When unit testing real-world code, there are many situations that make tests hard to write. How do you check if a function was called? How do you test an Ajax call? Or code using setTimeout? That’s when you use test doubles — replacement code that makes hard to test things easy to test.
For many years, Sinon.js has been the de-facto standard in JavaScript tests for creating test doubles. It’s a must-have tool for any JavaScript developer writing tests, as without it, writing tests for real applications would be nigh impossible.
Recently, a new library, aptly named testdouble.js, has been making waves. It boasts a similar feature set as Sinon.js, with a few differences here and there.
In this article, we’ll look into what both Sinon.js and testdouble.js offer, and compare their respective pros and cons. Will Sinon.js remain the superior choice, or will the challenger take the prize?
Note: If you’re unfamiliar with test doubles, I recommend reading my Sinon.js tutorial first. It will help you better understand the concepts we’ll be talking about here.