Jest Source Map Generation

Jest is a popular JavaScript testing framework known for its speed and simplicity. When debugging tests, especially in projects using transpiled languages like TypeScript, source maps are crucial for mapping executed JavaScript code back to the original source code. This challenge will test your understanding of how to configure Jest to generate and utilize source maps effectively.

Problem Description

Your task is to configure a Jest test environment for a TypeScript project such that Jest generates accurate source maps. These source maps should allow for effective debugging, enabling you to set breakpoints and inspect variables in your original TypeScript files when running tests.

Key Requirements:

1. TypeScript Compilation: Your project uses TypeScript, and you need to ensure it's transpiled correctly for Jest to execute.
2. Source Map Generation: Jest must be configured to generate source maps for the transpiled JavaScript code.
3. Source Map Utilization: When running tests, Jest should correctly interpret and use the generated source maps to provide accurate debugging information.
4. Verification: You need to demonstrate that source maps are working by successfully debugging a simple test case.

Expected Behavior:

When a debugger is attached (e.g., via VS Code's debugger for Jest), you should be able to set breakpoints in your .ts files and have the debugger pause execution at those points. The debugger should also correctly display variable values from your TypeScript code.

Edge Cases:

• Ensure source maps are generated even for files that are not directly imported but are used by your tests (e.g., utility files).
• Consider scenarios where there might be multiple levels of transpilation or build steps (though for this challenge, we'll keep it focused on Jest and TypeScript).

Examples

For this challenge, the "input" is the project setup and the "output" is the successful debugging experience. We'll illustrate with a hypothetical project structure.

Example 1: Basic Setup and Debugging

• Project Structure:

  my-ts-project/
  ├── src/
  │   └── calculator.ts
  ├── __tests__/
  │   └── calculator.test.ts
  ├── tsconfig.json
  └── jest.config.js
  

• src/calculator.ts:

  export function add(a: number, b: number): number {
      const result = a + b;
      return result; // Breakpoint here
  }
  

• __tests__/calculator.test.ts:

  import { add } from '../src/calculator';
  
  describe('Calculator', () => {
      test('should add two numbers correctly', () => {
          const sum = add(5, 3);
          expect(sum).toBe(8);
      });
  });
  

• tsconfig.json:

  {
    "compilerOptions": {
      "target": "es2016",
      "module": "commonjs",
      "strict": true,
      "esModuleInterop": true,
      "skipLibCheck": true,
      "forceConsistentCasingInFileNames": true,
      "outDir": "./dist", // Or wherever you want transpiled JS
      "sourceMap": true // Crucial for source map generation
    },
    "include": ["src/**/*.ts", "__tests__/**/*.ts"]
  }
  

• jest.config.js:

  /** @type {import('ts-jest').JestConfigWithTsJest} */
  module.exports = {
    preset: 'ts-jest',
    testEnvironment: 'node',
    transform: {
      '^.+\\.(ts|tsx)?$': 'ts-jest',
    },
    // Ensure Jest knows where to find source maps and respects them
    // For ts-jest, this is often handled by default when sourceMap is true in tsconfig.json
    // If not, you might explicitly configure:
    // globals: {
    //   'ts-jest': {
    //     useESM: true, // If using ES Modules
    //     // tsconfig: 'tsconfig.json' // Specify if not in root
    //   }
    // }
    // ensure cache is enabled and source maps are respected
    cache: true,
    // Optionally, you might need to tell Jest where to find source maps if they aren't inline or in the same directory
    // mapCoverage: true, // Useful for coverage reports that also respect source maps
  };
  

• Expected Outcome (Debugging):

  1. Run Jest with debugging enabled (e.g., node --inspect-brk node_modules/.bin/jest --runInBand).
  2. Attach a debugger (e.g., VS Code).
  3. Set a breakpoint on the return result; line in src/calculator.ts.
  4. The debugger should pause at this breakpoint when the test is executed, and you should be able to inspect the result variable.

Constraints

• The project will use TypeScript and Jest.
• You will be provided with tsconfig.json and jest.config.js files, which you may need to modify or create.
• The solution must work with the latest stable versions of Jest and ts-jest.
• Focus on Jest's configuration and its interaction with TypeScript source maps; avoid complex application logic.

Notes

• The sourceMap: true option in tsconfig.json is the primary driver for generating source maps.
• The ts-jest transformer typically handles the integration of source maps with Jest automatically.
• Debugging in Jest often involves launching Jest with Node.js's --inspect-brk flag and then attaching a debugger.
• Familiarize yourself with the jest.config.js options related to transformers and testing environments.
• Consider how source maps are generated (inline vs. separate .map files) and how Jest might consume them.