Added a test file for the start function

lib/core/src/client/preview/start.test.js

@@ -0,0 +1,101 @@
+import addons from '@storybook/addons';
+import Events from '@storybook/core-events';
+import { document } from 'global';
+
+import start from './start';
+
+jest.mock('@storybook/client-logger');
+jest.mock('@storybook/addons');
+jest.mock('global', () => ({
+  navigator: { userAgent: 'browser' },
+  window: {
+    addEventListener: jest.fn(),
+    location: { search: '' },
+    history: { replaceState: jest.fn() },
+  },
+  document: {
+    addEventListener: jest.fn(),
+    getElementById: jest.fn().mockReturnValue({}),
+    body: { classList: { add: jest.fn(), remove: jest.fn() } },
+    documentElement: {},
+  },
+}));
+
+function mockEmit() {
+  const emit = jest.fn();
+  addons.getChannel.mockReturnValue({ emit });
+
+  return emit;
+}
+
+it('renders nopreview when you have no stories', () => {
+  const emit = mockEmit();
+
+  const render = jest.fn();
+
+  start(render);
+
+  expect(render).not.toHaveBeenCalled();
+  expect(document.body.classList.add).toHaveBeenCalledWith('sb-show-nopreview');
+  expect(emit).toHaveBeenCalledWith(Events.STORY_RENDERED);
+});
+
+it('calls render when you add a story', () => {
+  const emit = mockEmit();
+
+  const render = jest.fn();
+
+  const { clientApi, configApi } = start(render);
+
+  emit.mockReset();
+  configApi.configure(() => {
+    clientApi.storiesOf('kind', {}).add('story', () => {});
+  }, {});
+
+  expect(render).toHaveBeenCalled();
+  expect(emit).toHaveBeenCalledWith(Events.STORY_RENDERED);
+});
+
+it('emits an exception and shows error when your story throws', () => {
+  const emit = mockEmit();
+
+  const render = jest.fn().mockImplementation(() => {
+    throw new Error('Some exception');
+  });
+
+  const { clientApi, configApi } = start(render);
+
+  emit.mockReset();
+  document.body.classList.add.mockReset();
+  configApi.configure(() => {
+    clientApi.storiesOf('kind', {}).add('story', () => {});
+  }, {});
+
+  expect(render).toHaveBeenCalled();
+  expect(document.body.classList.add).toHaveBeenCalledWith('sb-show-errordisplay');
+  expect(emit).toHaveBeenCalledWith(Events.STORY_THREW_EXCEPTION, expect.any(Error));
+});
+
+it('emits an error and shows error when your framework calls showError', () => {
+  const emit = mockEmit();
+
+  const error = {
+    title: 'Some error',
+    description: 'description',
+  };
+  const render = jest.fn().mockImplementation(({ showError }) => {
+    showError(error);
+  });
+
+  const { clientApi, configApi } = start(render);
+
+  emit.mockReset();
+  document.body.classList.add.mockReset();
+  configApi.configure(() => {
+    clientApi.storiesOf('kind', {}).add('story', () => {});
+  }, {});
+
+  expect(render).toHaveBeenCalled();
+  expect(document.body.classList.add).toHaveBeenCalledWith('sb-show-errordisplay');
+  expect(emit).toHaveBeenCalledWith(Events.STORY_ERRORED, error);
+});