Mocking Node.js Globals in Jest

When testing Node.js code, you often need to interact with Node.js built-in modules and global variables. Jest, by default, runs tests in a browser-like environment. This challenge requires you to configure Jest to simulate a Node.js environment, allowing you to test code that relies on these Node.js specific features.

Problem Description

Your task is to write a Jest test suite that correctly simulates a Node.js environment. This means that within your tests, you should be able to access and utilize Node.js global objects and modules as if you were running in a real Node.js process.

What needs to be achieved:

• Configure Jest to run tests in a Node.js environment.
• Write a test that successfully utilizes a Node.js global object.
• Write a test that successfully imports and uses a Node.js built-in module.

Key requirements:

• The solution must be written in TypeScript.
• You must demonstrate the ability to use at least one Node.js global (e.g., process, __dirname) and at least one Node.js built-in module (e.g., fs, path).

Expected behavior: Your tests should pass when run with Jest, indicating that the Node.js environment is correctly mocked or configured.

Important edge cases to consider:

• How different Jest configurations might affect the Node.js environment.

Examples

Example 1: Imagine you have a simple utility function that reads a file path.

// src/utils.ts
import path from 'path';

export function getAbsoluteFilePath(relativePath: string): string {
  return path.resolve(__dirname, relativePath);
}

Input for test: relativePath = '../data/config.json'

Expected test output: A Jest test that calls getAbsoluteFilePath('../data/config.json') and asserts that the returned path is an absolute path, likely containing the simulated __dirname. The exact output will depend on the Jest configuration and how __dirname is mocked, but it should be a valid absolute path.

// __tests__/utils.test.ts
import { getAbsoluteFilePath } from '../src/utils';

describe('Utils', () => {
  it('should resolve an absolute file path using __dirname', () => {
    // Jest will simulate __dirname in a Node.js environment
    const expectedPathPart = '/__dirname_simulation/'; // Example simulation
    const relativePath = '../data/config.json';
    const absolutePath = getAbsoluteFilePath(relativePath);

    expect(absolutePath).toContain(expectedPathPart);
    expect(absolutePath).toContain(relativePath);
    expect(absolutePath).toBeAbsolute(); // Custom matcher or check if it starts with '/'
  });
});

Example 2: Consider a scenario where you need to access environment variables.

// src/configReader.ts
export function getEnvVariable(key: string): string | undefined {
  return process.env[key];
}

Input for test: key = 'NODE_ENV'

Expected test output: A Jest test that sets a mock environment variable, calls getEnvVariable('NODE_ENV'), and asserts that it returns the mocked value.

// __tests__/configReader.test.ts
import { getEnvVariable } from '../src/configReader';

describe('ConfigReader', () => {
  it('should retrieve environment variables', () => {
    const mockEnvValue = 'test';
    process.env.NODE_ENV = mockEnvValue; // Mocking a global Node.js variable

    expect(getEnvVariable('NODE_ENV')).toBe(mockEnvValue);

    // Clean up the environment variable after the test
    delete process.env.NODE_ENV;
  });

  it('should return undefined for non-existent environment variables', () => {
    expect(getEnvVariable('NON_EXISTENT_VAR')).toBeUndefined();
  });
});

Constraints

• Your Jest configuration (jest.config.js or equivalent) must explicitly set the test environment to 'node'.
• You must use TypeScript for your tests and any helper files.
• The solution should not rely on external libraries specifically designed for Node.js environment mocking in Jest, beyond Jest's built-in capabilities and standard Node.js modules.

Notes

• To ensure your tests run in a Node.js environment, you'll need to configure Jest. Look for the testEnvironment option in your Jest configuration.
• You can import Node.js built-in modules directly in your TypeScript files as you would in a regular Node.js project (e.g., import fs from 'fs';).
• Global variables like process and __dirname are typically available when running in the 'node' environment.
• Consider how to effectively mock or manage global variables if your tests require specific states for them. For instance, process.env can be modified directly, but it's good practice to clean up after your tests.