How can you mock asynchronous functions in TypeScript using Mocha

1. Use Sinon.js for mocking:
Sinon.js is a popular library for creating stubs, spies, and mocks in JavaScript and TypeScript. It works well with Mocha and provides a flexible way to mock asynchronous functions.

2. Create a stub for the asynchronous function:
Use Sinon's `stub()` method to create a stub for the asynchronous function you want to mock.

typescript
import * as sinon from 'sinon';
import { expect } from 'chai';
import { myAsyncFunction } from './myModule';

describe('myAsyncFunction', () => {
  let sandbox: sinon.SinonSandbox;

  beforeEach(() => {
    sandbox = sinon.createSandbox();
  });

  afterEach(() => {
    sandbox.restore();
  });

  it('should return the expected value', async () => {
    // Create a stub for the asynchronous function
    const stub = sandbox.stub(this, 'myAsyncFunction').resolves('mocked value');

    const result = await myAsyncFunction();
    expect(result).to.equal('mocked value');
    expect(stub).to.have.been.called;
  });

  it('should handle errors', async () => {
    // Create a stub that rejects with an error
    const stub = sandbox.stub(this, 'myAsyncFunction').rejects(new Error('API error'));

    await expect(myAsyncFunction()).to.be.rejectedWith('API error');
    expect(stub).to.have.been.called;
  });
});

In this example, we use Sinon's `sandbox` to create and manage the stubs. In the `beforeEach` hook, we create a new sandbox, and in the `afterEach` hook, we restore the sandbox to clean up the mocks.

Inside the test cases, we use the `stub()` method to create stubs for the `myAsyncFunction`. The `resolves()` and `rejects()` methods are used to set the resolved and rejected values of the mocked function, respectively.

3. Verify the behavior:
In the test cases, you can use Chai's `expect` assertions to verify the behavior of the mocked asynchronous function, such as checking the returned value or ensuring that the function was called with the expected arguments.

By using Sinon.js with Mocha, you can effectively mock asynchronous functions in TypeScript and ensure that your tests cover the expected behavior, including error handling.

Remember to clean up the mocks after each test using the `sandbox.restore()` method to maintain a clean test environment.