implement private Object props, using Proxy.

privateProps.js

function proxied (obj) {
  const o = Object.assign({}, obj)

  function filterPrivate (key) {
    if (key[0] === '_') {
      throw Error('cannot access private key: ${key}')
    }
  }

  const handler = {
    get (target, key, receiver) {
      filterPrivate(key)

      const result = Reflect.get(target, key, receiver)
      if (isFunction(result)) {
        return result.bind(target)
      }

      if (isObject(result)) {
        return new Proxy(result, handler)
      }

      return result
    },

    set (target, key, value, receiver){
      filterPrivate(key)
      return Reflect.set(target, key, value, receiver)
    },

    deleteProperty (target, key) {
      filterPrivate(key)
      return Reflect.deleteProperty(target, key)
    },

    defineProperty (target, key, descriptor) {
      filterPrivate(key)
      return Reflect.defineProperty(target, key, descriptor)
    },

    has (target, key) {
      if (key[0] === '_') {
        return false
      }

      return Reflect.has(target, key)
    },

    ownKeys (target) {
      return Reflect.ownKeys(target).filter(key => key[0] !== '_')
    },

    getOwnPropertyDescriptor (target, key) {
      filterPrivate(key)
      return Reflect.getOwnPropertyDescriptor(target, key)
    }
  }

  return new Proxy(o, handler)
}

function isObject (obj) {
  return obj && typeof obj === 'object'
}

function isFunction (obj) {
  return typeof obj === 'function'
}


// ---------- example ---------
const obj = {
  o: {
    _v: 2,
    f () {
      console.log(this._v)
    }
  },
  f () {
    console.log(this._v)
  },
  _v: 1
}

const proxiedO = proxied(obj)

proxiedO.f() // 1
proxiedO.o.f() // 2

proxiedO._v // error
proxiedO.o._v // error