Cycle.js

Cycle.js is a functional and reactive JavaScript framework used to write predictable code. Cycle’s core abstraction is your application as a pure function main() where inputs are read effects (sources) from the external world and outputs (sinks) are write effects to affect the external world. These I/O effects in the external world are managed by drivers: plugins that handle DOM effects, HTTP effects, etc.

Cycle.js is a free and open-source library.