extendSchema: preserve "description" and "extensions" (#3776)

Co-authored-by: Ivan Goncharov <ivan.goncharov.ua@gmail.com>
Closes #3629

Author: igrlk

src/utilities/__tests__/extendSchema-test.ts

@@ -113,6 +113,17 @@ describe('extendSchema', () => {
     expect(extendedSchema.getDirectives()).to.have.members(specifiedDirectives);
   });
 
+  it('preserves original schema config', () => {
+    const description = 'A schema description';
+    const extensions = Object.freeze({ foo: 'bar' });
+    const schema = new GraphQLSchema({ description, extensions });
+
+    const extendedSchema = extendSchema(schema, parse('scalar Bar'));
+
+    expect(extendedSchema.description).to.equal(description);
+    expect(extendedSchema.extensions).to.deep.equal(extensions);
+  });
+
   it('extends objects by adding new fields', () => {
     const schema = buildSchema(`
       type Query {

src/utilities/extendSchema.ts

@@ -239,14 +239,14 @@ export function extendSchemaImpl(
 
   // Then produce and return a Schema config with these types.
   return {
-    description: schemaDef?.description?.value,
+    description: schemaDef?.description?.value ?? schemaConfig.description,
     ...operationTypes,
     types: Object.values(typeMap),
     directives: [
       ...schemaConfig.directives.map(replaceDirective),
       ...directiveDefs.map(buildDirective),
     ],
-    extensions: Object.create(null),
+    extensions: schemaConfig.extensions,
     astNode: schemaDef ?? schemaConfig.astNode,
     extensionASTNodes: schemaConfig.extensionASTNodes.concat(schemaExtensions),
     assumeValid: options?.assumeValid ?? false,