Unit and Integration Tests

MODULE 17 · LESSON 17.1

Test behavior at the smallest useful boundary and integrate real dependencies where risk justifies it.

Practice-firstBeginner-friendlyProduction-aware

The production problem this solves

Good work on Unit and Integration Tests leaves evidence: a visible behavior, a stable contract or a repeatable operational check. This lesson defines an application trust boundary, where an explicit contract is safer than framework convention or an undocumented assumption.

Here, that decision supports a specific checkpoint: Create a CI-ready test suite for CourseFlow's enrollment path. A reviewable result should include a repeatable request, automated test, query result and failure response rather than a claim that the feature simply works.

Unit and Integration Tests workflowA four-step visual showing test pyramid, arrange act assert, test doubles, database isolation.Unit and Integration Tests workflow1Test Pyramid2Arrange Act Assert3Test Doubles4Database Isolation

Unit and Integration Tests workflow

  1. 1Test Pyramid
  2. 2Arrange Act Assert
  3. 3Test Doubles
  4. 4Database Isolation
Unit and Integration Tests workflow: a practical sequence used in this lesson.

A practical model for unit and integration tests

Test behavior at the smallest useful boundary and integrate real dependencies where risk justifies it. The useful unit of understanding is the boundary: who owns the decision, which input crosses it, what result is visible and how a failure is reported.

  • Test Pyramid: Decide what belongs in code, configuration, data or documentation and explain why.
  • Arrange Act Assert: Name its input, observable result and most likely failure in this lesson.
  • Test Doubles: Locate this responsibility in CourseFlow and defend the boundary you chose.
  • Database Isolation: Implement one behavior that another learner can reproduce without reading your mind.

Explain each moving part

The sample is intentionally narrow. Its job is to expose test pyramid without hiding the decision behind unrelated setup.

TYPESCRIPT
it('rejects duplicate enrollment', async () => {
  await enroll(user, course);
  await expect(enroll(user, course)).rejects.toMatchObject({ code: 'already_enrolled' });
});
Review it as someone else's change

Explain what the sample proves, what it does not prove, and which test would increase your confidence in arrange act assert.

Trace the implementation boundary

  1. 1
    Test Pyramid

    Run the focused example and save the output, trace, query or screenshot that confirms the result.

  2. 2
    Arrange Act Assert

    Break one assumption on purpose, make recovery clear and record the trade-off you accepted.

  3. 3
    Test Doubles

    Name the caller and the owner of this behavior before changing the implementation.

  4. 4
    Database Isolation

    Compare expected and actual output before editing; the difference tells you where to investigate.

Mistakes that create hidden coupling

  • Treating test pyramid as vocabulary instead of defining the behavior it must produce.
  • Testing the expected path while ignoring an empty, invalid, repeated or unauthorized case around arrange act assert.
  • Allowing test doubles to cross a boundary without an explicit contract or useful error.
  • Changing several layers before capturing the first piece of evidence, which makes the original cause harder to see.

Debug from the boundary inward

  1. Reduce the problem to the smallest failing Unit and Integration Tests case.
  2. Capture the actual input and output at the test pyramid boundary.
  3. Read the first relevant error, request, trace or query rather than the loudest downstream symptom.
  4. Test one explanation for the failure in arrange act assert; avoid changing two variables together.
  5. Keep a regression check that would expose the same defect if it returned.

Security decision

Validate external input, authorize the requested action, use parameterized data access, and keep credentials out of responses, source control and logs.

Performance decision

Bound queries and collections, inspect the actual request or query plan, and optimize only the slow boundary confirmed by evidence.

PRACTICE

Build something you can inspect

Write unit tests for progress calculation and integration tests for enrollment constraints.

Stretch challenge

Ask another person to run the exercise from your README. Fix the first place where their result differs from yours.

Definition of done

  • The behavior around test pyramid works with realistic input.
  • A failure involving arrange act assert is handled clearly and without leaking sensitive detail.
  • The implementation remains keyboard-usable when it produces an interface.
  • Your evidence directly supports the claim made in the exercise.
  • The README records the important trade-off without pretending the solution is universal.

Check your reasoning

What makes a test double harmful when it copies implementation details?

Answer by naming the expected test pyramid behavior, the layer responsible for it and the evidence that would confirm your explanation.

Where would you investigate the first failure?

Start where arrange act assert crosses a boundary. Compare the actual input and output there before following downstream symptoms.

What would make this work reviewable?

Show the focused change, repeatable steps, the result of your check and one honest trade-off connected to test doubles.

What to carry into the next lesson

  • Test behavior at the smallest useful boundary and integrate real dependencies where risk justifies it.
  • Keep test pyramid visible at the boundary where it can be tested.
  • Use evidence from arrange act assert before widening the implementation.

References and related reading

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