Symbol.iterator.js

var myArray = [1,2,3];

// with `for of`
for(var value of myArray) {
    console.log(value);
}

// without `for of`
var _myArray = myArray[Symbol.iterator]();
while(var _iteration = _myArray.next()) {
    if (_iteration.done) {
        break;
    }
    var value = _iteration.value;
    console.log(value);
}


/**
----------------------------------------------
*/

class Collection {
  *[Symbol.iterator]() {
    var i = 0;
    while(this[i] !== undefined) {
      yield this[i];
      ++i;
    }
  }

}
var myCollection = new Collection();
myCollection[0] = 1;
myCollection[1] = 2;
for(var value of myCollection) {
    console.log(value); // 1, then 2
}