Mocking GraphQL API Responses in Jest for Unit Testing

When building applications that consume GraphQL APIs, it's crucial to have robust unit tests. These tests should verify your application's logic without relying on a live API connection, which can be slow, unreliable, and expensive. This challenge focuses on creating mock GraphQL responses within a Jest testing environment using TypeScript.

Problem Description

Your task is to implement a system for mocking GraphQL API responses within your Jest test suite. You'll need to define mock data and configure a mechanism to intercept GraphQL requests made by your application and return these predefined mock responses. This will allow you to test your frontend or backend components that interact with the GraphQL API in isolation.

Key Requirements:

1. Mock Data Definition: You should be able to define various mock responses for different GraphQL queries and mutations.
2. Request Interception: Implement a way to intercept outgoing GraphQL requests.
3. Response Matching: Match intercepted requests to the appropriate mock response based on the query/mutation being sent.
4. Error Mocking: Ability to mock GraphQL errors for testing error handling logic.
5. TypeScript Integration: The solution must be written in TypeScript and integrate seamlessly with Jest.

Expected Behavior:

When a GraphQL request is made by the application under test, and a corresponding mock is configured, the mock response (or error) should be returned instead of making an actual network request.

Edge Cases to Consider:

• Handling different query/mutation names.
• Handling queries with different variables.
• Mocking responses for a specific operation (e.g., a query named GetUser).
• Mocking both successful responses and GraphQL errors.

Examples

Example 1: Mocking a Simple Query

Scenario: Your application makes a GraphQL query to fetch a list of users.

Mock Configuration:

// Mock data for a query named 'GetUsers'
const mockGetUsersResponse = {
  data: {
    users: [
      { id: '1', name: 'Alice', email: 'alice@example.com' },
      { id: '2', name: 'Bob', email: 'bob@example.com' },
    ],
  },
};

Test Usage (Conceptual):

In your Jest test, you would configure your mocking setup to intercept a request containing the GetUsers query and return mockGetUsersResponse.

Expected Outcome:

Your application code that calls GetUsers will receive the mocked user data.

Example 2: Mocking a Query with Variables

Scenario: Your application makes a query to fetch a specific user by ID, passing an id variable.

Mock Configuration:

// Mock data for a query named 'GetUser' with variable '$userId'
const mockGetUserResponse = (userId: string) => ({
  data: {
    user: {
      id: userId,
      name: 'Charlie',
      email: 'charlie@example.com',
    },
  },
});

Test Usage (Conceptual):

Your mocking setup should be able to inspect the variables of an incoming GetUser query. If variables.userId is '3', return mockGetUserResponse('3').

Expected Outcome:

Your application code fetching user with ID '3' will receive the mocked user data for Charlie.

Example 3: Mocking a GraphQL Error

Scenario: Your application makes a query, and you want to test its error handling for a "Not Found" scenario.

Mock Configuration:

// Mock GraphQL error for a query named 'FindItem'
const mockFindItemError = {
  errors: [
    {
      message: 'Item not found',
      locations: [{ line: 2, column: 3 }],
      path: ['findItem'],
      extensions: {
        code: 'NOT_FOUND',
      },
    },
  ],
  data: {
    findItem: null,
  },
};

Test Usage (Conceptual):

Configure your mock to intercept the FindItem query and return mockFindItemError.

Expected Outcome:

Your application code handling the FindItem query should receive the error object and trigger its error handling logic.

Constraints

• The solution should be implemented using TypeScript.
• Jest must be the testing framework.
• The mocking solution should aim to be efficient for large test suites.
• Avoid making actual HTTP requests to a real GraphQL endpoint during tests.

Notes

Consider using a library specifically designed for mocking GraphQL requests in JavaScript/TypeScript environments. Libraries like jest-mock-extended or a dedicated GraphQL mocking library could be helpful. Think about how you will store and retrieve your mock definitions. A common approach is to have a separate file for mock data and a setup file in Jest to configure the mocks before tests run. For matching requests, consider inspecting the request body for the query string and variables object.