unit-testing Getting started with unit-testing A unit test with a spy (interaction test)

Example

Classic unit tests test state, but it can be impossible to properly test methods whose behavior depends on other classes through state. We test these methods through interaction tests, which verify that the system under test correctly calls its collaborators. Since the collaborators have their own unit tests, this is sufficient, and actually a better test of the actual responsibility of the tested method. We don't test that this method returns a particular result given an input, but instead that it correctly calls its collaborator(s).

// Test that squareOfDouble invokes square() with the doubled value

let systemUnderTest = new Calculator()          // Arrange - setup environment
let square = spy()
systemUnderTest.setSquare(square)               //   inject a spy

let actual = systemUnderTest.squareOfDouble(3)  // Act - Call system under test

assert(square.calledWith(6))                    // Assert - Validate expected interaction