diff --git a/src/type/definition.d.ts b/src/type/definition.d.ts
index 2f085c9c8a..42fb596024 100644
--- a/src/type/definition.d.ts
+++ b/src/type/definition.d.ts
@@ -306,7 +306,7 @@ export class GraphQLScalarType {
   parseLiteral: GraphQLScalarLiteralParser<any>;
   extensions: Maybe<Readonly<GraphQLScalarTypeExtensions>>;
   astNode: Maybe<ScalarTypeDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<ScalarTypeExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<ScalarTypeExtensionNode>;
 
   constructor(config: Readonly<GraphQLScalarTypeConfig<any, any>>);
 
@@ -409,7 +409,7 @@ export class GraphQLObjectType<TSource = any, TContext = any> {
   isTypeOf: Maybe<GraphQLIsTypeOfFn<TSource, TContext>>;
   extensions: Maybe<Readonly<GraphQLObjectTypeExtensions<TSource, TContext>>>;
   astNode: Maybe<ObjectTypeDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<ObjectTypeExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<ObjectTypeExtensionNode>;
 
   constructor(config: Readonly<GraphQLObjectTypeConfig<TSource, TContext>>);
 
@@ -616,7 +616,7 @@ export class GraphQLInterfaceType {
   resolveType: Maybe<GraphQLTypeResolver<any, any>>;
   extensions: Maybe<Readonly<GraphQLInterfaceTypeExtensions>>;
   astNode?: Maybe<InterfaceTypeDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<InterfaceTypeExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<InterfaceTypeExtensionNode>;
 
   constructor(config: Readonly<GraphQLInterfaceTypeConfig<any, any>>);
   getFields(): GraphQLFieldMap<any, any>;
@@ -692,7 +692,7 @@ export class GraphQLUnionType {
   resolveType: Maybe<GraphQLTypeResolver<any, any>>;
   extensions: Maybe<Readonly<GraphQLUnionTypeExtensions>>;
   astNode: Maybe<UnionTypeDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<UnionTypeExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<UnionTypeExtensionNode>;
 
   constructor(config: Readonly<GraphQLUnionTypeConfig<any, any>>);
   getTypes(): Array<GraphQLObjectType>;
@@ -762,7 +762,7 @@ export class GraphQLEnumType {
   description: Maybe<string>;
   extensions: Maybe<Readonly<GraphQLEnumTypeExtensions>>;
   astNode: Maybe<EnumTypeDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<EnumTypeExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<EnumTypeExtensionNode>;
 
   constructor(config: Readonly<GraphQLEnumTypeConfig>);
   getValues(): Array<GraphQLEnumValue>;
@@ -865,7 +865,7 @@ export class GraphQLInputObjectType {
   description: Maybe<string>;
   extensions: Maybe<Readonly<GraphQLInputObjectTypeExtensions>>;
   astNode: Maybe<InputObjectTypeDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<InputObjectTypeExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<InputObjectTypeExtensionNode>;
 
   constructor(config: Readonly<GraphQLInputObjectTypeConfig>);
   getFields(): GraphQLInputFieldMap;
diff --git a/src/type/definition.js b/src/type/definition.js
index fa37748347..04c5f05ef3 100644
--- a/src/type/definition.js
+++ b/src/type/definition.js
@@ -522,10 +522,6 @@ function resolveObjMapThunk<T>(thunk: ThunkObjMap<T>): ObjMap<T> {
   return typeof thunk === 'function' ? thunk() : thunk;
 }
 
-function undefineIfEmpty<T>(arr: ?$ReadOnlyArray<T>): ?$ReadOnlyArray<T> {
-  return arr && arr.length > 0 ? arr : undefined;
-}
-
 /**
  * Scalar Type Definition
  *
@@ -559,7 +555,7 @@ export class GraphQLScalarType {
   parseLiteral: GraphQLScalarLiteralParser<mixed>;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?ScalarTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<ScalarTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<ScalarTypeExtensionNode>;
 
   constructor(config: $ReadOnly<GraphQLScalarTypeConfig<mixed, mixed>>) {
     const parseValue = config.parseValue ?? identityFunc;
@@ -573,7 +569,7 @@ export class GraphQLScalarType {
       ((node, variables) => parseValue(valueFromASTUntyped(node, variables)));
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = undefineIfEmpty(config.extensionASTNodes);
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     devAssert(typeof config.name === 'string', 'Must provide name.');
 
