Hi there, thanks for the question! This is mostly a duplicate of the tech stack question above, which Poornima provided a pretty comprehensive answer to the unit test frameworks in use here at PayPal. However, I will also clarify that APIs are mostly written in Java, while the the front-end stack is written in Node.js. Outside of standard unit testing, there are functional tests, sandbox tests, and live tests, using a combination of off the shelf Java or Python frameworks and our own scripts.