model-something.js

const EventEmitter = require('events')

function ComputedModel (props) {
  const base = Object.create(new EventEmitter())
  const keys = Object.keys(props)
  for (let i = 0, len = keys.length; i < len; i++) {
    const key = keys[i]
    const prop = props[key]
    Object.defineProperty(base, `_${key}`, {writable: true, value: prop})
    if (typeof prop === 'function') {
      Object.defineProperty(base, key, {
        get: function () {
          return this[`_${key}`]()
        }
      })
    } else {
      Object.defineProperty(base, key, {
        enumerable: true,
        get: function () {
          return this[`_${key}`]
        },
        set: function (val) {
          this[`_${key}`] = val
          this.emit('set', {key: key, value: val})
        }
      })
    }
  }

  return base
}