1-2 Structure tests by the AAA pattern

⚪ ️ 1.2 Structure tests by the AAA pattern §

:white_check_mark: Do: Structure your tests with 3 well-separated sections Arrange, Act & Assert (AAA). Following this structure guarantees that the reader spends no brain-CPU on understanding the test plan:

1st A - Arrange: All the setup code to bring the system to the scenario the test aims to simulate. This might include instantiating the unit under test constructor, adding DB records, mocking/stubbing on objects, and any other preparation code

2nd A - Act: Execute the unit under test. Usually 1 line of code

3rd A - Assert: Ensure that the received value satisfies the expectation. Usually 1 line of code

❌ Otherwise: Not only do you spend hours understanding the main code but what should have been the simplest part of the day (testing) stretches your brain

✏ Code Examples

:clap: Doing It Right Example: A test structured with the AAA pattern §

describe("Customer classifier", () => {
  test("When customer spent more than 500$, should be classified as premium", () => {
    //Arrange
    const customerToClassify = { spent: 505, joined: new Date(), id: 1 };
    const DBStub = sinon.stub(dataAccess, "getCustomer").reply({ id: 1, classification: "regular" });
 
    //Act
    const receivedClassification = customerClassifier.classifyCustomer(customerToClassify);
 
    //Assert
    expect(receivedClassification).toMatch("premium");
  });
});

:thumbsdown: Anti-Pattern Example: No separation, one bulk, harder to interpret §

test("Should be classified as premium", () => {
  const customerToClassify = { spent: 505, joined: new Date(), id: 1 };
  const DBStub = sinon.stub(dataAccess, "getCustomer").reply({ id: 1, classification: "regular" });
  const receivedClassification = customerClassifier.classifyCustomer(customerToClassify);
  expect(receivedClassification).toMatch("premium");
});