FLAT - the "Frontend Layer API Toolkit" by Sevenval is designed for easy decoupling frontend from backend in modern web applications. It is the preview version of Couper. See couper.io for more details.
This developer's documention helps building maintainable JSON-based web APIs on top of FLAT.
Create your first FLAT application hands-on and learn the basics in Couper's Tutorial. This is the best place to start.
Browse the recipes in Couper's Cookbook to learn how to accomplish more common API development tasks with FLAT.
Look up detailed usage information about FLAT's nuts and bolts in Couper's Reference.