Have private properties & methods in ES6 classes [duplicate]

Answer 1

You can use symbols to give a sort-of private member.

const KEY = Symbol( 'key' )
const ANOTHER = Symbol( 'another' )

class Foo {
  constructor() {
    this[ KEY ] = 'private'
  }

  say() {
    console.log( this[ KEY ] )
  }

  [ ANOTHER ] = 'transpilation required'
}

The 2nd symbol is added to the class using a class field, this is only in proposal and will require transpilation to work anywhere, but the rest works in node and new browsers.

Answer 2

It seems like classes in ES6 doesn't explicitly provide anything to have private data or methods.

Correct. The class syntax is for normal classes with prototype methods. If you want private variables, you put them in the constructor as always:

class Deferred {
    constructor() {
        // initialise private data
        var isPending = true;
        var handlers = {
            resolve: [],
            reject: [],
            notify: []
        };

        // Private method
        function trigger(event, params) {
            ...
        }

        // initialise public properties
        this.promise = new Promise(this);

        // and create privileged methods
        this.resolve = trigger.bind(null, 'resolve');
        this.reject = trigger.bind(null, 'reject');
        this.notify = trigger.bind(null, 'notify');

        this.on = function(event, handler) {
            …
        };
    }
}