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Test Driven Development

By Srinesh Nisala (Senior Software Engineer @ iLabs)

1. Introduction to Vitest​

Objective: Explore Vitest as a unit testing tool.

Key Topics​

  • What a test file looks like:
    • describe(), it() and expect()
  • Sanity check:
    • Write a simple passing test to confirm the setup works

2. Hands-On Demo: Building a Password Validator with TDD​

Objective: Implement a TDD workflow using Vitest.

Pre-requisites​

Understanding the project structure​

  • Explain how to run tests
npm test

Write all tests first​

  • Create password.js and password.test.js
  • Write a test: short password returns { valid: false }
  • Write a test: 8-char password returns { valid: true }
  • Write a test: password with no uppercase returns { valid: false }
  • Write a test: password with no digit returns { valid: false }
  • Write a test: password with no special character returns { valid: false }
  • Run all tests — show all RED

Exercise​

  1. Minimum length — Implement minimum code: check password.length >= 8. Run tests — first two GREEN, rest still RED
  2. Uppercase — Add the uppercase check (/[A-Z]/). Run tests — show GREEN
  3. Digit — Add the digit check (/[0-9]/). Run tests — show GREEN
  4. Special character — Add the special character check (/[^a-zA-Z0-9]/). Run tests — all GREEN. Point out the smell: { valid: false, message } repeated four times

3. Introduction to TDD​

Objective: Understand the philosophy and purpose of TDD.

Key Topics​

  • What is TDD?
    • Test-Driven Development (TDD) is a Software development method in which you write Automation Tests before the actual development process starts.
  • Red-Green-Refactor:
    • Focus on testing behaviors (requirements) rather than implementation details.
    • Write minimal code to pass tests ("green").
    • Refactor without adding new tests.
  • Pros:
    • Forces you to think about the interface before the implementation.
    • Tests act as living documentation — they show exactly what the code is supposed to do.
    • Gives confidence to refactor without breaking existing behaviour.
  • Cons:
    • Can give false confidence if tests only cover happy paths.
  • Best Practices:
    • Never write code without a failing test: You may only write new production code to pass a unit test that has already failed.
    • Write minimal tests: Do not write more of a test than is necessary to fail. Compilation failures are considered test failures.
    • Code to pass: Write exactly the amount of production code needed to satisfy the failing test, not more.

4. References​