Jest Workspace Testing: Mocking and Isolation

This challenge focuses on testing code that interacts with a shared "workspace" environment. You'll learn how to isolate tests by mocking this workspace to ensure predictable and reliable test outcomes. This is crucial for building robust applications where different parts rely on a common, potentially external, context.

Problem Description

Your task is to write Jest tests for a WorkspaceService class that manages operations within a simulated workspace. The WorkspaceService depends on a WorkspaceAPI interface, which represents the actual interaction with the workspace (e.g., fetching files, saving data). For testing purposes, we want to mock the WorkspaceAPI to control its behavior and ensure our tests for WorkspaceService are isolated and don't depend on a real or complex workspace setup.

What needs to be achieved:

• Create Jest tests for the WorkspaceService class.
• Mock the WorkspaceAPI to simulate its responses during testing.
• Ensure each test for WorkspaceService has a clean and isolated workspace environment.

Key requirements:

• Implement tests for at least two methods of the WorkspaceService (e.g., loadConfiguration, saveDocument).
• Use Jest's mocking capabilities (e.g., jest.fn(), jest.spyOn(), or manual mock setup) to create a mock WorkspaceAPI.
• Assert that the WorkspaceService correctly calls the mocked WorkspaceAPI methods with the expected arguments.
• Assert that the WorkspaceService behaves correctly based on the mocked WorkspaceAPI's return values.

Expected behavior: Your tests should pass if:

• The WorkspaceService correctly interacts with the mocked WorkspaceAPI.
• The WorkspaceService produces the expected output or side effects based on controlled WorkspaceAPI behavior.

Edge cases to consider:

• What happens when the WorkspaceAPI returns an error or null/undefined values?
• How do you handle situations where a method might be called multiple times with different arguments?

Examples

Let's assume the following interfaces:

interface WorkspaceAPI {
  readFile(filePath: string): Promise<string | null>;
  writeFile(filePath: string, content: string): Promise<void>;
  getConfiguration(): Promise<Record<string, any> | null>;
  saveConfiguration(config: Record<string, any>): Promise<void>;
}

class WorkspaceService {
  constructor(private workspaceApi: WorkspaceAPI) {}

  async loadConfiguration(): Promise<Record<string, any> | null> {
    return this.workspaceApi.getConfiguration();
  }

  async saveDocument(filePath: string, content: string): Promise<boolean> {
    try {
      await this.workspaceApi.writeFile(filePath, content);
      return true;
    } catch (error) {
      console.error(`Failed to save document: ${error}`);
      return false;
    }
  }

  async readFileContent(filePath: string): Promise<string | null> {
    return this.workspaceApi.readFile(filePath);
  }
}

Example 1: Testing loadConfiguration (successful case)

Input:
- WorkspaceAPI.getConfiguration() is mocked to return Promise.resolve({ theme: 'dark', fontSize: 14 })

Output:
- WorkspaceService.loadConfiguration() returns { theme: 'dark', fontSize: 14 }

Explanation:
The test will mock `WorkspaceAPI.getConfiguration` to return a specific configuration object. The test will then call `WorkspaceService.loadConfiguration` and assert that it returns the exact same configuration object.

Example 2: Testing saveDocument (successful case)

Input:
- WorkspaceAPI.writeFile is mocked to resolve immediately (Promise.resolve())
- filePath = 'my-notes.txt'
- content = 'This is my important note.'

Output:
- WorkspaceService.saveDocument('my-notes.txt', 'This is my important note.') returns true
- WorkspaceAPI.writeFile is called with arguments: ['my-notes.txt', 'This is my important note.']

Explanation:
The test will mock `WorkspaceAPI.writeFile` to do nothing (just resolve). It will then call `WorkspaceService.saveDocument` with a file path and content. The test will assert that the method returns `true` and that `WorkspaceAPI.writeFile` was called with the correct arguments.

Example 3: Testing saveDocument (error case)

Input:
- WorkspaceAPI.writeFile is mocked to reject with an Error('Disk full')
- filePath = 'important-data.json'
- content = '{"data": "sensitive"}'

Output:
- WorkspaceService.saveDocument('important-data.json', '{"data": "sensitive"}') returns false
- WorkspaceAPI.writeFile is called with arguments: ['important-data.json', '{"data": "sensitive"}']
- console.error is called with a message containing "Failed to save document: Error: Disk full"

Explanation:
The test will mock `WorkspaceAPI.writeFile` to reject with an error. It will call `WorkspaceService.saveDocument` and assert that it returns `false` due to the caught error. It should also verify that the underlying `writeFile` was attempted and that the error was logged to `console.error`.

Constraints

• You must use TypeScript.
• You must use Jest for testing.
• Mocking of WorkspaceAPI should be done without relying on any external libraries beyond Jest.
• Tests should be organized into logical describe blocks and individual it or test cases.
• Ensure proper use of async/await for asynchronous operations.

Notes

Consider how to set up your mocks. You might want to create a mock implementation of WorkspaceAPI that uses jest.fn() for each of its methods. Alternatively, you could use jest.mock('./workspace-api') if WorkspaceAPI was in its own module. For this challenge, focus on creating the mock within your test file or describe block.

Think about how to reset mocks between tests to ensure isolation. beforeEach or afterEach hooks in Jest are useful for this.

Success looks like a suite of Jest tests that reliably verify the WorkspaceService's functionality, demonstrating a strong understanding of mocking and test isolation in a TypeScript environment.