Parallelize Jest Tests for Improved Performance

Modern applications demand efficient testing workflows. Slow test suites can hinder rapid development cycles and introduce delays in the CI/CD pipeline. This challenge focuses on optimizing Jest test execution by leveraging its built-in parallelization capabilities, allowing multiple test files to run concurrently.

Problem Description

Your task is to implement parallel testing for a given Jest test suite written in TypeScript. You will be provided with a set of test files that are currently configured to run sequentially. Your goal is to configure Jest to execute these tests in parallel, significantly reducing the overall execution time without compromising test accuracy or introducing race conditions.

Key Requirements:

• Modify the Jest configuration to enable parallel test execution.
• Ensure that the tests still pass when run in parallel.
• No changes should be made to the individual test logic within the provided .test.ts files.

Expected Behavior:

When the Jest tests are run with your parallel configuration, the total execution time should be noticeably less than running them sequentially. Jest's output should indicate that tests are running in multiple worker processes.

Edge Cases to Consider:

• Tests that rely on shared global state or singletons might exhibit unexpected behavior in parallel execution. While you are not expected to fix such issues within the tests themselves, be aware that they could cause failures. The provided test files are designed to be independent.
• The number of parallel workers should be configurable or adapt to the available system resources.

Examples

For this challenge, we'll consider a hypothetical project structure with a few test files.

Project Structure:

/src
  - dataProcessor.ts
/tests
  - dataProcessor.test.ts
  - anotherFeature.test.ts
  - apiService.test.ts
jest.config.js

jest.config.js (Initial - Sequential):

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  // No parallelization configuration
};

tests/dataProcessor.test.ts (Illustrative - Actual test logic will be provided separately):

// Assume this file contains tests for a data processing module.
// For example:
describe('Data Processor', () => {
  it('should correctly process simple data', () => {
    // ... test implementation
  });
  it('should handle empty input gracefully', () => {
    // ... test implementation
  });
});

tests/anotherFeature.test.ts (Illustrative):

// Assume this file contains tests for another feature.
describe('Another Feature', () => {
  it('should perform operation X correctly', () => {
    // ... test implementation
  });
});

tests/apiService.test.ts (Illustrative):

// Assume this file contains tests for an API service.
describe('API Service', () => {
  it('should fetch data successfully', async () => {
    // ... test implementation
  });
});

Scenario:

If running these three tests sequentially takes T seconds, after correctly configuring parallel execution, the time should be significantly less than T. For example, if you have 4 CPU cores and Jest uses 4 workers, the execution time could theoretically approach T / 4 (though actual performance will vary due to overhead).

Constraints

• The solution must be implemented using TypeScript.
• The provided test files are independent and do not have explicit dependencies or shared mutable state that would cause race conditions when run in parallel.
• The Jest configuration file (jest.config.js or equivalent) is the primary point of modification.
• No modifications to the existing test files' logic (*.test.ts) are allowed.

Notes

• Jest offers several options for controlling parallelization. Research the maxWorkers option in Jest's configuration.
• Consider how Jest determines the number of workers. It often defaults to the number of logical CPU cores.
• The primary goal is to demonstrate the configuration for parallel testing, not to optimize individual test performance (which is outside the scope of this challenge).
• To observe the effect of parallelization, you'll need a sufficient number of test files and/or tests within those files that take a noticeable amount of time to execute.