@@ -608,7 +604,7 @@ export class GraphQLScalarType {
       parseLiteral: this.parseLiteral,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes ?? [],
+      extensionASTNodes: this.extensionASTNodes,
     };
   }
 
@@ -706,7 +702,7 @@ export class GraphQLObjectType {
   isTypeOf: ?GraphQLIsTypeOfFn<any, any>;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?ObjectTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<ObjectTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<ObjectTypeExtensionNode>;
 
   _fields: ThunkObjMap<GraphQLField<any, any>>;
   _interfaces: ThunkArray<GraphQLInterfaceType>;
@@ -717,7 +713,7 @@ export class GraphQLObjectType {
     this.isTypeOf = config.isTypeOf;
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = undefineIfEmpty(config.extensionASTNodes);
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     this._fields = defineFieldMap.bind(undefined, config);
     this._interfaces = defineInterfaces.bind(undefined, config);
@@ -752,7 +748,7 @@ export class GraphQLObjectType {
       isTypeOf: this.isTypeOf,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes || [],
+      extensionASTNodes: this.extensionASTNodes,
     };
   }
 
@@ -1023,7 +1019,7 @@ export class GraphQLInterfaceType {
   resolveType: ?GraphQLTypeResolver<any, any>;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?InterfaceTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<InterfaceTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<InterfaceTypeExtensionNode>;
 
   _fields: ThunkObjMap<GraphQLField<any, any>>;
   _interfaces: ThunkArray<GraphQLInterfaceType>;
@@ -1034,7 +1030,7 @@ export class GraphQLInterfaceType {
     this.resolveType = config.resolveType;
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = undefineIfEmpty(config.extensionASTNodes);
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     this._fields = defineFieldMap.bind(undefined, config);
     this._interfaces = defineInterfaces.bind(undefined, config);
@@ -1069,7 +1065,7 @@ export class GraphQLInterfaceType {
       resolveType: this.resolveType,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes ?? [],
+      extensionASTNodes: this.extensionASTNodes,
     };
   }
 
@@ -1140,7 +1136,7 @@ export class GraphQLUnionType {
   resolveType: ?GraphQLTypeResolver<any, any>;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?UnionTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<UnionTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<UnionTypeExtensionNode>;
 
   _types: ThunkArray<GraphQLObjectType>;
 
@@ -1150,7 +1146,7 @@ export class GraphQLUnionType {
     this.resolveType = config.resolveType;
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = undefineIfEmpty(config.extensionASTNodes);
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     this._types = defineTypes.bind(undefined, config);
     devAssert(typeof config.name === 'string', 'Must provide name.');
@@ -1176,7 +1172,7 @@ export class GraphQLUnionType {
       resolveType: this.resolveType,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes ?? [],
+      extensionASTNodes: this.extensionASTNodes,
     };
   }
 
@@ -1253,7 +1249,7 @@ export class GraphQLEnumType /* <T> */ {
   description: ?string;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?EnumTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<EnumTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<EnumTypeExtensionNode>;
 
   _values: Array<GraphQLEnumValue /* <T> */>;
   _valueLookup: Map<any /* T */, GraphQLEnumValue>;
@@ -1264,7 +1260,7 @@ export class GraphQLEnumType /* <T> */ {
     this.description = config.description;
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = undefineIfEmpty(config.extensionASTNodes);
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     this._values = defineEnumValues(this.name, config.values);
     this._valueLookup = new Map(
@@ -1354,7 +1350,7 @@ export class GraphQLEnumType /* <T> */ {
       values,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes ?? [],
+      extensionASTNodes: this.extensionASTNodes,
     };
   }
 
@@ -1466,7 +1462,7 @@ export class GraphQLInputObjectType {
   description: ?string;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?InputObjectTypeDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<InputObjectTypeExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<InputObjectTypeExtensionNode>;
 
   _fields: ThunkObjMap<GraphQLInputField>;
 
