equal(0, XMLHttpRequest.UNSENT);
+assert.equal(1, XMLHttpRequest.OPENED);
+assert.equal(2, XMLHttpRequest.HEADERS_RECEIVED);
+assert.equal(3, XMLHttpRequest.LOADING);
+assert.equal(4, XMLHttpRequest.DONE);
+
+// Test constant values in instance
 assert.equal(0, xhr.UNSENT);
 assert.equal(1, xhr.OPENED);
 assert.equal(2, xhr.HEADERS_RECEIVED);