Interfaces cannot be instantiated

lib/interface.js

@@ -45,8 +45,13 @@ exports.extend = function()
 function Interface() {}
 
 
-function extend()
+var extend = ( function( extending )
 {
+    return function extend()
+    {
+        // ensure we'll be permitted to instantiate interfaces for the base
+        extending = true;
+
         var args      = Array.prototype.slice.call( arguments ),
             props     = args.pop() || {},
             base      = args.pop() || Interface,
@@ -55,7 +60,7 @@ function extend()
         // sanity check
         inheritCheck( prototype );
 
-    var new_interface = function() {};
+        var new_interface = createInterface();
 
         util.propCopy( props, prototype, {
             each: function( name, value )
@@ -77,8 +82,33 @@ function extend()
         // freeze the interface (preventing additions), if supported
         util.freeze( new_interface );
 
+        // we're done; let's not allow interfaces to be instantiated anymore
+        extending = false;
+
         return new_interface;
-}
+    };
+
+
+    /**
+     * Creates a new interface constructor function
+     *
+     * @return  {function()}
+     */
+    function createInterface()
+    {
+        return function()
+        {
+            // allows us to extend the interface without throwing an exception
+            // (since the prototype requires an instance)
+            if ( !extending )
+            {
+                // only called if someone tries to create a new instance of an
+                // interface
+                throw Error( "Interfaces cannot be instantiated" );
+            }
+        };
+    }
+} )( false );
 
 
 /**

test/test-interface-extend.js

@@ -75,7 +75,7 @@ var SubType = Interface.extend( BaseType,
 });
 
 assert.ok(
-    ( new SubType() instanceof BaseType ),
+    ( SubType.prototype instanceof BaseType ),
     "Generic interface extend method can extend from other interfaces"
 );
 
@@ -102,7 +102,7 @@ var SubType2 = BaseType.extend(
 });
 
 assert.ok(
-    ( new SubType2 instanceof BaseType ),
+    ( SubType2.prototype instanceof BaseType ),
     "Interface extend method can extend interfaces"
 );
 

test/test-interface.js

@@ -35,6 +35,12 @@ assert.ok(
     "Interface extend method creates a new interface object"
 );
 
+assert.throws( function()
+{
+    new FooType();
+}, Error, "Interfaces cannot be instantiated" );
+
+
 // only perform check if supported by the engine
 if ( Object.isFrozen )
 {