Ensured abstract classes cannot be instantiated and permitted their instantiation during extending so that it may be used in the subclass's prototype

lib/class.js

@@ -211,6 +211,14 @@ function prop_copy( props, dest, result_data )
 }
 
 
+/**
+ * Set to TRUE when class is being extended to allow the instantiation of
+ * abstract classes (for use in prototypes)
+ *
+ * @var  {boolean}
+ */
+var extending = false;
+
 /**
  * Mimics class inheritance
  *
@@ -225,18 +233,13 @@ function prop_copy( props, dest, result_data )
  */
 function extend()
 {
-    var args  = Array.prototype.slice.call( arguments ),
+    // ensure we'll be permitted to instantiate abstract classes for the base
-        props = args.pop() || {},
+    extending = true;
-        base  = args.pop() || Class,
 
-        prototype = new base(),
+    var args      = Array.prototype.slice.call( arguments ),
-        new_class = function()
+        props     = args.pop() || {},
-        {
+        base      = args.pop() || Class,
-            if ( this.__construct instanceof Function )
+        prototype = new base();
-            {
-                this.__construct.apply( this, arguments );
-            }
-        };
 
     // copy the given properties into the new prototype
     var result_data = {
@@ -247,11 +250,37 @@ function extend()
     // reference to the parent prototype (for more experienced users)
     prototype.parent = base.prototype;
 
+    var new_class = ( result_data.abstractMethods.length === 0 )
+        ? (
+            // concrete class
+            function()
+            {
+                if ( this.__construct instanceof Function )
+                {
+                    // call the constructor
+                    this.__construct.apply( this, arguments );
+                }
+            }
+        )
+        : (
+            // do not allow abstract classes to be instantiated
+            function ()
+            {
+                if ( !extending )
+                {
+                    throw new Error( "Abstract classes cannot be instantiated" );
+                }
+            }
+        );
+
     // set up the new class
     setup_props( new_class, result_data );
     new_class.prototype   = prototype;
     new_class.constructor = new_class;
 
+    // we're done with the extension process
+    extending = false;
+
     return new_class;
 }
 

test/test-class-abstract.js

@@ -133,3 +133,15 @@ assert.equal(
     "Subtypes of abstract types are not abstract if they provide concrete " +
         "implementations of all abstract methods"
 );
+
+assert.throws( function()
+{
+    new AbstractFoo();
+    new SubAbstractFoo();
+}, Error, "Abstract classes cannot be instantiated" );
+
+assert.ok(
+    new ConcreteFoo(),
+    "Concrete subclasses can be instantiated"
+);
+