Why and How you should use a Sinon Spy
Sinon spys and stubs are very using in testing code. Spys and Stubs allow you to verify if a function was called and how it was called (args).
Thus allowing you to easily test your code.
Import sinon
import * as sinon from 'sinon';
Declare the Spy
const spy: sinon.SinonSpy = sinon.spy();
Test the spy
You can now test against the spy.
if(spy.called) {
// spy has been called
}