em.js

function CompileFunction(body) {
  return (new Function(`return (${body})`))()
}

function CompileEntityConstructor(props) {
  return CompileFunction(`
    function Entity() {
      this._mask = 0
      ${props.map(prop => `this._${prop} = undefined`).join(";")}
    }
  `)
}

function CompileGetComponent(prop) {
  return CompileFunction(`
    function ${prop}$get() {
      return this._${prop}
    }
  `)
}

function CompileSetComponent(prop, index) {
  return CompileFunction(`
    function ${prop}$set(component) {
      if (component === null || component === undefined) {
        this._${prop} = undefined
        this._mask = this._mask & ${~(1 << index)}
      } else {
        this._${prop} = component
        this._mask = this._mask | ${1 << index}
      }
    }
  `)
}

function createManager(comps) {
  class EntityManager {
    constructor() {
      this._pool = []
    }
    
    create() {
      return this._pool.length > 0
        ? this._pool.pop()
        : new Entity()
    }

    destroy(entity) {
      this._pool.push(entity)
    }
  }

  const keys = Object.keys(comps)
  const Entity = CompileEntityConstructor(keys)

  Entity.prototype.destroy = function () {
    this._manager.destroy(this)
    this._mask = 0
  }

  for (let i = 0; i < keys.length; i++) {
    Object.defineProperty(Entity.prototype, keys[i], {
      enumerable: true,
      get: CompileGetComponent(keys[i]),
      set: CompileSetComponent(keys[i], i),
    })
  }

  const em = new EntityManager()

  Object.defineProperty(Entity.prototype, "_manager", {
    value: em,
  })

  return em
}