Add PersistentUserCallout factory method

Use factory method to DRY safe instantiation.
Add tests for PersistentUserCallout.

app/assets/javascripts/persistent_user_callout.js

@@ -31,4 +31,12 @@ export default class PersistentUserCallout {
         Flash(__('An error occurred while dismissing the alert. Refresh the page and try again.'));
       });
   }
+
+  static factory(container) {
+    if (!container) {
+      return undefined;
+    }
+
+    return new PersistentUserCallout(container);
+  }
 }

spec/javascripts/persistent_user_callout_spec.js

+import MockAdapter from 'axios-mock-adapter';
+import axios from '~/lib/utils/axios_utils';
+import PersistentUserCallout from '~/persistent_user_callout';
+import setTimeoutPromise from 'spec/helpers/set_timeout_promise_helper';
+
+describe('PersistentUserCallout', () => {
+  const dismissEndpoint = '/dismiss';
+  const featureName = 'feature';
+
+  function createFixture() {
+    const fixture = document.createElement('div');
+    fixture.innerHTML = `
+      <div
+        class="container"
+        data-dismiss-endpoint="${dismissEndpoint}"
+        data-feature-id="${featureName}"
+      >
+        <button type="button" class="js-close"></button>
+      </div>
+    `;
+
+    return fixture;
+  }
+
+  describe('dismiss', () => {
+    let button;
+    let mockAxios;
+    let persistentUserCallout;
+
+    beforeEach(() => {
+      const fixture = createFixture();
+      const container = fixture.querySelector('.container');
+      button = fixture.querySelector('.js-close');
+      mockAxios = new MockAdapter(axios);
+      persistentUserCallout = new PersistentUserCallout(container);
+      spyOn(persistentUserCallout.container, 'remove');
+    });
+
+    afterEach(() => {
+      mockAxios.restore();
+    });
+
+    it('POSTs endpoint and removes container when clicking close', done => {
+      mockAxios.onPost(dismissEndpoint).replyOnce(200);
+
+      button.click();
+
+      setTimeoutPromise()
+        .then(() => {
+          expect(persistentUserCallout.container.remove).toHaveBeenCalled();
+          expect(mockAxios.history.post[0].data).toBe(
+            JSON.stringify({ feature_name: featureName }),
+          );
+        })
+        .then(done)
+        .catch(done.fail);
+    });
+
+    it('invokes Flash when the dismiss request fails', done => {
+      const Flash = spyOnDependency(PersistentUserCallout, 'Flash');
+      mockAxios.onPost(dismissEndpoint).replyOnce(500);
+
+      button.click();
+
+      setTimeoutPromise()
+        .then(() => {
+          expect(persistentUserCallout.container.remove).not.toHaveBeenCalled();
+          expect(Flash).toHaveBeenCalledWith(
+            'An error occurred while dismissing the alert. Refresh the page and try again.',
+          );
+        })
+        .then(done)
+        .catch(done.fail);
+    });
+  });
+
+  describe('factory', () => {
+    it('returns an instance of PersistentUserCallout with the provided container property', () => {
+      const fixture = createFixture();
+
+      expect(PersistentUserCallout.factory(fixture) instanceof PersistentUserCallout).toBe(true);
+    });
+
+    it('returns undefined if container is falsey', () => {
+      expect(PersistentUserCallout.factory()).toBe(undefined);
+    });
+  });
+});