Bill Sorensen shares an open-source library he authored called tzientist. It's a "Scientist-like library for Node.js, implemented in TypeScript. It permits comparing legacy and refactored code paths in production environments, verifying both functional and non-functional requirements. This is also known as the Parallel Run pattern."

Bill goes into what inspired him to author the library, what it's been like to maintain it, and why you should consider using it if traditional testing strategies won't handle the refactoring you're trying to do.