Jest setupFilesAfterEnv: Global Setup for Your Tests

In testing, it's often necessary to perform some setup after the environment has been initialized by Jest. This could involve setting global variables, mocking modules that are used across multiple test files, or performing other one-time configurations. Jest provides the setupFilesAfterEnv option in its configuration to handle such scenarios efficiently. This challenge will test your understanding of how to use this Jest feature effectively.

Problem Description

Your task is to implement a solution that leverages Jest's setupFilesAfterEnv configuration. You need to create a setup file that will be executed after Jest's environment has been set up, but before any test files are run. This setup file should perform a specific action, such as defining a global variable or adding a helper function to the global scope, which will then be accessible within your test files.

What needs to be achieved:

• Create a Jest configuration that uses setupFilesAfterEnv.
• Implement a TypeScript file that serves as the setupFilesAfterEnv script.
• Ensure that the actions performed in the setup file are available in subsequent test files.

Key requirements:

• The setup file must be written in TypeScript.
• The setup file should modify the global scope in a discernible way (e.g., define a global variable).
• You must demonstrate that this global modification is accessible within a Jest test file.

Expected behavior: When Jest is run with the specified configuration, tests should be able to access the global modifications made by the setupFilesAfterEnv script.

Edge cases to consider:

• What happens if the setup file has an error? (Jest should report it).
• How does setupFilesAfterEnv differ from setupFiles? (Though you don't need to implement both, understanding this is key).

Examples

Example 1: Setting a Global Variable

Imagine you have a common configuration value that many of your tests need. Instead of importing it repeatedly, you want to set it globally.

Input (Conceptual):

• jest.config.ts:

  import type { JestConfigWithTsJest } from 'ts-jest';
  
  const config: JestConfigWithTsJest = {
    // ... other Jest configurations
    globalSetup: undefined, // Not used for this example
    setupFilesAfterEnv: ['./jest.setup.ts'],
    testEnvironment: 'node',
    preset: 'ts-jest',
  };
  
  export default config;
  

• jest.setup.ts:

  // This file is executed after Jest's environment is set up.
  global.MY_GLOBAL_CONFIG = { apiKey: 'test-api-key-123' };
  console.log('Global config set!');
  

• my.test.ts:

  describe('Global variable accessibility', () => {
    it('should have access to MY_GLOBAL_CONFIG', () => {
      expect(global.MY_GLOBAL_CONFIG).toBeDefined();
      expect(global.MY_GLOBAL_CONFIG.apiKey).toBe('test-api-key-123');
    });
  });
  

Output (Conceptual during Jest run):

PASS  ./my.test.ts
  Global variable accessibility
    ✓ should have access to MY_GLOBAL_CONFIG (Xms)

Console output during run:
Global config set!

Explanation: The jest.setup.ts file is executed by Jest due to the setupFilesAfterEnv configuration. It defines a property MY_GLOBAL_CONFIG on the global object. The my.test.ts file then successfully accesses this global variable, verifying the setup.

Example 2: Adding a Global Helper Function

You might want to add a utility function that can be used across all tests for common assertions or data manipulation.

Input (Conceptual):

• jest.config.ts:

  import type { JestConfigWithTsJest } from 'ts-jest';
  
  const config: JestConfigWithTsJest = {
    // ... other Jest configurations
    setupFilesAfterEnv: ['./jest.setup.ts'],
    testEnvironment: 'node',
    preset: 'ts-jest',
  };
  
  export default config;
  

• jest.setup.ts:

  // This file is executed after Jest's environment is set up.
  global.expectWithCustomMessage = (received: any, expected: any, message: string) => {
    expect(received).toBe(expected);
    console.log(`Custom message: ${message}`);
  };
  console.log('Global helper function added!');
  

• my.helper.test.ts:

  describe('Global helper function', () => {
    it('should be able to use the custom expect function', () => {
      const name = 'Hone';
      // @ts-ignore // Ignoring because the global function is not typed in this context by default
      expectWithCustomMessage(name, 'Hone', 'Name should be Hone');
    });
  });
  

Output (Conceptual during Jest run):

PASS  ./my.helper.test.ts
  Global helper function
    ✓ should be able to use the custom expect function (Xms)

Console output during run:
Global helper function added!
Custom message: Name should be Hone

Explanation: The jest.setup.ts file adds a function expectWithCustomMessage to the global scope. The test file my.helper.test.ts calls this function, demonstrating its availability and correct execution, including the logged custom message.

Constraints

• The jest.config.ts file must be used to configure Jest.
• The setupFilesAfterEnv array in the Jest configuration must contain at least one relative path to a TypeScript file.
• The setup file must be a valid TypeScript file and compile without errors.
• The test environment should be set to 'node' for simplicity, though other environments may also work.
• The solution must use ts-jest as the preset.

Notes

• Remember that setupFilesAfterEnv runs after Jest has set up the test environment. This is crucial for scenarios where you need to interact with the environment itself (e.g., modifying prototypes).
• In contrast, setupFiles runs before the test environment is set up. This is typically used for polyfills or other early-stage configurations.
• When adding global variables or functions, be mindful of potential naming conflicts.
• For better type safety, consider defining global types in a *.d.ts file that Jest can pick up, especially if you are using the ts-jest preset. However, for this challenge, using @ts-ignore or simply relying on JavaScript's dynamic typing for the global object is acceptable to demonstrate the core concept.
• The primary goal is to demonstrate the mechanism of setupFilesAfterEnv and its effect on test files.