I'm not the author of [Hecs](https://github.com/gohyperr/hecs/tree/master/packages/hecs), but I'm really digging it. I think it is under-appreciated tech, and thought others might like to take a look (especially if you're weighing javascript ECS libraries at the moment).

HECS is a [very fast](https://github.com/gohyperr/hecs/blob/master/packages/hecs/TECHNICAL.md) Entity-Component-System that is based on some of the same principles as Unity DOTS. I couldn't be happier with having chosen it for our in-browser virtual world (www.relm.us).

Some of the things I've appreciated:

1. The queries are fast but flexible. You can query for entities with a particular set of components, or add `Not()` as well as `Modified()` to your query as modifiers. This is extremely helpful in managing the full life-cycle of a component as it travels through states like being "added", "active" or "removed."
2. It has a plugin system which allows you to pull in others' work, as well as group components and systems together into logical containers. For example, I [implemented](https://github.com/relm-us/relm-experimental/tree/main/src/ecs/plugins/rapier) the [Rapier3D physics engine](https://rapier.rs/) as a HECS plugin.
3. It runs on both server (node) and client (browser). This means you can run physics simulations in both places for a networked multiplayer game, for instance.
4. Small, extremely well-written codebase. In addition, the developer is very responsive & friendly.

I hope you like it as much as I do!