Jest mock/spy returns undefined even when set up

Explanation of the problem

When using the Jest mock system, a common issue that developers may encounter is the mocking spy return doing nothing. This issue can arise when developers copy-paste examples from the Jest documentation and find that the mocked function does not return the expected result. In the code snippet provided, the issue is with the function someUtil.m1() returning undefined instead of the expected value of 42.

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Problem solution for: Jest mock/spy returns undefined even when set up

To address this issue, developers can take several steps. First, they should ensure that the mocked module is imported correctly and that the mock is actually being used in the test. In the provided example, the module is mocked using jest.mock('./someUtil'). However, if the import statement for the module is incorrect, the mock will not be used, and the original implementation will be called instead. Additionally, developers can use the toHaveBeenCalled() method to check if the mocked function has been called. For example:

// someUtil.test.js import someUtil from ‘./someUtil’;

jest.mock(‘./someUtil’)

console.log(someUtil.m1) // expected spy stuff console.log(someUtil.m1()) // undefined 🤯 expect(someUtil.m1).toHaveBeenCalled()

Another potential solution is to use the mockReturnValue() method to specify the return value of the mocked function. This method can be used to override the default implementation of the mocked function and specify the desired return value. For example:

// someUtil.test.js import someUtil from ‘./someUtil’;

jest.mock(‘./someUtil’, () => ({ m1: jest.fn().mockReturnValue(42), }));

console.log(someUtil.m1) // expected spy stuff console.log(someUtil.m1()) // should return 42 expect(someUtil.m1).toHaveBeenCalled()

By using mockReturnValue(), developers can ensure that the mocked function returns the expected value and avoid issues with the mock system not working as expected.

Other popular problems with jest

// Example of creating a new mock function with jest.fn()
const myMock = jest.fn(() => 'Hello, World!');
console.log(myMock()); // Output: 'Hello, World!'

// Example of specifying the return value for an existing mock function
const myMock = jest.fn();
myMock.mockReturnValue('Hello, World!');
console.log(myMock()); // Output: 'Hello, World!'

jest.mock('./myModule');

test('asynchronous test', (done) => {
  setTimeout(() => {
    expect(1 + 2).toBe(3);
    done();
  }, 1000);
}, 5000);

A brief introduction to jest

Most popular use cases for jest

1. Jest can be used for unit testing and integration testing in JavaScript projects. Jest provides a comprehensive set of features for writing and running tests, such as assertions, mocks, and code coverage reporting. Here’s an example of a simple test using Jest:

test('adds 1 + 2 to equal 3', () => {
  expect(1 + 2).toBe(3);
});

This test checks whether the expression 1 + 2 evaluates to 3 using the toBe() matcher. Jest allows you to write tests in a variety of styles and supports a wide range of matchers for asserting the behavior of your code.

2. Jest can be used for snapshot testing, which allows you to compare the current output of a component or function to a previously saved snapshot. Snapshot testing can help catch regressions and ensure that your code produces consistent output over time. Here’s an example of a snapshot test:

test('renders a button', () => {
  const button = renderer.create(<Button label="Click me" />).toJSON();
  expect(button).toMatchSnapshot();
});

This test creates a snapshot of the rendered output of a Button component and compares it to a previously saved snapshot. If the output has changed, Jest will show a diff and prompt you to update the snapshot if the change is expected.

3. Jest can be used for end-to-end testing with tools like Puppeteer or WebDriver. This allows you to simulate user interactions with your application and verify that it behaves correctly in a real-world environment. Here’s an example of an end-to-end test using Puppeteer:

test('clicks a button', async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  await page.goto('https://example.com');
  await page.click('button');
  const buttonText = await page.$eval('button', el => el.textContent);
  expect(buttonText).toBe('Clicked!');
  await browser.close();
});