Answer by Paul Sweatte for What's the difference between a mock & stub?

A stub is an empty function which is used to avoid unhandled exceptions during tests:

function foo(){}

A mock is an artificial function which is used to avoid OS, environment or hardware dependencies during tests:

function foo(bar){ window = this; return window.toString(bar); }

In terms of assertions and state:

• Mocks are asserted before an event or state change
• Stubs are not asserted, they provide state before an event to avoid executing code from unrelated units
• Spies are setup like stubs, then asserted after an event or state change
• Fakes are not asserted, they run after an event with hardcoded dependencies to avoid state

References

• Geek Glossary: Mock
• Geek Glossary: Stub
• Geek Glossary: Spy
• Test Doubles: Fakes, Mocks and Stubs