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

Answer 1

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
    }
}

Answer 2

I came up with what I feel is a much better solution allowing:

• no need for 'this._', that/self, weakmaps, symbols etc. Clear and straightforward 'class' code

• private variables and methods are really private and have the correct 'this' binding

• No use of 'this' at all which means clear code that is much less error prone

• public interface is clear and separated from the implementation as a proxy to private methods

• allows easy composition

with this you can do:

function Counter() {
  // public interface
  const proxy = {
    advance,  // advance counter and get new value
    reset,    // reset value
    value     // get value
  }
	
  // private variables and methods
  let count=0;
    
  function advance() {
    return ++count;
  }
    	
  function reset(newCount) {
    count=(newCount || 0);
  }
    	
  function value() {
    return count;
  }
    
  return proxy;
}
    	
let counter=Counter.New();
console.log(counter instanceof Counter); // true
counter.reset(100);
console.log('Counter next = '+counter.advance()); // 101
console.log(Object.getOwnPropertyNames(counter)); // ["advance", "reset", "value"]

<script src="https://cdn.rawgit.com/kofifus/New/7987670c/new.js"></script>

see New for the code and more elaborate examples including constructor and composition

Answer 3

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'