How to make an iterator out of an ES6 class

You need to specify Symbol.iterator property for SomeClass which returns iterator for class instances. Iterator must have next() method, witch in turn returns object with done and value fields. Simplified example:

function SomeClass() {
  this._data = [1,2,3,4];
}

SomeClass.prototype[Symbol.iterator] = function() {
  var index = 0;
  var data  = this._data;

  return {
    next: function() {
      return { value: data[++index], done: !(index in data) }
    }
  };
};

Or using ES6 classes and arrow functions:

class SomeClass {
  constructor() {
    this._data = [1,2,3,4];
  }

  [Symbol.iterator]() {
    var index = -1;
    var data  = this._data;

    return {
      next: () => ({ value: data[++index], done: !(index in data) })
    };
  };
}

And usage:

var obj = new SomeClass();
for (var i of obj) { console.log(i) }

In your updated question you realized class iterator through generator function. You can do so, but you must understand that iterator COULD NOT BE a generator. Actually iterator in es6 is any object that has specific next() method

Define a suitable iterator method. For example:

class C {
  constructor() { this.a = [] }
  add(x) { this.a.push(x) }
  [Symbol.iterator]() { return this.a.values() }
}

Edit: Sample use:

let c = new C
c.add(1); c.add(2)
for (let i of c) console.log(i)