Jest Cross-Package Testing: Building a Monorepo Testing Strategy

In modern JavaScript development, monorepos are increasingly popular for managing multiple related packages within a single repository. Testing the interactions and dependencies between these packages is crucial for ensuring the stability and correctness of the entire system. This challenge focuses on setting up and executing effective cross-package tests using Jest.

Problem Description

Your task is to configure Jest to run tests that span across different packages within a monorepo. This involves understanding how to import and test code from one package within the test files of another, and ensuring Jest can correctly resolve these dependencies. You will be provided with a simple monorepo structure, and your goal is to write tests that demonstrate the successful integration between these packages.

What needs to be achieved:

• Set up Jest in a way that it can discover and run tests located in different packages.
• Write tests in one package that import and utilize functionality from another package.
• Ensure these cross-package imports resolve correctly and tests pass.

Key requirements:

• Use Jest as the testing framework.
• Assume a monorepo structure (e.g., using Yarn Workspaces, Lerna, or pnpm workspaces).
• The solution should be written in TypeScript.
• Tests should verify that functionality exported from one package can be successfully used in another.

Expected behavior: When Jest is run, it should discover all test files across the monorepo and execute them. Tests that import code from other local packages within the monorepo should pass.

Edge cases to consider:

• Packages with different build configurations (though for this challenge, we'll keep them simple).
• Circular dependencies between packages (though we will aim to avoid this in the provided structure).

Examples

Let's imagine a simple monorepo with two packages: core-utils and user-service.

Package Structure:

/monorepo
  /packages
    /core-utils
      index.ts (exports a utility function)
      package.json
      tsconfig.json
      /src
        utils.ts
    /user-service
      index.ts (exports a service that uses core-utils)
      package.json
      tsconfig.json
      /src
        userService.ts
      /tests
        userService.test.ts
  package.json (monorepo root)
  jest.config.js

Example 1: Basic Cross-Package Import

packages/core-utils/src/utils.ts:

export function greet(name: string): string {
  return `Hello, ${name}!`;
}

packages/core-utils/index.ts:

export * from './src/utils';

packages/user-service/src/userService.ts:

import { greet } from 'core-utils'; // Importing from another package

export function getUserGreeting(userName: string): string {
  return greet(userName);
}

packages/user-service/index.ts:

export * from './src/userService';

packages/user-service/tests/userService.test.ts:

import { getUserGreeting } from 'user-service'; // Importing the service itself
import { greet } from 'core-utils'; // Directly importing from core-utils

describe('UserService', () => {
  it('should correctly greet a user using the core-utils function', () => {
    const userName = 'Alice';
    // This test uses the direct import from core-utils
    expect(greet(userName)).toBe('Hello, Alice!');
  });

  it('should return the correct greeting from getUserGreeting', () => {
    const userName = 'Bob';
    // This test uses the functionality exposed by user-service, which internally uses core-utils
    expect(getUserGreeting(userName)).toBe('Hello, Bob!');
  });
});

Explanation: The userService.test.ts file successfully imports functions from both user-service and core-utils. The tests verify that the greet function from core-utils works as expected and that getUserGreeting from user-service correctly leverages core-utils to produce the desired output. This demonstrates that Jest can resolve and use dependencies between local packages.

Constraints

• The monorepo structure will be pre-defined and simplified for this challenge.
• You will be provided with a basic package.json for the monorepo root, core-utils, and user-service.
• You will need to configure jest.config.js and potentially tsconfig.json files within the packages.
• The primary goal is demonstrating the test setup, not complex package logic.
• All code and tests must be in TypeScript.

Notes

• Consider how Jest's module resolution works, especially in monorepos. You might need to configure moduleNameMapper in your Jest configuration.
• Pay attention to tsconfig.json settings, particularly paths or baseUrl, if you are using them to map module names.
• Ensure that your package managers (Yarn Workspaces, pnpm, Lerna) are set up to link local packages correctly.
• The solution should focus on the Jest configuration and test writing to achieve cross-package testing.