An example of extending a prototypical game Entity to have different functionality (via adding functions to the prototype) in its server environment than on the game client.

client.js

/* ClientSide usage test, via browserify */
var Entity = require('./Entity')
require('./EntityClientSide')(Entity)

// the client receives data about a new entity:
var ent = Entity.newFromJson({ name: 'newbie', x: 12, y: -6 })
console.log(ent.name, ent.x, ent.y) // outputs "newbie 12 -6"

// an existing entity receives an update from the server:
ent.updateFromJson({ name: 'newbie the renamed', x: 2, y: 2 })
console.log(ent.name, ent.x, ent.y) // outputs "newbie the renamed 2 2"

Entity.js

// a simple game entity having a name and positional data
function Entity(name) {
	this.name = name
	this.x = 0
	this.y = 0
	this.speed = 5
}

module.exports = Entity

EntityClientSide.js

// adds render, newFromJson, and updateFromJson to entity
module.exports = function(Entity) {

	// draws an Entity
	Entity.prototype.render = function(drawDelta, context) {
		console.log('drawing', this.name)
		// actual drawing code would go here
	}

	// creates an Entity from json data
	Entity.newFromJson = function(entityJson) {
		var entity = new Entity(entityJson.name)
		entity.x = entityJson.x
		entity.y = entityJson.y
		return entity
	}

	// updates an entity from json data
	Entity.prototype.updateFromJson = function(entityJson) {
		this.name = entityJson.name
		this.x = entityJson.x
		this.y = entityJson.y
	}
}

EntityServerSide.js

// adds an update(delta) to entity, which runs its ai logic
module.exports = function(Entity) {
	// updates an entity from json data
	Entity.prototype.update = function(delta) {
		console.log(this.name, 'updating')
		this.ai.update(delta)
	}
}

PatrolState.js

// AI for an entity that patrols from right to left
// the actual behavior is irrelevant, I just wanted to have a complex behavior for the serverside example
function PatrolState(entity, distance) {
	this.entity = entity
	this.distance = distance
	this.x0 = entity.x
	this.y0 = entity.y
	this.movingRight = true
}

PatrolState.prototype.update = function(delta) {	

	if (this.movingRight) {
		// travel to the right
		this.entity.x += delta * this.entity.speed
		// until distance reached
		if (this.entity.x > this.x0 + this.distance) {
			// then turn around
			this.movingRight = false
		}
	} else {
		// travel to the left
		this.entity.x += delta * this.entity.speed * -1
		// until distance reached (negative)
		if (this.entity.x < this.x0 - this.distance) {
			// then turn around
			this.movingRight = true
		}
	}
}

module.exports = PatrolState

server.js

/* ServerSide usage example */
var PatrolState = require('./PatrolState')
var Entity = require('./Entity')
require('./EntityServerSide')(Entity)

// create an entity and give it the patroller AI
var sEnt = new Entity('Jimmy')
sEnt.ai = new PatrolState(sEnt, 5)

// run a game loop @ 20 FPS, updating our entity Jimmy
var tickLengthMs = 1000 / 20
var previousTick = Date.now()
var main = function () {
  var now = Date.now()
  if (previousTick + tickLengthMs <= now) {
    var delta = (now - previousTick) / 1000
    previousTick = now
    sEnt.update(delta)
    console.log(sEnt.x)
  }
  setImmediate(main)
}

main() // output: counts from ~ -5 to 5 as Jimmy patrols back and forth