Fix stitchSchemas for when empty array of typeDefs are passed (#1575)

If an empty array is passed, the GraphQL parser fails, because the
source is empty. Switch the typeDef check to check for length as well,
since an empty array is truthy.

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Fix visitEnumValue to allow modifying the enum value <br/>
   [@danielrearden](https://github.com/danielrearden) in [#1003](https://github.com/ardatan/graphql-tools/pull/1391)
 - Export `generateProxyingResolvers` from `@graphql-tools/wrap`.
+- Fix `stitchSchemas` from `@graphql-tools/stitch` from the case there the typeDefs array is empty. [#1575](https://github.com/ardatan/graphql-tools/pull/1575)
 
 ### 5.0.0
 

packages/stitch/src/stitchSchemas.ts

@@ -51,7 +51,7 @@ export function stitchSchemas({
   }
 
   let schemaLikeObjects: Array<GraphQLSchema | SubschemaConfig | DocumentNode | GraphQLNamedType> = [...subschemas];
-  if (typeDefs) {
+  if ((typeDefs && !Array.isArray(typeDefs)) || (Array.isArray(typeDefs) && typeDefs.length)) {
     schemaLikeObjects.push(buildDocumentFromTypeDefinitions(typeDefs, parseOptions));
   }
   if (types != null) {

packages/stitch/tests/stitchSchemas.test.ts

@@ -2982,6 +2982,36 @@ fragment BookingFragment on Booking {
     });
   });
 
+  describe('empty typeDefs array', () => {
+    test('works', async () => {
+      const typeDefs = `
+      type Query {
+        book: Book
+      }
+      type Book {
+        category: String!
+      }
+    `;
+      let schema = makeExecutableSchema({ typeDefs });
+
+      const resolvers = {
+        Query: {
+          book: () => ({ category: 'Test' }),
+        },
+      };
+
+      schema = stitchSchemas({
+        schemas: [schema],
+        resolvers,
+        typeDefs: [],
+      });
+
+      const result = await graphql(schema, '{ book { cat: category } }');
+
+      expect(result.data.book.cat).toBe('Test');
+    });
+  });
+
   describe('new root type name', () => {
     test('works', async () => {
       let bookSchema = makeExecutableSchema({