Nestjs + Jest: Unraveling the Mystery of expect().toHaveReturnedWith() Returning an Empty Object

Are you tired of banging your head against the wall, trying to figure out why Jest’s expect().toHaveReturnedWith() is returning an empty object in your NestJS application? You’re not alone! In this comprehensive guide, we’ll delve into the world of Jest and NestJS, exploring the reasons behind this frustrating issue and providing step-by-step solutions to get you back on track.

Understanding Jest’s expect().toHaveReturnedWith()

Jest’s expect().toHaveReturnedWith() is a powerful expectation that allows you to verify the return value of a function. It’s a crucial tool in your testing arsenal, ensuring that your functions are behaving as expected. However, when used in conjunction with NestJS, things can get a bit hairy.

import { Test, TestingModule } from '@nestjs/testing';
import { MyService } from './my.service';

describe('MyService', () => {
  let service: MyService;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [MyService],
    }).compile();

    service = module.get<MyService>(MyService);
  });

  it('should return an object', async () => {
    const result = await service.MyMethod();
    expect(result).toHaveReturnedWith({ foo: 'bar' });
  });
});

In the example above, we’re using Jest’s expect().toHaveReturnedWith() to verify that the MyMethod() function returns an object with the property foo set to ‘bar’. Sounds simple, right? But what if, instead of getting the expected result, we’re faced with an empty object?

The Culprits: Async/Await and Jest’s Expectation

To understand why expect().toHaveReturnedWith() might be returning an empty object, we need to take a closer look at how async/await and Jest’s expectation work.

When you use async/await in your tests, Jest wraps your test function in a Promise. This allows Jest to handle async operations correctly. However, this also means that Jest’s expectation is evaluated before the async operation has a chance to complete.

In our example, when we call service.MyMethod(), it returns a Promise. Jest’s expectation is evaluated immediately, before the Promise has resolved. As a result, expect().toHaveReturnedWith() returns an empty object, because the Promise has not yet resolved.

Solution 1: Using callbacks

One way to solve this issue is by using callbacks. Instead of using async/await, we can pass a callback function to our service method. This allows us to wait for the async operation to complete before evaluating the expectation.

it('should return an object', done => {
  service.MyMethod((result) => {
    expect(result).toHaveReturnedWith({ foo: 'bar' });
    done();
  });
});

In this example, we’re passing a callback function to service.MyMethod(). The callback function is called when the async operation has completed, and we can then evaluate the expectation using expect().toHaveReturnedWith().

Solution 2: Using async/await with await expect()

Another solution is to use async/await with await expect(). This allows us to wait for the async operation to complete before evaluating the expectation.

it('should return an object', async () => {
  const result = await service.MyMethod();
  await expect(result).resolves.toHaveReturnedWith({ foo: 'bar' });
});

In this example, we’re using async/await to wait for the async operation to complete. We then use await expect() to evaluate the expectation. The resolves matcher ensures that the Promise has resolved before evaluating the expectation.

Solution 3: Using jest.runOnlyPendingTimers()

A third solution is to use jest.runOnlyPendingTimers(). This method allows you to run all pending timers, ensuring that any async operations have completed before evaluating the expectation.

it('should return an object', async () => {
  const result = await service.MyMethod();
  jest.runOnlyPendingTimers();
  expect(result).toHaveReturnedWith({ foo: 'bar' });
});

In this example, we’re using jest.runOnlyPendingTimers() to ensure that all pending timers have been executed. We can then evaluate the expectation using expect().toHaveReturnedWith().

Conclusion

In this article, we’ve explored the reasons behind Jest’s expect().toHaveReturnedWith() returning an empty object in NestJS applications. We’ve also provided three solutions to overcome this issue: using callbacks, async/await with await expect(), and jest.runOnlyPendingTimers(). By understanding the intricacies of Jest’s expectation and async/await, you can write more effective tests for your NestJS applications.

Remember, when working with async operations and Jest’s expectation, it’s essential to ensure that the async operation has completed before evaluating the expectation. By using one of the solutions outlined above, you can overcome the issue of expect().toHaveReturnedWith() returning an empty object and write more reliable tests for your NestJS applications.

Best Practices

To avoid running into issues with expect().toHaveReturnedWith() returning an empty object, follow these best practices:

• Use async/await with await expect() to ensure that the async operation has completed before evaluating the expectation.
• Use jest.runOnlyPendingTimers() to ensure that all pending timers have been executed before evaluating the expectation.
• Avoid using expect().toHaveReturnedWith() without ensuring that the async operation has completed.
• Use callbacks when working with async operations to ensure that the expectation is evaluated after the async operation has completed.

Common Pitfalls

Be aware of the following common pitfalls when using expect().toHaveReturnedWith() with async operations:

• Forgetting to use async/await or callbacks to ensure that the async operation has completed before evaluating the expectation.
• Not using jest.runOnlyPendingTimers() to ensure that all pending timers have been executed before evaluating the expectation.
• Using expect().toHaveReturnedWith() without verifying that the async operation has completed.

Frequently Asked Questions

Here are some frequently asked questions related to expect().toHaveReturnedWith() returning an empty object:

Question	Answer
Why is expect().toHaveReturnedWith() returning an empty object?	The async operation has not completed before evaluating the expectation.
How do I fix expect().toHaveReturnedWith() returning an empty object?	Use one of the solutions outlined above: using callbacks, async/await with await expect(), or jest.runOnlyPendingTimers().
Can I use expect().toHaveReturnedWith() without await?	No, expect().toHaveReturnedWith() requires await to ensure that the async operation has completed before evaluating the expectation.

By following the solutions and best practices outlined in this article, you can overcome the issue of expect().toHaveReturnedWith() returning an empty object and write more effective tests for your NestJS applications.

Happy testing!

Frequently Asked Question

Stuck with Nestjs and Jest? Worry not! Get answers to the most frequently asked questions about `expect().toHaveReturnedWith()` returning an empty object.