1 files changed, 94 insertions, 0 deletions
diff --git a/spec/frontend/helpers/event_hub_factory_spec.js b/spec/frontend/helpers/event_hub_factory_spec.js
new file mode 100644
index 00000000000..dcfec6b836a
--- /dev/null
+++ b/spec/frontend/helpers/event_hub_factory_spec.js
@@ -0,0 +1,94 @@
+import createEventHub from '~/helpers/event_hub_factory';
+
+describe('event bus factory', () => {
+  let eventBus;
+
+  beforeEach(() => {
+    eventBus = createEventHub();
+  });
+
+  afterEach(() => {
+    eventBus = null;
+  });
+
+  describe('underlying module', () => {
+    let mitt;
+
+    beforeEach(() => {
+      jest.resetModules();
+      jest.mock('mitt');
+
+      // eslint-disable-next-line global-require
+      mitt = require('mitt');
+      mitt.mockReturnValue(() => ({}));
+
+      const createEventHubActual = jest.requireActual('~/helpers/event_hub_factory').default;
+      eventBus = createEventHubActual();
+    });
+
+    it('creates an emitter', () => {
+      expect(mitt).toHaveBeenCalled();
+    });
+  });
+
+  describe('instance', () => {
+    it.each`
+      method
+      ${'on'}
+      ${'once'}
+      ${'off'}
+      ${'emit'}
+    `('binds $$method to $method ', ({ method }) => {
+      expect(typeof eventBus[method]).toBe('function');
+      expect(eventBus[method]).toBe(eventBus[`$${method}`]);
+    });
+  });
+
+  describe('once', () => {
+    const event = 'foobar';
+    let handler;
+
+    beforeEach(() => {
+      jest.spyOn(eventBus, 'on');
+      jest.spyOn(eventBus, 'off');
+      handler = jest.fn();
+      eventBus.once(event, handler);
+    });
+
+    it('calls on internally', () => {
+      expect(eventBus.on).toHaveBeenCalled();
+    });
+
+    it('calls handler when event is emitted', () => {
+      eventBus.emit(event);
+      expect(handler).toHaveBeenCalled();
+    });
+
+    it('calls off when event is emitted', () => {
+      eventBus.emit(event);
+      expect(eventBus.off).toHaveBeenCalled();
+    });
+
+    it('calls the handler only once when event is emitted multiple times', () => {
+      eventBus.emit(event);
+      eventBus.emit(event);
+      expect(handler).toHaveBeenCalledTimes(1);
+    });
+
+    describe('when the handler thows an error', () => {
+      beforeEach(() => {
+        handler = jest.fn().mockImplementation(() => {
+          throw new Error();
+        });
+        eventBus.once(event, handler);
+      });
+
+      it('calls off when event is emitted', () => {
+        expect(() => {
+          eventBus.emit(event);
+        }).toThrow();
+        expect(eventBus.off).toHaveBeenCalled();
+      });
+    });
+  });
+});