ct.js has a "Core" library that provides basic drawing functions, room and asset management, and mouse interactions. Any other functions are added to projects as "catmods", or simply modules. These modules can be enabled or disabled in one click, and can inject their code in different game loop stages, e.g. after drawing all the objects, leaving a room, or when a new object is created.

v1.3 brings a particle system editor, which displays a preview sprite for proper attachment of emitter to visual elements, and allows combining more than one emitter with different particles into one effect. With these, even the creation of complex, multi-step effects becomes easy. The editor comes with dozens ready-made textures for faster prototyping.

This means that no one will ever put any features behind a paywall and that you can reliably use ct.js in any projects without worrying about licensing. And you can hack on ct.js!
The repo is at https://github.com/ct-js/ct-js

ct.js is bundled with examples, docs, and easy to follow tutorials. Documentation and tutorials are available in a side panel on every screen.