Private attributes and attribute accessors using decorators

babel.js

import * from './privatize.js';

let token = Symbol(); // this is a class secret!

export class Babel {
  @attr_accessor // <- create getters and setters
  @privatize(token) // <- private attributes turns into functions `fun (token, ...args)`
  lang;
  
  @attr_accessor
  @privatize(token)
  country;
  
  @privatize(token)
  phrase;
  
  constructor () {
    this.phrase(token, ' speaks ');
  }
  
  public_method () {
    this.private_method(token);
  }
  
  @privatize(token)
  private_method () {
    console.log(`${this.country}${this.phrase(token)}${this.lang}`);
  }
}

privatize.js

export function privatize (TOKEN) {
  return function (t, n, d) {
    let pass = function (value) {
      if (value !== TOKEN)
        throw new Error(`"${n}" is private method`);
    }
    
    var original = d.value;
      
    // variable
    if ('initializer' in d) {
      delete d.initializer;
      
      let key = Symbol();
      
      d.value = function (token, value) {
        pass(token);
        
        if (value === undefined)
          return this[key];
        else
          this[key] = value;
      }
      
      return;
    }
    
    // method
    d.value = function () {
      pass(arguments[0]);
      this::original(arguments);
    }
  }
}

export function attr_accessor (t, n, d) {
  delete d.initializer;
  delete d.value;
  
  let key = Symbol();
  
  d.get = function () {
    return this[key];
  }
  
  d.set = function (value) {
    this[key] = value;
  }
}

export function attr_reader (t, n, d) {
  delete d.initializer;
  delete d.value;
  
  let key = Symbol();
  
  d.get = function () {
    return this[key];
  }
}

export function attr_writer (t, n, d) {
  delete d.initializer;
  delete d.value;
  
  let key = Symbol();
  
  d.set = function (value) {
    this[key] = value;
  }
}

test.js

import {Babel} from './babel.js';

var b = new Babel();
b.country = 'brazil';
b.lang = 'portuguese'
console.log(b.public_method()); 

try {
  b.private_method();
} catch (e) {
  console.log(e.message); // "private_method" is private method
}

try {
  b.phrase = 'hey';
} catch (e) {
  console.log(e.message); // Cannot assign to read only property 'phrase' of [object Object]
}