Test-Driven Development (TDD) Foundations

Foundations

Test-Driven Development (TDD)

Writing an automated test before the code that makes it pass, using tests to drive design and quality.

Test-First Principle

Specifying how you will know code works before writing it, clarifying intent and design.

Benefits of TDD

Well-tested, well-designed code, fast feedback, and confidence to change code safely.

Quality Built In

TDD creates quality as code is written rather than inspecting it afterward.

The Cycle

Red-Green-Refactor

Write a failing test, make it pass simply, then improve the design.

Red

A small failing test specifying the next desired behavior.

Green

The simplest code that makes the test pass.

Refactor

Improving structure without changing behavior, with tests ensuring safety.

Baby Steps

Working in tiny increments for fast feedback and easy error detection.

Unit Testing

Unit Test

An automated test verifying a small unit of code in isolation.

Arrange-Act-Assert

A structure for tests: set up, act, then assert the expected result.

Good Test Qualities

Tests should be fast, isolated, repeatable, self-validating, and timely (FIRST).

One Assert per Concept

Keeping tests focused so failures clearly indicate what broke.

Test Naming

Naming tests to describe the behavior they verify, aiding readability.

Test Doubles

Test Doubles

Stand-ins for real dependencies used to test code in isolation.

Stub

A double that returns canned responses to calls made during a test.

Mock

A double that verifies expected interactions occurred.

Fake

A working but simplified implementation used for testing.

Design

Emergent Design

Letting design evolve through refactoring as tests drive out behavior.

Testable Design

Structuring code with clear responsibilities and dependencies so it is easy to test.

Refactoring

Improving code structure safely because passing tests catch regressions.

Applying It

TDD Discipline

Consistently following the cycle to gain its benefits and build the habit.