Fix: Serialize arrays as JSON on fetch in RESTDataSource (#2219)

Fixes issue introduced in #1125 and logged in #2186 and #2196.

Arrays are valid JSON per spec: https://tools.ietf.org/html/rfc7159

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### vNEXT
 
+- Fix: Serialize arrays as JSON on fetch in `RESTDataSource`. [PR #2219](https://github.com/apollographql/apollo-server/pull/2219)
+
 ### v2.3.3
 
 - `apollo-server` (only): Stop double-invocation of `serverWillStart` life-cycle event.  (More specific integrations - e.g. Express, Koa, Hapi, etc. - were unaffected.) [PR #2239](https://github.com/apollographql/apollo-server/pull/2239)

packages/apollo-datasource-rest/src/RESTDataSource.ts

@@ -218,11 +218,12 @@ export abstract class RESTDataSource<TContext = any> extends DataSource {
       url.searchParams.append(name, value);
     }
 
-    // We accept arbitrary objects as body and serialize them as JSON
+    // We accept arbitrary objects and arrays as body and serialize them as JSON
     if (
       options.body !== undefined &&
       options.body !== null &&
       (options.body.constructor === Object ||
+        Array.isArray(options.body) ||
         ((options.body as any).toJSON &&
           typeof (options.body as any).toJSON === 'function'))
     ) {

packages/apollo-datasource-rest/src/__tests__/RESTDataSource.test.ts

@@ -268,6 +268,31 @@ describe('RESTDataSource', () => {
       );
     });
 
+    it('serializes a request body that is an array as JSON', async () => {
+      const dataSource = new class extends RESTDataSource {
+        baseURL = 'https://api.example.com';
+
+        postFoo(foo) {
+          return this.post('foo', foo);
+        }
+      }();
+
+      dataSource.httpCache = httpCache;
+
+      fetch.mockJSONResponseOnce();
+
+      await dataSource.postFoo(['foo', 'bar']);
+
+      expect(fetch.mock.calls.length).toEqual(1);
+      expect(fetch.mock.calls[0][0].url).toEqual('https://api.example.com/foo');
+      expect(fetch.mock.calls[0][0].body).toEqual(
+        JSON.stringify(['foo', 'bar']),
+      );
+      expect(fetch.mock.calls[0][0].headers.get('Content-Type')).toEqual(
+        'application/json',
+      );
+    });
+
     it('serializes a request body that has a toJSON method as JSON', async () => {
       const dataSource = new class extends RESTDataSource {
         baseURL = 'https://api.example.com';