JSPM

Middleware for AB testing in Express

Simple curve-fitting algorithm

Lightweight split-testing library (A/B or multivariant)

Simple feature, multi-variant and A/B testing

A feature toggling/throttling and multivariate testing tool

A/B Test Development Toolkit

An extensive charting library with minimalistic background

Expressjs middleware for testing multiple route handlers (think A/B testing). Middleware will round-robin each route handler for a given test, setting a cookie for each new request in order to match the user to the same handler on subsequent requests.