How do I use Jest for mocking in Node.js?

How do I use Jest for mocking in Node.js?

You can use Jest's built-in mocking functions like jest.mock() to mock modules and functions in your tests.

Can you give me an example of mocking a function?

Sure! Here's a simple example:

// myModule.js
const fetchData = () => {
  return 'real data';
};

module.exports = fetchData;

// myModule.test.js
const fetchData = require('./myModule');

jest.mock('./myModule');

fetchData.mockImplementation(() => 'mocked data');

test('fetchData returns mocked data', () => {
  expect(fetchData()).toBe('mocked data');
});

What if I want to mock a specific implementation?

You can use mockImplementationOnce() to specify a different implementation for a single call. For example:

fetchData.mockImplementationOnce(() => 'first call data');
fetchData.mockImplementationOnce(() => 'second call data');