Approval Testing

I need to change this code, but it has no test!

This is the title of a talk I gave at ConFoo 2020. Can you relate?

I present a technique known under different names:

• Approval Tests
• Characterization Tests
• Golden Master
• Snapshot Tests

The purpose of the technique is to capture the existing behavior of system to create regression tests that will tell you if it changes.

It's used to quickly put Legacy Code under tests so you can refactor it safely.

📖 Synopsis

Have you ever felt helpless, facing some cryptic code you had to change? You know, the kind of code that has no test to tell you if something breaks…

This is a scenario I frequently faced. And I learned better ways to work with such Legacy Code.

In this session, I'll show you how to write tests on existing code, even when you don't understand what it does. You'll learn how to modify this code and know that nothing broke when you’re done.

🧰 Resources

• Slides of my talk
• Jest, the JavaScript test runner I used in the presentation
• jest-extended-snapshot which provides Jest matchers to write Approval Tests
• approvaltests.com which contains resources to do the same if you're not using JavaScript and Jest
• My Gilded Rose kata starter in JavaScript. You can find the Approval Tests solution in the `approval-testing` branch.
• Emily Bache's Gilded Rose kata starter in many, many languages

🎓 Related articles

• Approval Tests, TCR, and Menders

  5 great talks on Legacy Code that I the pleasure to host. Learn how to test existing code and approach unfamiliar codebases.

• 5 coding exercises to practice refactoring Legacy Code

  Feeling overwhelmed by your legacy codebase? These katas will help you learn how to tackle it.

• 3 steps to add tests on existing code when you have short deadlines

  Here's a recipe you can follow when you want to add tests on Legacy Code, but you don't have much time to do so.

Written by Nicolas Carlo who lives and works in Montreal, Canada 🍁
He founded the Software Crafters Montreal community which cares about building maintainable softwares.