What is Test-Driven Development (TDD)?

Question

Accepted Answer

**Test-Driven Development (TDD)** is a practice where you write **tests before the code** — following a short, repeating cycle: write a failing test, write code to pass it, then refactor. It drives design through tests and ensures code is testable and well-tested.

## The TDD cycle: Red, Green, Refactor

```text
1. RED → write a FAILING test for the behavior you want (it fails — the code doesn't exist yet)
2. GREEN → write the MINIMUM code to make the test PASS (just enough, don't over-build)
3. REFACTOR → improve/clean up the code (and tests) while keeping tests GREEN
→ Repeat this short cycle for each small piece of behavior.
```

```text
Write test (RED) → write code (GREEN) → clean up (REFACTOR) → repeat
```

## Example flow

```js
// 1. RED — write the test first (fails: add doesn't exist / is wrong)
test('add(2,3) === 5', () => expect(add(2, 3)).toBe(5));

// 2. GREEN — write minimal code to pass
function add(a, b) { return a + b; }

// 3. REFACTOR — improve if needed (test still passes); then write the next test
```

## Benefits and considerations

```text
✓ Ensures code is TESTABLE (you design for testing) and TESTED (tests exist by definition)
✓ Drives DESIGN — thinking about behavior/interface first → cleaner, focused code
✓ Fast FEEDBACK; confidence to refactor; avoids over-engineering (write only what's needed)
✓ Good test coverage as a natural result
⚠️ A discipline/skill that takes practice; not always applied dogmatically (use judgment)
```

## Why it matters

Understanding Test-Driven Development is valuable because it's an **influential, widely-discussed practice** that shapes how many developers approach testing and design, so it's important knowledge for professional development.

TDD's core idea — **writing tests before the code**, following the **Red-Green-Refactor cycle** (write a failing test, write minimal code to pass it, then refactor while keeping tests green) — is a distinctive approach that inverts the usual order.

Understanding the cycle is the key practical knowledge.

Understanding the **benefits** explains why TDD is valued: it ensures code is **testable** (by designing for testing from the start) and **tested** (tests exist by definition), **drives design** (thinking about behavior and interface first leads to cleaner, more focused code), provides fast feedback and confidence to refactor, avoids over-engineering (writing only what's needed to pass tests), and naturally produces good test coverage.

These benefits make TDD influential as both a testing and a design practice.

Understanding the **considerations** — that TDD is a discipline requiring practice, and that it's not always applied dogmatically (developers use judgment about when it fits) — reflects a balanced, realistic view.

Even developers who don't always practice TDD benefit from understanding it, as it shapes thinking about testing and design and is a common topic in professional development and interviews.

Since TDD is an influential, widely-known practice that shapes testing and design approaches (with real benefits in testability, design, and coverage) and is a common professional and interview topic, and since understanding the cycle, benefits, and considerations reflects testing maturity, understanding Test-Driven Development is valuable, relevant knowledge — an important testing practice and philosophy worth understanding for professional development, influential in how quality-focused developers approach building software.