unit-testing Getting started with unit-testing A unit test with stubbed dependency

Example

Good unit tests are independent, but code often has dependencies. We use various kinds of test doubles to remove the dependencies for testing. One of the simplest test doubles is a stub. This is a function with a hard-coded return value called in place of the real-world dependency.

// Test that oneDayFromNow returns a value 24*60*60 seconds later than current time

let systemUnderTest = new FortuneTeller()       // Arrange - setup environment
systemUnderTest.setNow(() => {return 10000})    //   inject a stub which will 
                                                //   return 10000 as the result

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

assert.equals(actual, 10000 + 24 * 60 * 60)     // Assert - Validate expected result

In production code, oneDayFromNow would call Date.now(), but that would make for inconsistent and unreliable tests. So here we stub it out.