diff --git a/src/type/definition.js b/src/type/definition.js
index 428304f71d8..b7795b5a4ba 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -563,7 +563,7 @@ export class GraphQLObjectType {
   name: string;
   description: ?string;
   astNode: ?ObjectTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<ObjectTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<ObjectTypeExtensionNode>;
   isTypeOf: ?GraphQLIsTypeOfFn<*, *>;
 
   _typeConfig: GraphQLObjectTypeConfig<*, *>;
@@ -574,7 +574,7 @@ export class GraphQLObjectType {
     this.name = config.name;
     this.description = config.description;
     this.astNode = config.astNode;
-    this.extensionASTNodes = config.extensionASTNodes;
+    this.extensionASTNodes = config.extensionASTNodes || [];
     this.isTypeOf = config.isTypeOf;
     this._typeConfig = config;
     invariant(typeof config.name === 'string', 'Must provide name.');
@@ -820,7 +820,7 @@ export class GraphQLInterfaceType {
   name: string;
   description: ?string;
   astNode: ?InterfaceTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<InterfaceTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<InterfaceTypeExtensionNode>;
   resolveType: ?GraphQLTypeResolver<*, *>;
 
   _typeConfig: GraphQLInterfaceTypeConfig<*, *>;
@@ -830,7 +830,7 @@ export class GraphQLInterfaceType {
     this.name = config.name;
     this.description = config.description;
     this.astNode = config.astNode;
-    this.extensionASTNodes = config.extensionASTNodes;
+    this.extensionASTNodes = config.extensionASTNodes || [];
     this.resolveType = config.resolveType;
     this._typeConfig = config;
     invariant(typeof config.name === 'string', 'Must provide name.');
diff --git a/src/type/schema.js b/src/type/schema.js
index 1b2ea738d14..58d71acf070 100644
--- a/src/type/schema.js
+++ b/src/type/schema.js
@@ -83,7 +83,7 @@ export class GraphQLSchema {
   // Used as a cache for validateSchema().
   __validationErrors: ?$ReadOnlyArray<GraphQLError>;
   // Referenced by validateSchema().
-  __allowedLegacyNames: ?$ReadOnlyArray<string>;
+  __allowedLegacyNames: $ReadOnlyArray<string>;
 
   constructor(config: GraphQLSchemaConfig): void {
     // If this schema was built from a source known to be valid, then it may be
@@ -113,7 +113,7 @@ export class GraphQLSchema {
       );
     }
 
-    this.__allowedLegacyNames = config.allowedLegacyNames;
+    this.__allowedLegacyNames = config.allowedLegacyNames || [];
     this._queryType = config.query;
     this._mutationType = config.mutation;
     this._subscriptionType = config.subscription;
diff --git a/src/type/validate.js b/src/type/validate.js
index d65b0dc87c6..99f13a7918f 100644
--- a/src/type/validate.js
+++ b/src/type/validate.js
@@ -218,10 +218,7 @@ function validateName(
 ): void {
   // If a schema explicitly allows some legacy name which is no longer valid,
   // allow it to be assumed valid.
-  if (
-    context.schema.__allowedLegacyNames &&
-    context.schema.__allowedLegacyNames.indexOf(node.name) !== -1
-  ) {
+  if (context.schema.__allowedLegacyNames.indexOf(node.name) !== -1) {
     return;
   }
   // Ensure names are valid, however introspection types opt out.
@@ -564,10 +561,8 @@ function getAllObjectNodes(
   type: GraphQLObjectType,
 ): $ReadOnlyArray<ObjectTypeDefinitionNode | ObjectTypeExtensionNode> {
   return type.astNode
-    ? type.extensionASTNodes
-      ? [type.astNode].concat(type.extensionASTNodes)
-      : [type.astNode]
-    : type.extensionASTNodes || [];
+    ? [type.astNode].concat(type.extensionASTNodes)
+    : type.extensionASTNodes;
 }
 
 function getAllObjectOrInterfaceNodes(
@@ -579,10 +574,8 @@ function getAllObjectOrInterfaceNodes(
   | InterfaceTypeExtensionNode,
 > {
   return type.astNode
-    ? type.extensionASTNodes
-      ? [type.astNode].concat(type.extensionASTNodes)
-      : [type.astNode]
-    : type.extensionASTNodes || [];
+    ? [type.astNode].concat(type.extensionASTNodes)
+    : type.extensionASTNodes;
 }
 
 function getImplementsInterfaceNode(
diff --git a/src/utilities/__tests__/extendSchema-test.js b/src/utilities/__tests__/extendSchema-test.js
index 8731c65cfd7..71f9a10e891 100644
--- a/src/utilities/__tests__/extendSchema-test.js
+++ b/src/utilities/__tests__/extendSchema-test.js
@@ -245,7 +245,7 @@ describe('extendSchema', () => {
     const testDirective = extendedTwiceSchema.getDirective('test');
 
     expect(query.extensionASTNodes).to.have.lengthOf(2);
-    expect(testType.extensionASTNodes).to.equal(undefined);
+    expect(testType.extensionASTNodes).to.have.lengthOf(0);
 
     const restoredExtensionAST = {
       kind: Kind.DOCUMENT,
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 237473b35b1..ce7a073b2ec 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -241,10 +241,9 @@ export function extendSchema(
   // Support both original legacy names and extended legacy names.
   const schemaAllowedLegacyNames = schema.__allowedLegacyNames;
   const extendAllowedLegacyNames = options && options.allowedLegacyNames;
-  const allowedLegacyNames =
-    schemaAllowedLegacyNames && extendAllowedLegacyNames
-      ? schemaAllowedLegacyNames.concat(extendAllowedLegacyNames)
-      : schemaAllowedLegacyNames || extendAllowedLegacyNames;
+  const allowedLegacyNames = extendAllowedLegacyNames
+    ? schemaAllowedLegacyNames.concat(extendAllowedLegacyNames)
+    : schemaAllowedLegacyNames;
 
   // Then produce and return a Schema with these types.
   return new GraphQLSchema({
@@ -309,9 +308,7 @@ export function extendSchema(
   function extendObjectType(type: GraphQLObjectType): GraphQLObjectType {
     const name = type.name;
     const extensionASTNodes = typeExtensionsMap[name]
-      ? type.extensionASTNodes
-        ? type.extensionASTNodes.concat(typeExtensionsMap[name])
-        : typeExtensionsMap[name]
+      ? type.extensionASTNodes.concat(typeExtensionsMap[name])
       : type.extensionASTNodes;
     return new GraphQLObjectType({
       name,
@@ -329,9 +326,7 @@ export function extendSchema(
   ): GraphQLInterfaceType {
     const name = type.name;
     const extensionASTNodes = typeExtensionsMap[name]
-      ? type.extensionASTNodes
-        ? type.extensionASTNodes.concat(typeExtensionsMap[name])
-        : typeExtensionsMap[name]
+      ? type.extensionASTNodes.concat(typeExtensionsMap[name])
       : type.extensionASTNodes;
     return new GraphQLInterfaceType({
       name: type.name,