A way to handle sequences of events - which is really powerful abstraction. Look at this (shameless self-promotion) presentation https://slides.com/bahmutov/javascript-journey-boston-code-c... and the companion blog post http://glebbahmutov.com/blog/journey-from-procedural-to-reac... plus repo of the code that shows every style of JS, including Redux and Reactive https://github.com/bahmutov/javascript-journey