Show HN: Easily Add a CMS to Your JavaScript App

Looks very similar to https://github.com/asm-products/buckets

Not exactly. Cosmic JS is a flexible, platform-agnostic cloud-hosted CMS, whereas the one you linked to is installed and locked into Node / MongoDB.

FYI, there's also a browser version: https://github.com/cosmicjs/cosmicjs-browser

An example React app that uses the Cosmic JS browser client: https://github.com/tonyspiro/react-flux-cosmicjs

How does this integrate into existing Content Management Systems like WordPress, Drupal, and such?

Cosmic JS allows you to create an on-the-fly CMS / API to power your full app or website or you could also integrate it with only a part of your website.

Check out the Docs to learn about the Cosmic JS API: https://cosmicjs.com/docs

There's also an easy export plugin available for an existing WordPress site: https://github.com/cosmicjs/cosmicjs-wp-export