@@ -1475,7 +1471,7 @@ export class GraphQLInputObjectType {
     this.description = config.description;
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = undefineIfEmpty(config.extensionASTNodes);
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     this._fields = defineInputFieldMap.bind(undefined, config);
     devAssert(typeof config.name === 'string', 'Must provide name.');
@@ -1503,7 +1499,7 @@ export class GraphQLInputObjectType {
       fields,
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes ?? [],
+      extensionASTNodes: this.extensionASTNodes,
     };
   }
 
diff --git a/src/type/schema.d.ts b/src/type/schema.d.ts
index c4c366f737..9e751faff0 100644
--- a/src/type/schema.d.ts
+++ b/src/type/schema.d.ts
@@ -62,7 +62,7 @@ export class GraphQLSchema {
   description: Maybe<string>;
   extensions: Maybe<Readonly<GraphQLSchemaExtensions>>;
   astNode: Maybe<SchemaDefinitionNode>;
-  extensionASTNodes: Maybe<ReadonlyArray<SchemaExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<SchemaExtensionNode>;
 
   constructor(config: Readonly<GraphQLSchemaConfig>);
   getQueryType(): Maybe<GraphQLObjectType>;
@@ -136,6 +136,6 @@ export interface GraphQLSchemaNormalizedConfig extends GraphQLSchemaConfig {
   types: Array<GraphQLNamedType>;
   directives: Array<GraphQLDirective>;
   extensions: Maybe<Readonly<GraphQLSchemaExtensions>>;
-  extensionASTNodes: Maybe<ReadonlyArray<SchemaExtensionNode>>;
+  extensionASTNodes: ReadonlyArray<SchemaExtensionNode>;
   assumeValid: boolean;
 }
diff --git a/src/type/schema.js b/src/type/schema.js
index 5b2e834865..be67ffd270 100644
--- a/src/type/schema.js
+++ b/src/type/schema.js
@@ -121,7 +121,7 @@ export class GraphQLSchema {
   description: ?string;
   extensions: ?ReadOnlyObjMap<mixed>;
   astNode: ?SchemaDefinitionNode;
-  extensionASTNodes: ?$ReadOnlyArray<SchemaExtensionNode>;
+  extensionASTNodes: $ReadOnlyArray<SchemaExtensionNode>;
 
   _queryType: ?GraphQLObjectType;
   _mutationType: ?GraphQLObjectType;
@@ -157,7 +157,7 @@ export class GraphQLSchema {
     this.description = config.description;
     this.extensions = config.extensions && toObjMap(config.extensions);
     this.astNode = config.astNode;
-    this.extensionASTNodes = config.extensionASTNodes;
+    this.extensionASTNodes = config.extensionASTNodes ?? [];
 
     this._queryType = config.query;
     this._mutationType = config.mutation;
@@ -337,7 +337,7 @@ export class GraphQLSchema {
       directives: this.getDirectives().slice(),
       extensions: this.extensions,
       astNode: this.astNode,
-      extensionASTNodes: this.extensionASTNodes ?? [],
+      extensionASTNodes: this.extensionASTNodes,
       assumeValid: this.__validationErrors !== undefined,
     };
   }
diff --git a/src/type/validate.js b/src/type/validate.js
index 4d39256ac4..bc0763a0f7 100644
--- a/src/type/validate.js
+++ b/src/type/validate.js
@@ -261,10 +261,10 @@ function validateFields(
 
   // Objects and Interfaces both must define one or more fields.
   if (fields.length === 0) {
-    context.reportError(
-      `Type ${type.name} must define one or more fields.`,
-      [type.astNode].concat(type.extensionASTNodes),
-    );
+    context.reportError(`Type ${type.name} must define one or more fields.`, [
+      type.astNode,
+      ...type.extensionASTNodes,
+    ]);
   }
 
   for (const field of fields) {
@@ -364,7 +364,7 @@ function validateTypeImplementsInterface(
     if (!typeField) {
       context.reportError(
         `Interface field ${iface.name}.${fieldName} expected but ${type.name} does not provide it.`,
-        [ifaceField.astNode, type.astNode].concat(type.extensionASTNodes),
+        [ifaceField.astNode, type.astNode, ...type.extensionASTNodes],
       );
       continue;
     }
@@ -464,7 +464,7 @@ function validateUnionMembers(
   if (memberTypes.length === 0) {
     context.reportError(
       `Union type ${union.name} must define one or more member types.`,
-      [union.astNode].concat(union.extensionASTNodes),
+      [union.astNode, ...union.extensionASTNodes],
     );
   }
 
@@ -497,7 +497,7 @@ function validateEnumValues(
   if (enumValues.length === 0) {
     context.reportError(
       `Enum type ${enumType.name} must define one or more values.`,
-      [enumType.astNode].concat(enumType.extensionASTNodes),
+      [enumType.astNode, ...enumType.extensionASTNodes],
     );
   }
 
@@ -524,7 +524,7 @@ function validateInputFields(
   if (fields.length === 0) {
     context.reportError(
       `Input Object type ${inputObj.name} must define one or more fields.`,
-      [inputObj.astNode].concat(inputObj.extensionASTNodes),
+      [inputObj.astNode, ...inputObj.extensionASTNodes],
     );
   }
 
