An example set of component tests

microservice.component.test.js

/* ----- Your Microservice -----  */

// An imaginary database
const DATABASE = [{ id: 2, name: "Lou" }]

// library.js
function databaseLibrary() {
    return {
        findByID: (searched_id) => DATABASE.find(id => searched_id)
    }
}

// models/user.js
function getUser(searched_id) {
    return databaseLibrary().findByID(searched_id)
}

// index.js
function microservice({ searched_id: id }) {
    if (!id) throw new Error("User ID must be passed");
    return getUser(id);
}

/* ----- Your Component Tests -----  */

let mockDatabase = {
    findByID: jest.fn()
}

beforeEach(() => {
    databaseLibrary = jest.fn().mockReturnValue(mockDatabase);
})

test('Throws error user ID is not set', () => {
    expect(() => microservice({ searched_id: null })).toThrow("User ID must be passed");
});

test('Calls the database with passed user ID', () => {
    microservice({ searched_id: 1 })
    expect(mockDatabase.findByID).toHaveBeenCalledWith(1);
});