## Mocking function > If you look closely, you might realize that spyOn is replacing the original function with a spy that intercepts the function calls and tracks a lot of potentially useful information about them. The problem we run into above is that once we’ve replaced the original function, we’ve lost its capabilities. We can remedy that with andCallThrough. If you chain andCallThrough() after calling spyOn, the spy will then pass any calls to it through to the original function [http://www.joezimjs.com/javascript/javascript-unit-testing-with-jasmine-part-2/](http://www.joezimjs.com/javascript/javascript-unit-testing-with-jasmine-part-2/) `spyOn(FilteredMembersProvider.prototype, "_initFirefly").and.callThrough();`