JavaScript heap out of memory after upgrade to Jest 26

Explanation of the problem

describe('when item ids are in sessionStorage', () => {
  const itemIds = [333, 222, 111];

  beforeEach(() => {
    // Mocking functions
    parseLocationToQueries.mockImplementation(() => ({
      queue_id: testQueueId
    }));
    isAdHocReviewByItemId.mockReturnValue(false);
    isAdHocReviewByObjId.mockReturnValue(false);
    setItemsToBeReviewed(itemIds);
  });

  it('initial fetch', () => {
    const wrapper = tf.render();
    expect(wrapper.state('itemIds')).toEqual([]);
    expect(axios.post).toBeCalledWith('/review/items', { item_ids: itemIds });
  });

  // More test cases follow...
});

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Problem solution for: JavaScript heap out of memory after upgrade to Jest 26

Other popular problems with Jest

test('asynchronous test using async/await', async () => {
  // Perform asynchronous operations
  await doAsyncOperation();

  // Make assertions after the asynchronous operations complete
  expect(someValue).toBe(expectedValue);
});

Problem 2: Mocking External Dependencies

Description: When writing tests, it’s often necessary to mock external dependencies, such as API calls, databases, or third-party libraries. Jest provides powerful mocking capabilities, but configuring mocks for complex dependencies or handling specific scenarios can be challenging.

Solution: Jest offers different mocking techniques, such as manual mocking using the jest.mock function, automatic mocking using the __mocks__ directory, and module mocking with jest.fn or jest.spyOn. These techniques allow developers to replace actual dependencies with mock implementations or stubs, ensuring controlled test environments.

Example:

// Mocking an API call
jest.mock('axios');
const axios = require('axios');
axios.get.mockResolvedValue({ data: { id: 1, name: 'Mocked Data' } });

test('testing API call', async () => {
  const response = await fetchDataFromAPI();
  expect(response).toEqual({ id: 1, name: 'Mocked Data' });
});

Problem 3: Code Coverage and Test Reporting

Description: Ensuring adequate code coverage and generating comprehensive test reports are crucial aspects of testing. However, configuring Jest to collect code coverage information and generate detailed reports can be challenging, especially in complex projects.

Solution: Jest provides built-in support for code coverage and offers various configuration options to customize the coverage reports. By setting the collectCoverage option to true in the Jest configuration, Jest will collect coverage information during test execution. Additionally, Jest supports generating coverage reports in different formats, such as HTML, text, or JSON, using plugins like jest-html-reporter or jest-junit. These reports provide insights into which parts of the code are covered by tests and can help identify areas that need additional test coverage.

Example:

// jest.config.js
module.exports = {
  // Enable code coverage collection
  collectCoverage: true,

  // Configure coverage report
  coverageReporters: ['html', 'text'],

  // Configure additional plugins or reporters
  reporters: ['default', 'jest-html-reporter'],
};

Jest is a popular JavaScript testing framework developed by Facebook. It provides a comprehensive and intuitive testing solution for JavaScript applications, enabling developers to write unit tests, integration tests, and snapshot tests with ease. Jest is designed to be fast, reliable, and easy to set up, making it a preferred choice for many developers.

One of the key features of Jest is its support for automatic mocking. Jest allows developers to easily mock dependencies and external modules, enabling isolated testing of individual components or functions. With Jest’s mocking capabilities, developers can simulate behavior and control the inputs and outputs of mocked functions, ensuring predictable and controlled test environments.

Another notable feature of Jest is its snapshot testing functionality. Snapshot testing allows developers to capture the current state of a component or data structure and compare it against a previously stored snapshot. This enables quick and efficient regression testing, ensuring that changes in the codebase do not unintentionally modify the expected output. Jest’s snapshot testing feature simplifies the process of detecting unexpected changes in the application’s UI or data structures, helping to identify potential bugs or regressions early in the development cycle.

Overall, Jest provides a comprehensive and powerful testing framework for JavaScript applications. With its intuitive API, robust mocking capabilities, and snapshot testing functionality, Jest simplifies the process of writing and maintaining tests, promoting code quality and reliability in JavaScript projects.

Most popular use cases for Jest

function add(a, b) {
  return a + b;
}

test('add function correctly adds two numbers', () => {
  expect(add(2, 3)).toBe(5);
  expect(add(-1, 7)).toBe(6);
});

2. Integration Testing: Jest is also suitable for integration testing, where multiple components or modules are tested together to ensure their seamless interaction. With Jest’s test runner and powerful mocking capabilities, developers can simulate the behavior of dependencies and external APIs, enabling thorough testing of the integration points. This allows for the detection of potential issues that may arise from the interaction between different parts of the application.
3. Snapshot Testing: Jest provides snapshot testing functionality, which enables developers to capture the rendered output of components, data structures, or JSON objects and compare them against stored snapshots. This is useful for detecting unintended changes in the rendered UI or data over time. Here’s an example of snapshot testing with Jest:

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

In this test, Jest captures the rendered output of the Button component and compares it with the stored snapshot. If any changes are detected, Jest will notify the developer, helping to prevent unexpected regressions.

Overall, Jest is a versatile testing framework that can be used for unit testing, integration testing, and snapshot testing in JavaScript applications. Its extensive feature set, ease of use, and robust testing capabilities make it a popular choice among developers.