Choosing a Test Type

When to Use

When you know what you need to verify but are not sure which test type to reach for. This is the master decision matrix — map a need to the right test type(s) and the right guide.

Master Decision Matrix

I need to verify...	Primary test type	Secondary / complement	Cross-link
A function's return value for given inputs	Unit	Parameterized unit for edge cases	Unit Testing Concepts
A class behaves correctly when dependencies return specific values	Unit (with stubs)	—	Test Doubles
A service correctly queries and transforms database data	Integration (real test DB)	Unit for transformation logic	Integration Testing Concepts
An HTTP endpoint returns the correct response shape	Integration (HTTP layer)	Unit for business logic inside	Integration Testing Concepts
A multi-step workflow end-to-end within one process	Functional / system	Integration for individual seams	Functional Testing Concepts
A critical user journey through a real browser	E2E	Integration for logic; unit for components	E2E Testing Concepts
A UI component renders correctly without regression	Visual regression	Unit/Integration for behavior	Visual Regression Concepts
An API consumer and provider stay in sync	Contract test	Integration for each side independently	Contract & API Testing Concepts
System behavior under concurrent users / load	Load/performance test	Unit for algorithmic bottlenecks	Performance Testing Concepts
A page has no WCAG violations	Automated a11y (axe)	Manual keyboard + screen reader	Accessibility Testing Concepts
A React/Next.js component's interactive behavior	Integration (Testing Library)	Unit for pure logic	Integration Testing Concepts
Drupal form submission, validation, redirects	Drupal Functional (BrowserTestBase)	Kernel for service logic	drupal/testing
Drupal plugin / hook / service behavior	Drupal Kernel (KernelTestBase)	Unit for pure logic	drupal/testing
Drupal E2E with Playwright against a DDEV site	Playwright + ATK	VR for visual stability	testing/playwright, testing/atk
AI-generated tests match spec and cover edge cases	AI test generation workflow	Unit + integration for gap coverage	testing/ai-test-generation

Test type is not test discipline

The matrix above picks a type. It does not say which loop the test belongs to, and mixing the two is how a suite ends up with a healthy count and unspecified behavior.

A test written before the code and watched to fail is a specification, produced by the TDD loop: unit, integration, contract, property-based, and a framework's in-process functional or browser tier all qualify. A test that can only be written against a system that already stands is outer verification: browser E2E, visual regression, load and performance, accessibility and security scans, mutation testing, fuzzing. Both are necessary. Only the first can drive a design decision, and only the first should be counted as TDD coverage.

The discriminator is not whether a browser is involved. Drupal's BrowserTestBase and WebDriverTestBase tiers drive real requests and a real browser, and are written red-green before the code — they are inside the loop. A Playwright journey against a running site is not, however similar it looks.

See TDD — What TDD Covers for the full table, and TDD — When Not to Write a Test for what the loop requires from a change and what counts as excess.

When One Test Type Is Not Enough

Most features need a combination. Use the pyramid/trophy shape as a guide:

Feature: User Registration

Unit tests:
  - Email format validation logic
  - Password strength rules
  - Hash function produces correct output format

Integration tests:
  - Registration endpoint: saves user to DB, returns 201
  - Duplicate email: returns 409 with correct error body
  - Weak password: returns 400 with validation errors

Functional tests:
  - Full registration form submission through Drupal
  - Email confirmation flow

E2E test:
  - One test: user fills form, submits, sees confirmation page

Security tests (at integration level):
  - SQL injection payload returns 400, not 500
  - Token in response is signed and contains correct claims

Quick Reference by Stack

Stack	Unit	Integration	E2E
PHP/Drupal	PHPUnit Unit	PHPUnit Kernel	PHPUnit Functional (in the TDD loop) / Playwright (outer)
JavaScript/React	Vitest/Jest	React Testing Library	Playwright
TypeScript/Next.js	Vitest/Jest	Supertest + Testing Library	Playwright
Python	pytest	pytest with real DB	Playwright / Selenium
Go	`testing` package	`httptest` + real DB	Playwright