Stub a function that runs a promise - reactjs

Question

I've been attempting to write unit tests for a registration page on a new react application
However I am very new to the concept of Sinon stubs/spies and have been having issues with intercepting a function call and forcing a resolve.
This is my initial test:
test('Success notification is displayed if all information is valid', () => {
wrapper.setState({ username: 'test', password: 'Test1234!#', confirmPassword: 'Test1234!#' });
const stub = sinon.stub(Register.prototype, 'signUp').resolves('Test');
wrapper.find('#submitRegistration').simulate('click');
});
onClick runs this event handler: (Simplified)
public onSubmitHandler(e: React.FormEvent) {
// Perform password match check
if (password === confirmPassword) {
this.signUp(this.state.username, password);
} else {
// Set success state
}
} else {
// Set error state
}
}
And finally, Signup:
public signUp(username: string, password: string) {
// Perform Cognito Signup
return Auth.signUp({
password,
username,
})
.then(() => {
// Set success State
})
.catch((err) => {
// Set error state
});
}
How can I intercept the call to signUp and force it down the resolve path, Currently due to the fact i do not configure my AWS Amplify Auth module it catches every time with "No userPool"

Answer 1

Jest provides many ways of mocking out dependencies, primarily there are 2 flavours:
Manual Mocks - These let you control mocks in far more details.
Mock functions - If you need a simple mock for a common problem, translation for example, these are very helpful.
For your case, you will also need to handle Promises, for this reason, you need to follow the advice on testing async behaviour.
For your case, I am assuming:
Auth is a module named 'auth-module'
you just need to setup the mock once and have 1 test data which resolves to success and 1 for failure.
// telling jest to mock the entire module
jest.mock("auth-module", () => ({
// telling jest which method to mock
signUp: ({ password, username }) =>
new Promise((resolve, reject) => {
// setting the test expectations / test data
process.nextTick(
() =>
username === "SuccessUser"
? resolve(userName)
: reject({
error: "failure message"
})
);
})
}));
Note : Adding the snippet even though it will not run, as the formatting seems to be broken with code block.

Answer 2

With a little thought push from dubes' answer I managed to get the following:
const signUpSpy = jest.spyOn(Auth, 'signUp').mockImplementation((user, password) => {
return new Promise((res, rej) => {
res();
});
});
Instead of spying directly onto the function I wrote and call moved it to the Modules function and resolved the promise from there!