[Feature Request] Allow for easy mocking

Is your feature request related to a problem? Please describe.

Mocking an SDK function is difficult. If you use a system like Sinon that depends on default exports, it doesn't work, since we don't do default exports. Even if you use a system like Jest that allows you to mock the whole package, there are two issues:

• Unless your code only uses a single function, you have to mock or add back in the other functions. This, for instance, creates a package that only exports proxyActivities:

const mockRequest = jest.fn<Promise<string>, []>();
mockRequest.mockResolvedValue('3');

test('httpWorkflow with mocked proxyActivities', async () => {
  jest.mock('@temporalio/workflow', () => ({
    proxyActivities: mockRequest,
  }));

  expect(await httpWorkflow()).toBe('The answer is 3');
});

• When you mock, you have to recreate the environment. For instance, if you run the above, you get:

 FAIL  src/workflows.test.ts (9.489 s)
  ● httpWorkflow with mocked proxyActivities

    TypeError: exports.storage.getStore is not a function

      14 |
      15 | export async function httpWorkflow(): Promise<string> {
    > 16 |   const answer = await makeHTTPRequest();
         |                        ^
      17 |   return `The answer is ${answer}`;
      18 | }
      19 |

      at Function.current (node_modules/@temporalio/workflow/src/cancellation-scope.ts:156:20)
      at node_modules/@temporalio/workflow/src/workflow.ts:151:37
      at scheduleActivityNextHandler (node_modules/@temporalio/workflow/src/workflow.ts:150:10)
      at scheduleActivity (node_modules/@temporalio/workflow/src/workflow.ts:259:10)
      at node_modules/@temporalio/workflow/src/workflow.ts:493:18
      at httpWorkflow (src/workflows.ts:16:24)
      at Object.<anonymous> (src/workflows.test.ts:67:28)

Solution

The current solution in the case of proxyActivities is to run a Worker with a test connection and mock activities (like this), and to start a workflow using the test client. If you want to test a helper function used by workflows, then tell the Worker the helper is a workflow and start it, like this:

// workflow-helpers.ts
export function myHelper() { ... }

// workflow-helpers.test.ts
  const worker = await Worker.create({
    workflowsPath: require.resolve('./workflow-helpers'),
    ...
  });
  await withWorker(worker, async () => {
    const result = await workflowClient.execute(myHelper, {
      workflowId: uuid4(),
      taskQueue: 'test',
    });
    expect(result).toEqual('The answer is 99');
  });  

Feedback

If the current solution described above doesn't work for your case, or isn't ideal for you, let us know in this thread! ☺️

Start with the mocked workflow example in src/workflows.test.ts and compare it with the Worker-based testing approach described in the issue. Define the desired mocking behavior and identify tests that demonstrate it without requiring callers to recreate the workflow environment; the issue does not name an implementation entry point or specific completion criteria.