State verification is similar to result verification but rather than checking the result of SUT, we check the state of an object or something similar.
Consider an example of testing a function that closes the database connection
module databaseManager
isOpen
function close():
isOpen = false
When we call this function, it is not going to return anything, instead it is going to change the state of isOpen field
So we need to verify the state of isOpen field to ensure the correctness of close function
function test_close():
// arrange
databaseManager.isOpen = true
// act
databaseManager.close()
// assert
areEqual(false, databaseManager.isOpen) // false is expected