@@ -611,10 +611,13 @@ function getAllImplementsInterfaceNodes(
   type: GraphQLObjectType | GraphQLInterfaceType,
   iface: GraphQLInterfaceType,
 ): $ReadOnlyArray<NamedTypeNode> {
+  const { astNode, extensionASTNodes } = type;
+  const nodes =
+    astNode != null ? [astNode, ...extensionASTNodes] : extensionASTNodes;
+
   // istanbul ignore next (See: 'https://github.com/graphql/graphql-js/issues/2203')
-  return [type.astNode]
-    .concat(type.extensionASTNodes)
-    .flatMap((typeNode) => typeNode?.interfaces ?? [])
+  return nodes
+    .flatMap((typeNode) => typeNode.interfaces ?? [])
     .filter((ifaceNode) => ifaceNode.name.value === iface.name);
 }
 
@@ -622,10 +625,13 @@ function getUnionMemberTypeNodes(
   union: GraphQLUnionType,
   typeName: string,
 ): ?$ReadOnlyArray<NamedTypeNode> {
+  const { astNode, extensionASTNodes } = union;
+  const nodes =
+    astNode != null ? [astNode, ...extensionASTNodes] : extensionASTNodes;
+
   // istanbul ignore next (See: 'https://github.com/graphql/graphql-js/issues/2203')
-  return [union.astNode]
-    .concat(union.extensionASTNodes)
-    .flatMap((unionNode) => unionNode?.types ?? [])
+  return nodes
+    .flatMap((unionNode) => unionNode.types ?? [])
     .filter((typeNode) => typeNode.name.value === typeName);
 }
 
diff --git a/src/utilities/__tests__/buildASTSchema-test.js b/src/utilities/__tests__/buildASTSchema-test.js
index b6369be924..1fa28dc073 100644
--- a/src/utilities/__tests__/buildASTSchema-test.js
+++ b/src/utilities/__tests__/buildASTSchema-test.js
@@ -58,7 +58,7 @@ function printASTNode(obj: ?{ +astNode: ?ASTNode, ... }): string {
 }
 
 function printAllASTNodes(obj: GraphQLNamedType): string {
-  invariant(obj.astNode != null && obj.extensionASTNodes != null);
+  invariant(obj.astNode != null);
   return print({
     kind: Kind.DOCUMENT,
     definitions: [obj.astNode, ...obj.extensionASTNodes],
diff --git a/src/utilities/__tests__/extendSchema-test.js b/src/utilities/__tests__/extendSchema-test.js
index 1c7c920cd1..1d8470cd7a 100644
--- a/src/utilities/__tests__/extendSchema-test.js
+++ b/src/utilities/__tests__/extendSchema-test.js
@@ -38,8 +38,7 @@ import { extendSchema } from '../extendSchema';
 import { buildSchema } from '../buildASTSchema';
 
 function printExtensionNodes(obj: ?GraphQLNamedType | GraphQLSchema): string {
-  // istanbul ignore next (FIXME)
-  invariant(obj?.extensionASTNodes != null);
+  invariant(obj != null);
   return print({
     kind: Kind.DOCUMENT,
     definitions: obj.extensionASTNodes,
@@ -442,18 +441,11 @@ describe('extendSchema', () => {
       extendedTwiceSchema.getDirective('test'),
     );
 
-    expect(testType).to.include({ extensionASTNodes: undefined });
-    expect(testEnum).to.include({ extensionASTNodes: undefined });
-    expect(testUnion).to.include({ extensionASTNodes: undefined });
-    expect(testInput).to.include({ extensionASTNodes: undefined });
-    expect(testInterface).to.include({ extensionASTNodes: undefined });
-
-    invariant(query.extensionASTNodes);
-    invariant(someScalar.extensionASTNodes);
-    invariant(someEnum.extensionASTNodes);
-    invariant(someUnion.extensionASTNodes);
-    invariant(someInput.extensionASTNodes);
-    invariant(someInterface.extensionASTNodes);
+    expect(testType.extensionASTNodes).to.deep.equal([]);
+    expect(testEnum.extensionASTNodes).to.deep.equal([]);
+    expect(testUnion.extensionASTNodes).to.deep.equal([]);
+    expect(testInput.extensionASTNodes).to.deep.equal([]);
+    expect(testInterface.extensionASTNodes).to.deep.equal([]);
 
     expect([
       testInput.astNode,