How to implement private method in ES6 class with Traceur [duplicate]

There are no private, public or protected keywords in current ECMAScript 6 specification.

So Traceur does not support private and public. 6to5 (currently it's called "Babel") realizes this proposal for experimental purpose (see also this discussion). But it's just proposal, after all.

So for now you can just simulate private properties through WeakMap (see here). Another alternative is Symbol - but it doesn't provide actual privacy as the property can be easily accessed through Object.getOwnPropertySymbols.

IMHO the best solution at this time - just use pseudo privacy. If you frequently use apply or call with your method, then this method is very object specific. So it's worth to declare it in your class just with underscore prefix:

class Animal {

    _sayHi() {
        // do stuff
    }
}

You can always use normal functions:

function myPrivateFunction() {
  console.log("My property: " + this.prop);
}

class MyClass() {
  constructor() {
    this.prop = "myProp";
    myPrivateFunction.bind(this)();
  }
}

new MyClass(); // 'My property: myProp'

Although currently there is no way to declare a method or property as private, ES6 modules are not in the global namespace. Therefore, anything that you declare in your module and do not export will not be available to any other part of your program, but will still be available to your module during run time. Thus, you have private properties and methods :)

Here is an example (in test.js file)

function tryMe1(a) {
  console.log(a + 2);
}

var tryMe2 = 1234;

class myModule {
  tryMe3(a) {
    console.log(a + 100);
  }

  getTryMe1(a) {
    tryMe1(a);
  }

  getTryMe2() {
    return tryMe2;
  }
}

// Exports just myModule class. Not anything outside of it.
export default myModule; 

In another file

import MyModule from './test';

let bar = new MyModule();

tryMe1(1); // ReferenceError: tryMe1 is not defined
tryMe2; // ReferenceError: tryMe2 is not defined
bar.tryMe1(1); // TypeError: bar.tryMe1 is not a function
bar.tryMe2; // undefined

bar.tryMe3(1); // 101
bar.getTryMe1(1); // 3
bar.getTryMe2(); // 1234

You can use Symbol

var say = Symbol()

function Cat(){
  this[say]() // call private methos
}

Cat.prototype[say] = function(){ alert('im a private') }

P.S. alexpods is not correct. he get protect rather than private, since inheritance is a name conflict

Actually you can use var say = String(Math.random()) instead Symbol

IN ES6:

var say = Symbol()

class Cat {

  constructor(){
    this[say]() // call private
  }

  [say](){
    alert('im private')
  }

}

I hope this can be helpful. :)

I. Declaring vars, functions inside IIFE(Immediately-invoked function expression), those can be used only in the anonymous function. (It can be good to use "let, const" keywords without using 'var' when you need to change code for ES6.)

let Name = (function() {
  const _privateHello = function() {
  }
  class Name {
    constructor() {
    }
    publicMethod() {
      _privateHello()
    }
  }
  return Name;
})();

II. WeakMap object can be good for memoryleak trouble.

Stored variables in the WeakMap will be removed when the instance will be removed. Check this article. (Managing the private data of ES6 classes)

let Name = (function() {
  const _privateName = new WeakMap();
})();

III. Let's put all together.

let Name = (function() {
  const _privateName = new WeakMap();
  const _privateHello = function(fullName) {
    console.log("Hello, " + fullName);
  }

  class Name {
    constructor(firstName, lastName) {
      _privateName.set(this, {firstName: firstName, lastName: lastName});
    }
    static printName(name) {
      let privateName = _privateName.get(name);
      let _fullname = privateName.firstName + " " + privateName.lastName;
      _privateHello(_fullname);
    }
    printName() {
      let privateName = _privateName.get(this);
      let _fullname = privateName.firstName + " " + privateName.lastName;
      _privateHello(_fullname);
    }
  }

  return Name;
})();

var aMan = new Name("JH", "Son");
aMan.printName(); // "Hello, JH Son"
Name.printName(aMan); // "Hello, JH Son"