diff --git a/lib/jsduck/ast.rb b/lib/jsduck/ast.rb
index 6f03a80b5f219133a8504c18ab5e16472a4cda37..053df19e2f8524bd04c12ae33b814a67c21a7ad6 100644
--- a/lib/jsduck/ast.rb
+++ b/lib/jsduck/ast.rb
@@ -196,12 +196,7 @@ module JsDuck
     # given cls Hash
     def detect_ext_define(cls, ast)
       # defaults
-      cls[:extends] = "Ext.Base"
-      cls[:requires] = []
-      cls[:uses] = []
-      cls[:alternateClassNames] = []
-      cls[:mixins] = []
-      cls[:aliases] = []
+      cls.merge!(BuiltinsRegistry.ext_define_defaults)
       cls[:members] = []
       cls[:code_type] = :ext_define
 
diff --git a/lib/jsduck/builtins/alias.rb b/lib/jsduck/builtins/alias.rb
index f4bacc29a4cb2d053ee4f98b8c1af1c318ae3604..e384a6177c5d765b7681b5cab8eca5b7b378c7bf 100644
--- a/lib/jsduck/builtins/alias.rb
+++ b/lib/jsduck/builtins/alias.rb
@@ -6,6 +6,7 @@ module JsDuck::Builtins
     def initialize
       @pattern = "alias"
       @ext_define_pattern = "alias"
+      @ext_define_default = {:aliases => []}
     end
 
     # For backwards compatibility decide whether the @alias was used
diff --git a/lib/jsduck/builtins/alternate_class_names.rb b/lib/jsduck/builtins/alternate_class_names.rb
index 221ea020f9f82d2e25d92f130c5bf128c6f22c94..2afd96588f4c28bb86491ee49d659bca8e647d0e 100644
--- a/lib/jsduck/builtins/alternate_class_names.rb
+++ b/lib/jsduck/builtins/alternate_class_names.rb
@@ -6,6 +6,7 @@ module JsDuck::Builtins
     def initialize
       @pattern = ["alternateClassName", "alternateClassNames"]
       @ext_define_pattern = "alternateClassName"
+      @ext_define_default = {:alternateClassNames => []}
     end
 
     # @alternateClassNames classname1 classname2 ...
diff --git a/lib/jsduck/builtins/extends.rb b/lib/jsduck/builtins/extends.rb
index de4668ce1cb8e4c038fa43802e1d8a9b3f699b17..37fb023050e15b6505e5f7786ee7127b0ae8cd8f 100644
--- a/lib/jsduck/builtins/extends.rb
+++ b/lib/jsduck/builtins/extends.rb
@@ -6,6 +6,7 @@ module JsDuck::Builtins
     def initialize
       @pattern = ["extend", "extends"]
       @ext_define_pattern = "extend"
+      @ext_define_default = {:extends => "Ext.Base"}
     end
 
     # @extends classname
diff --git a/lib/jsduck/builtins/mixins.rb b/lib/jsduck/builtins/mixins.rb
index 01f0c4255ac993c0bdca51671f66d6768f0d5f41..eabf824bb34e9983740f433462a89655449790d9 100644
--- a/lib/jsduck/builtins/mixins.rb
+++ b/lib/jsduck/builtins/mixins.rb
@@ -5,6 +5,7 @@ module JsDuck::Builtins
     def initialize
       @pattern = ["mixin", "mixins"]
       @ext_define_pattern = "mixins"
+      @ext_define_default = {:mixins => []}
     end
 
     # @mixins classname1 classname2 ...
diff --git a/lib/jsduck/builtins/requires.rb b/lib/jsduck/builtins/requires.rb
index 2dd4e4dd8bc225c72e34b99a4135cabc973bae7a..9cd4f23598d5819aed05b48eb0ee902be9ba267d 100644
--- a/lib/jsduck/builtins/requires.rb
+++ b/lib/jsduck/builtins/requires.rb
@@ -6,6 +6,7 @@ module JsDuck::Builtins
     def initialize
       @pattern = "requires"
       @ext_define_pattern = "requires"
+      @ext_define_default = {:requires => []}
     end
 
     # @requires classname1 classname2 ...
diff --git a/lib/jsduck/builtins/tag.rb b/lib/jsduck/builtins/tag.rb
index 9fa559ca73b11f6bdf8eca0bee5fdfa20ec2b14f..f2f5c1667945637d74f5b7b76689438fbb51c636 100644
--- a/lib/jsduck/builtins/tag.rb
+++ b/lib/jsduck/builtins/tag.rb
@@ -16,6 +16,11 @@ module JsDuck::Builtins
     # #parse_ext_define method to be invoked.
     attr_reader :ext_define_pattern
 
+    # The default value to use when Ext.define is encountered, but the
+    # key in the config object itself is not found.
+    # This must be a Hash defining the key and value.
+    attr_reader :ext_define_default
+
     # Called by Ast class to parse a config in Ext.define().
     # @param {Hash} cls A simple Hash representing a class on which
     # various properties can be set.
diff --git a/lib/jsduck/builtins/uses.rb b/lib/jsduck/builtins/uses.rb
index 75d6cb5de1d4c6beb409638718b75a6f68469d3b..fa767a1743877d700f0c34fdfed41e860c5dfacd 100644
--- a/lib/jsduck/builtins/uses.rb
+++ b/lib/jsduck/builtins/uses.rb
@@ -6,6 +6,7 @@ module JsDuck::Builtins
     def initialize
       @pattern = "uses"
       @ext_define_pattern = "uses"
+      @ext_define_default = {:uses => []}
     end
 
     # @uses classname1 classname2 ...
diff --git a/lib/jsduck/builtins_registry.rb b/lib/jsduck/builtins_registry.rb
index a988c764aaca90315198a96d7ec8272d21746249..f00934e21853c4957db139b8816cb7cc29d76e71 100644
--- a/lib/jsduck/builtins_registry.rb
+++ b/lib/jsduck/builtins_registry.rb
@@ -10,6 +10,7 @@ module JsDuck
     def initialize
       @patterns = {}
       @ext_define_patterns = {}
+      @ext_define_defaults = {}
       load_tag_classes(File.dirname(__FILE__) + "/builtins")
       instantiate_tags
     end
@@ -29,6 +30,9 @@ module JsDuck
         Array(tag.ext_define_pattern).each do |pattern|
           @ext_define_patterns[pattern] = tag
         end
+        if tag.ext_define_default
+          @ext_define_defaults.merge!(tag.ext_define_default)
+        end
       end
     end
 
@@ -42,6 +46,9 @@ module JsDuck
       @ext_define_patterns[name]
     end
 
+    # Default values for class config when Ext.define is encountered.
+    attr_reader :ext_define_defaults
+
   end
 
 end