A mod is contained in a single directory under server/mods/, which makes it easy to manage (for example as a git repository).
If you want your server to run a mod called "MyMod" for example, you must create a directory MyMod. Then in build_config.json, you must set the value mod to "MyMod" (the default mod is "capture").
Your mod must contain the files enemy.js, engine.js, knife.js, lmg.js, on_message.js, planet.js, player.js, rapid_fire_weapon.js, shotgun.js, shot.js, smg.js and weapon.js.
Your files must export at least the same things as the default mod capture, and your own version of classes must have the same signatures and return types as capture's.
It is advised you copy the example mod example, then modify it to make it into whatever you want it to be! To do so, you can have a look at how the default mod capture is made.
Note that when you modify something, it only applies to the server. We will never serve client code from your mod for obvious security reasons.
Feedback from modders is appreciated. Besides, this API is yet quite new, might lacks features, etc. If you think it could be improved, you are welcome to open an issue or a pull request.