Test-Driven Development (TDD)
All Jest topics∙ Jest
Test-Driven Development (TDD) focuses on the JavaScript behavior described by Test-Driven Development (TDD). It uses `test()` with `expect()` and a focused matcher to confirm the observed value matching the stated expectation.
Syntax
test("behavior", () => { expect(actual).toBe(expected); });📝 Jest Example
👁 Expected Result
💡 Run the test from isolated state and read the matcher diff when it fails.
Output
Test-Driven Development (TDD): pASS — adds two values
Line-by-Line Explanation
| Line | Meaning |
|---|---|
test('adds two values', () => { | In Test-Driven Development (TDD), line 2 declares a named Jest test. |
expect(2 + 3).toBe(5); | In Test-Driven Development (TDD), line 3 creates an expectation for the received value. |
}); | In Test-Driven Development (TDD), line 4 implements setup, action, or verification for this example. |
Real-World Uses
- 1Use Test-Driven Development (TDD) to verify the JavaScript behavior described by Test-Driven Development (TDD).
- 2Test-Driven Development (TDD) is valuable in professional test engineering when the test must prove the observed value matching the stated expectation.
- 3A useful failure record for Test-Driven Development (TDD) contains the assertion message, stack trace, and relevant test output.
Common Mistakes
- 1Test-Driven Development (TDD) commonly fails because of testing implementation details instead of externally meaningful behavior.
- 2Starting Test-Driven Development (TDD) without a deterministic input and isolated test state makes the result nondeterministic.
- 3For Test-Driven Development (TDD), executing code without asserting the observed value matching the stated expectation is incomplete.
- 4Using Test-Driven Development (TDD) to cover browser rendering, production infrastructure, or non-JavaScript behavior outside this unit creates the wrong test boundary.
Best Practices
- 1Prepare a deterministic input and isolated test state before running Test-Driven Development (TDD).
- 2Implement Test-Driven Development (TDD) with `test()` with `expect()` and a focused matcher.
- 3Make the central Test-Driven Development (TDD) assertion prove the observed value matching the stated expectation.
- 4Preserve the assertion message, stack trace, and relevant test output whenever Test-Driven Development (TDD) fails.
Core behavior
- 1Test-Driven Development (TDD) target: the JavaScript behavior described by Test-Driven Development (TDD).
- 2Test-Driven Development (TDD) API: `test()` with `expect()` and a focused matcher.
- 3Test-Driven Development (TDD) expected result: the observed value matching the stated expectation.
- 4Test-Driven Development (TDD) primary risk: testing implementation details instead of externally meaningful behavior.
Implementation steps
- 1Set up Test-Driven Development (TDD) with a deterministic input and isolated test state.
- 2For Test-Driven Development (TDD), invoke the behavior that produces the JavaScript behavior described by Test-Driven Development (TDD).
- 3In Test-Driven Development (TDD), apply `test()` with `expect()` and a focused matcher to the observed result.
- 4Finish Test-Driven Development (TDD) by asserting the observed value matching the stated expectation.
Verification
- 1Run Test-Driven Development (TDD) once with input that should satisfy the observed value matching the stated expectation.
- 2Add a negative Test-Driven Development (TDD) case that must produce a readable failure.
- 3Repeat Test-Driven Development (TDD) from fresh state to reveal shared-data or ordering dependencies.
- 4Diagnose Test-Driven Development (TDD) through the assertion message, stack trace, and relevant test output.
Scope
- 1Test-Driven Development (TDD) covers the JavaScript behavior described by Test-Driven Development (TDD).
- 2Test-Driven Development (TDD) does not directly prove browser rendering, production infrastructure, or non-JavaScript behavior outside this unit.
- 3Mocks and fixtures used by Test-Driven Development (TDD) must continue to match its real dependency contracts.
- 4For evidence outside the Test-Driven Development (TDD) process boundary, prefer an integration, end-to-end, contract, performance, or manual test.
Summary
- Test-Driven Development (TDD) setup: a deterministic input and isolated test state.
- Test-Driven Development (TDD) action: `test()` with `expect()` and a focused matcher.
- Test-Driven Development (TDD) assertion: the observed value matching the stated expectation.
- Test-Driven Development (TDD) diagnostics: the assertion message, stack trace, and relevant test output.
- Test-Driven Development (TDD) boundary: choose an integration, end-to-end, contract, performance, or manual test for browser rendering, production infrastructure, or non-JavaScript behavior outside this unit.
Interview Questions
Q1. What does Test-Driven Development (TDD) verify?
Answer: Test-Driven Development (TDD) verifies the JavaScript behavior described by Test-Driven Development (TDD).
Q2. Which Jest API is central to Test-Driven Development (TDD)?
Answer: The central Test-Driven Development (TDD) API is `test()` with `expect()` and a focused matcher.
Q3. What proves Test-Driven Development (TDD) passed?
Answer: A passing Test-Driven Development (TDD) test shows the observed value matching the stated expectation.
Q4. What makes Test-Driven Development (TDD) unreliable?
Answer: A common Test-Driven Development (TDD) cause is testing implementation details instead of externally meaningful behavior.
Q5. When should another test type replace Test-Driven Development (TDD)?
Answer: Replace Test-Driven Development (TDD) with an integration, end-to-end, contract, performance, or manual test for browser rendering, production infrastructure, or non-JavaScript behavior outside this unit.
Quick Quiz
Which approach correctly implements Test-Driven Development (TDD)?