Add test for select2_utils axios transport helper

spec/frontend/lib/utils/select2_utils_spec.js

+import MockAdapter from 'axios-mock-adapter';
+import $ from 'jquery';
+import { setHTMLFixture } from 'helpers/fixtures';
+import waitForPromises from 'helpers/wait_for_promises';
+import axios from '~/lib/utils/axios_utils';
+import { select2AxiosTransport } from '~/lib/utils/select2_utils';
+
+import 'select2/select2';
+
+const TEST_URL = '/test/api/url';
+const TEST_SEARCH_DATA = { extraSearch: 'test' };
+const TEST_DATA = [{ id: 1 }];
+const TEST_SEARCH = 'FOO';
+
+describe('lib/utils/select2_utils', () => {
+  let mock;
+  let resultsSpy;
+
+  beforeEach(() => {
+    setHTMLFixture('<div><input id="root" /></div>');
+
+    mock = new MockAdapter(axios);
+
+    resultsSpy = jest.fn().mockReturnValue({ results: [] });
+  });
+
+  afterEach(() => {
+    mock.restore();
+  });
+
+  const setupSelect2 = (input) => {
+    input.select2({
+      ajax: {
+        url: TEST_URL,
+        quietMillis: 250,
+        transport: select2AxiosTransport,
+        data(search, page) {
+          return {
+            search,
+            page,
+            ...TEST_SEARCH_DATA,
+          };
+        },
+        results: resultsSpy,
+      },
+    });
+  };
+
+  const setupSelect2AndSearch = async () => {
+    const $input = $('#root');
+
+    setupSelect2($input);
+
+    $input.select2('search', TEST_SEARCH);
+
+    jest.runOnlyPendingTimers();
+    await waitForPromises();
+  };
+
+  describe('select2AxiosTransport', () => {
+    it('uses axios to make request', async () => {
+      // setup mock response
+      const replySpy = jest.fn();
+      mock.onGet(TEST_URL).reply((...args) => replySpy(...args));
+
+      await setupSelect2AndSearch();
+
+      expect(replySpy).toHaveBeenCalledWith(
+        expect.objectContaining({
+          url: TEST_URL,
+          method: 'get',
+          params: {
+            page: 1,
+            search: TEST_SEARCH,
+            ...TEST_SEARCH_DATA,
+          },
+        }),
+      );
+    });
+
+    it.each`
+      headers                                | pagination
+      ${{}}                                  | ${{ more: false }}
+      ${{ 'X-PAGE': '1', 'x-next-page': 2 }} | ${{ more: true }}
+    `(
+      'passes results and pagination to results callback, with headers=$headers',
+      async ({ headers, pagination }) => {
+        mock.onGet(TEST_URL).reply(200, TEST_DATA, headers);
+
+        await setupSelect2AndSearch();
+
+        expect(resultsSpy).toHaveBeenCalledWith(
+          { results: TEST_DATA, pagination },
+          1,
+          expect.anything(),
+        );
+      },
+    );
+  });
+});