How can I mock a module that exports both a function and namespace in Node.js using Jest?

Below is one workable solution. Because the exported ytdl is both a function and a namespace, you can’t simply override its properties afterward. Instead, you create a new mock function, attach mocks for its submethods, and tell Jest to replace the module with that mock. Importantly, you must call jest.mock() before any module that imports ytdl-core is loaded (or use jest.resetModules). For instance:

JAVASCRIPTCopied
// In your test file (at the very top before any other imports)
jest.mock('@distube/ytdl-core', () => {
  // Create a mock function that will be returned when @distube/ytdl-core is required.
  const ytdlMock = jest.fn((link, options) => {
    // You can create and return a dummy stream (e.g., a PassThrough stream) if needed.
    const { PassThrough } = require('stream');
    const stream = new PassThrough();
    // Optionally simulate async behavior if your code calls getInfo().
    process.nextTick(() => {
      ytdlMock.getInfo(link, options).then(
        info => {
          // Optionally, simulate callback behavior on stream.
          stream.emit('info', info);
        },
        error => stream.emit('error', error)
      );
    });
    return stream;
  });

  // Attach mocked sub-methods to the function (namespace)
  ytdlMock.getInfo = jest.fn((link, options) => {
    // Return a promise that resolves to some fake info
    return Promise.resolve({ title: 'Fake Title', link });
  });
  ytdlMock.getBasicInfo = jest.fn((link, options) => {
    // Return fake basic info
    return Promise.resolve({ title: 'Fake Basic Title', link });
  });

  // Return the mock function when the module is imported.
  return ytdlMock;
});

// Now import modules that use ytdl-core (or your express app)
const app = require('./path/to/your/express/app');
const inject = require('light-my-request');

// Your tests...
it('should treat empty strings as no value', async () => {
  const PLAYLIST_URL = 'https://youtube.com/playlist?list=';
  const VALID_PLAYLIST_ID = 'FAKE_ID';
  
  const result = await inject(app, {
    method: 'POST',
    url: '/',
    headers: { 'Accept': 'multipart/form-data' },
    payload: {
      url: PLAYLIST_URL + VALID_PLAYLIST_ID,
      limit: ''
    }
  });
  
  // Assertions based on your test expectations.
  expect(result).toBeArray();
});

Key Points

1. jest.mock Timing:
   Since your express server imports ytdl-core at module load time, you must call jest.mock() before that happens.

2. Mock as Function with Properties:
   Create a function (ytdlMock) and add properties (e.g. getInfo) to it. This mimics the dual-nature (callable + namespace) of ytdl-core.

3. Simulate Asynchronous Behavior:
   If your code depends on how ytdl.getInfo works asynchronously, you can simulate it using process.nextTick or similar patterns.

4. Alternative Approaches:
   If you run into caching issues, consider using mocks folder with a manual mock or using jest.resetModules() to clear the module cache before each test.

This approach should let you fully control the behavior of both the ytdl() function and its sub-methods in your tests.