Interaction verification is completely different from result and state verification. With this technique, we will be checking whether an interaction under a specific condition has happened or not.
Consider the example below, the databaseManager module offers a function to close database, but it neither returns any result nor changes the state. All this function does is that it calls a function in database module.
module databaseManager
function close():
database.close()
So, the only thing we can do to verify the correctness of this function is to check whether it calls close() function in database module or not
function test_close():
// arrange
mockDatabase = mock(database)
database = mockDatabase
// act
databaseManager.close()
// assert
mockDatabase.close().calledOnce()