Creating mock objects to abstract layers
Copilot Chat can help with creating mock objects that you can use for unit tests.
In this article
When creating unit tests, it's important to ensure they're isolated and not depending on external services. One way to achieve this is to create mock objects that abstract layers of your application. Copilot Chat can help you generate the code you need to create these mock objects.
Example scenario
Imagine a website built with TypeScript which displays a list of runners. You have a service which fetches the runners from a database, and the server-side code which uses this service. You want to test the server-side code, but you don't want to make a call to the database. You can ask Copilot Chat to generate a mock object for the service.
// SvelteKit example at +page.server.tsimport servicefrom'./service';exportasyncfunctionload({ params }) {const runner =await service.getRunner(params.id);return { runner };}Example prompt
This example assumes you are usingvitest for your tests, but will work with other test runners.
/tests Create a unit test to ensure the service is called correctly. Mock the service object.
/tests Create a unit test to ensure the service is called correctly. Mock the service object.If you have an existing test you wish Copilot to use as a model, you can use#file: path/to/test-file.js to provide the path to the file, and include this information in the prompt.
Example response
Note
The following response is an example. Copilot Chat responses are non-deterministic, so you may get a different response from the one shown here.
Copilot provides the plan it will implement and the code text which includes the mocks.
import { describe, it, expect, vi, beforeEach }from'vitest';import { load }from'./+page.server.ts';// Mock the entire service modulevi.mock('./service',() => ({default: {getRunner: vi.fn() }}));// Import the mocked serviceimport servicefrom'./service';describe('load function',() => {const mockRunner = {id:'123',name:'Test Runner',status:'active' };beforeEach(() => {// Reset mock before each test vi.resetAllMocks();// Setup default mock implementation (service.getRunnerasany).mockResolvedValue(mockRunner); });it('should fetch runner with correct id',async () => {const params = {id:'123' };const result =awaitload({ params });expect(service.getRunner).toHaveBeenCalledWith('123');expect(service.getRunner).toHaveBeenCalledTimes(1);expect(result).toEqual({runner: mockRunner }); });});