When testing my StencilJs application with Jest, I encountered an issue with mocking a service class method used in a component. The service class has only one function that prints text:
The Component class:
import {sayHello} from './helloworld-service';
@Component({
tag: 'comp-home',
styleUrl: 'comp-home.scss',
shadow: false,
scoped: true
})
export class MyComponent {
public callHelloWorldService () {
return sayHello();
} }
The Service class code is as follows:
export function sayHello () {
return "Hello!";
}
I needed to mock this function in my test method for proper testing, with the following setup:
jest.mock('../helloworld/helloworld-service', () => ({
sayHello: jest.fn()
}));
import { sayHello } from '../helloworld/helloworld-service';
const mockSayHello = sayHello as jest.Mock;
describe('mycomp-tests', () => {
let compInstance;
beforeEach(() => {
mockSayHello.mockClear();
compInstance = new MyComponent();
});
afterEach(() => {
jest.clearAllMocks();
});
it('should return mock value', () => {
mockSayHello.mockReturnValue("yahoo");
expect(compInstance.callHelloWorldService()).toBe("yahoo");
});
Even after setting up the mocks, calling the service method directly in the test works fine, but when calling it through the component, the original method is invoked instead of the mocked one:
it('should return mock value', () => {
mockSayHello.mockReturnValue("yahoo");
expect(compInstance.callHelloWorldService()).toBe("yahoo");//Fails ,return value is Hello !
});
To ensure that the components also use the mocked methods during testing, additional steps may be required. This behavior is crucial for correctly simulating REST calls in tests.