js class with private fields

unit.js

"use strict"

class Foo {
  constructor() {
    //# make object with private properties atop { this }
    const privateObject = {
      __proto__: this, 
      _a: 1
    }
    //# and bind every method to { privateObject }
    Object.getOwnPropertyNames(Object.getPrototypeOf(this)).forEach((name) => {
      if(this[name] instanceof Function) {
        this[name] = this[name].bind(privateObject)
      }
      else {
        
      }
    })
  }
  incA() {
    this._a += 1
  }
  getA() {
    return this._a
  }
  
}

const assert = function(expr) {
  if(!expr) {
    throw new Error("Assert failed")
  }
}

if(1) {
  var foo = new Foo()
  assert(foo._a == undefined)
  assert(foo.getA() == 1)
  foo.incA()
  assert(foo.getA() == 2)
  assert(foo._a == undefined)
  console.log("test ok")
}