Distributing Test Execution in Jest

Imagine you have a large suite of tests that, when run together, take a significant amount of time to complete. To improve developer productivity and faster feedback loops, you want to distribute the execution of these tests across multiple parallel processes. Jest provides built-in mechanisms to achieve this.

Problem Description

Your task is to configure Jest to distribute the execution of your test files across available worker processes. You will need to create a jest.config.ts file and set the appropriate configuration option to enable test distribution. This involves understanding how Jest manages worker processes and how to control the number of workers used.

Key Requirements:

• Create a jest.config.ts file in the root of your project.
• Configure Jest to use a specific number of worker processes for test execution.
• Ensure that Jest successfully launches and utilizes these worker processes.

Expected Behavior:

When you run jest in your terminal, Jest should launch the specified number of worker processes and distribute the test files among them for parallel execution. You should observe that tests are running concurrently, rather than sequentially.

Edge Cases to Consider:

• What happens if you specify more workers than available CPU cores?
• What if your test files have dependencies that might be affected by parallel execution (though for this specific challenge, we'll assume independence)?

Examples

For this challenge, we won't have specific input/output in terms of data. Instead, the "output" is the observed behavior of Jest during test execution.

Example 1: Basic Configuration

Scenario: You want to run tests using 4 worker processes.

Configuration in jest.config.ts:

import type { JestConfigWithTsESM } from 'ts-jest';

const config: JestConfigWithTsESM = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  // Other Jest configurations...
  maxWorkers: 4, // This is the key setting
};

export default config;

Observed Behavior: When you run jest, you will see output indicating that tests are being run in parallel, potentially with messages like "Tests completed in X.Y seconds" and the task runner showing activity across multiple worker threads. The number of concurrent tests running should be close to your maxWorkers setting.

Example 2: Dynamic Worker Count (Less Common for this Challenge)

Scenario: You want Jest to automatically determine the number of workers based on available CPU cores.

Configuration in jest.config.ts:

import type { JestConfigWithTsESM } from 'ts-jest';

const config: JestConfigWithTsESM = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  // Other Jest configurations...
  maxWorkers: '80%', // Or a number like 2, 3, etc.
};

export default config;

Observed Behavior: Jest will use a percentage of your available CPU cores (or a fixed number if you provide one) to determine the worker count. This provides a good balance for resource utilization.

Example 3: Minimal Configuration

Scenario: You want to ensure that test distribution is enabled, even if you don't explicitly set a number.

Configuration in jest.config.ts:

import type { JestConfigWithTsESM } from 'ts-jest';

const config: JestConfigWithTsESM = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  // Other Jest configurations...
  // If maxWorkers is not explicitly set, Jest defaults to a reasonable number based on CPU cores.
  // For this challenge, we want to explicitly set it to demonstrate control.
};

export default config;

Observed Behavior: While this configuration will result in parallel execution by default, the goal of the challenge is to explicitly control the number of workers. For this example, we'll assume the user needs to demonstrate setting a specific number.

Constraints

• You must use TypeScript for your Jest configuration file (jest.config.ts).
• The solution must involve configuring Jest's maxWorkers option.
• Your project should have at least a few dummy test files (e.g., example.test.ts) so that Jest has something to distribute.
• Performance expectations are related to observing parallel execution, not necessarily achieving a specific speedup (though that's the ultimate benefit).

Notes

• Jest uses a worker pool to run tests in parallel. The maxWorkers option directly controls the size of this pool.
• Setting maxWorkers to true (or not setting it at all) tells Jest to determine the number of workers based on the number of CPU cores.
• Setting maxWorkers to a string like '50%' or '80%' allows Jest to use a percentage of available CPU cores.
• You can use ts-jest as a preset for your TypeScript tests, which simplifies the configuration.
• To test this, create a few simple .test.ts files that contain basic describe and it blocks, perhaps with a small delay (e.g., using setTimeout within a test) to make the parallel execution more apparent.
• The primary goal is to demonstrate the correct configuration of maxWorkers in jest.config.ts.