A mock object is a simulated object that mimics the behavior of a real object in controlled ways. Mock objects are often employed in unit testing to scrutinize the performance of actual objects. In this context, an object is the smallest testable part of an application. A mock object makes use of the same interface as the element of code it is intended to imitate.

A mock object can be useful in place of a real object that:

• Runs slowly or inefficiently in practical situations
• Occurs rarely and is difficult to produce artificially
• Produces non-deterministic results
• Does not yet exist in a practical sense
• Is intended mainly or exclusively for conducting tests.

Read more about it:
>>	IBM discusses some practical aspects of unit testing with mock objects.
>>	DevX offers alternatives to the use of mock objects in Extreme Programming.
>>	JSMock is an open-source library for testing JavaScript code using mock objects.