Mocking External Dependencies with Jest Spies

In modern software development, it's crucial to write robust and maintainable code. A key practice for achieving this is thorough testing. When your code relies on external services, modules, or complex objects, directly testing these dependencies can be difficult, slow, or even impossible. This is where test doubles come in. This challenge will focus on using Jest spies to create controlled replacements for these external dependencies, allowing you to isolate your code under test and ensure predictable test outcomes.

Problem Description

You are tasked with testing a UserService class that has a dependency on an external DatabaseService. The DatabaseService is responsible for interacting with a real database and can be slow or have side effects that we want to avoid during unit testing. Your goal is to create test doubles for the DatabaseService methods using Jest spies.

Specifically, you need to:

1. Isolate UserService: Write unit tests for the UserService class without actually calling the real DatabaseService implementation.
2. Mock Database Operations: Use Jest spies to simulate the behavior of the DatabaseService's getUserById and saveUser methods.
3. Assert Interactions: Verify that UserService correctly calls the DatabaseService methods with the expected arguments.
4. Control Return Values: Configure the spies to return specific, predictable values when they are called, allowing you to test different scenarios within UserService.

The UserService class has the following methods:

• getUserById(userId: string): Promise<User | null>: Fetches a user from the database.
• saveUser(user: User): Promise<void>: Saves a user to the database.
• updateUserEmail(userId: string, newEmail: string): Promise<boolean>: Updates a user's email. This method internally uses getUserById and then saveUser.

The User interface is defined as:

interface User {
  id: string;
  name: string;
  email: string;
}

Examples

Example 1: Mocking getUserById and verifying its call

Input:

• UserService is initialized with a mocked DatabaseService where getUserById is spied upon.
• The getUserById spy is configured to return a specific User object when called with userId = '123'.
• userService.getUserById('123') is called.

Output:

• The userService.getUserById('123') should return the User object that the spy was configured to return.
• The DatabaseService.getUserById spy should have been called exactly once with the argument '123'.

Explanation: This tests that UserService can successfully retrieve user data from its dependency and that the correct method on the dependency is invoked with the right parameters.

Example 2: Mocking saveUser and verifying its call

Input:

• UserService is initialized with a mocked DatabaseService where saveUser is spied upon.
• A User object is prepared.
• userService.saveUser(user) is called.

Output:

• The DatabaseService.saveUser spy should have been called exactly once with the provided User object.

Explanation: This verifies that UserService correctly persists user data by calling the appropriate method on its dependency.

Example 3: Mocking multiple database calls within a single UserService method

Input:

• UserService is initialized with a mocked DatabaseService.
• DatabaseService.getUserById is spied upon and configured to return a User object when called with '456'.
• DatabaseService.saveUser is spied upon and configured to resolve immediately.
• userService.updateUserEmail('456', 'new.email@example.com') is called.

Output:

• The DatabaseService.getUserById spy should have been called once with '456'.
• The DatabaseService.saveUser spy should have been called once with a User object whose id is '456' and whose email is 'new.email@example.com'.
• The userService.updateUserEmail method should return true.

Explanation: This complex scenario tests the orchestration of multiple dependency calls within UserService and ensures data integrity across these operations.

Constraints

• The DatabaseService implementation is not provided and should be treated as an external, untestable unit in this challenge. You will only interact with its interface.
• All tests must be written using Jest.
• You must use Jest's spying capabilities (jest.spyOn) to mock the DatabaseService methods.
• Avoid using jest.mock() for the entire module, as the focus is on selective spying.
• Tests should be asynchronous where necessary, given that database operations are typically asynchronous.

Notes

• Consider how you will set up your UserService with the mocked DatabaseService in your test suite. Dependency Injection is a common pattern here.
• Remember to reset your spies between tests to ensure test isolation.
• Think about how to handle cases where DatabaseService.getUserById might return null or undefined.
• For saveUser and updateUserEmail, consider the success and potential failure scenarios you might want to simulate.