Mocking in jest and express - node.js

Question

I have some code in an Express route which talks to AWS Cognito and am having trouble working out how to mock it in tests.
cognitoExpress.validate(accessTokenFromClient, (err, response) => {
if (err) return res.status(401).json({ error: err });
res.json({ data: `Hello ${response.username}!` });
});
Then in my test I want to say cognitoExpress.validate should be called once and return {username: 'test user'} so that it doesnt hit the network and doesnt actually call AWS Cognito
it('It should returns 200 with a valid token', async done => {
const { cognitoExpress } = require('../helpers/cognitoExpress');
// I have tried
jest.mock('../helpers/cognitoExpress');
// and this
jest.mock('../helpers/cognitoExpress', () => ({
validate: jest.fn()
}));
const token = 'sfsfdsfsdfsd';
const response = await request.get('/').set('Authorization', token);
expect(cognitoExpress.validate).toHaveBeenCalledWith(token);
expect(response.body).toEqual({ data: 'Hello test user' });
done();
});
Thanks in advance....

Answer 1

let spyInstance = undefined;
beforeAll(() => {
spyInstance = jest.spyOn(cognitoExpress.prototype, "validate").mockImplementation(() => {
// Replace the body of 'validate' here, ensure it sets
// response body to {username: 'test user'} without calling AWS
...
});
});
afterAll(() => {
expect(spyInstance).toBeDefined();
expect(spyInstance).toHaveBeenCalledTimes(1);
jest.restoreAllMocks();
});
it("It should call mocked cognitoExpress.validate once", async done => {
...
});
A similar and working test in my project. Instead of cognitoExpress.validate it mocks and tests SampleModel.getData

Answer 2

Create file ../helpers/__mocks__/cognitoExpress.js with mocked function you want to use. It is essential to call the folder __mocks__. You can modify a functions and return any data you want.
example
module.exports = {
validate: () => { username: 'test user' }
}
Now you can use jest.mock('../helpers/cognitoExpress'), but I recommend you to place it to some global or test setup file, not to separate tests.
Jest Manual Mocks