Iterator object is an object that obeys the Iterator Protocol.

Iterator Protocol :

A Standard way of producing a sequence of values.

These values can be finite or infinite.

An iterator object implements the next method with the following rules:

next-method next()

A function that accepts zero or one argument and returns an object with the following interface.

done - optional

It is a boolean value. When there are no more values to iterate or the stop condition has been reached it turns false.

value - optional

The next value during iteration

const fruits = ["apple", "oranges", "grapes", "watermelon", "papaya"];

function fruitsIterator(frValues) {
  let nextFruitIndex = 0;

  return {
    next: function () {
      if (nextFruitIndex < frValues.length) {
        return {
          value: frValues[nextFruitIndex++],
          done: false
        };
      } else {
        return {
          done: true
        };
      }
    }
  };
}

// using an iterator
const fruitsIt = fruitsIterator(fruits);

console.log(fruitsIt.next());
console.log(fruitsIt.next());
console.log(fruitsIt.next());
console.log(fruitsIt.next());
console.log(fruitsIt.next());
console.log(fruitsIt.next());

You can create your iterators and make them iterate as per any business logic.

All iterable items (Arrays, Maps, Sets etc) have [Symbol.iterator] property which can be used to design a custom iterator.

Objects do not have [Symbol.Iterator] property and hence we cannot use for .. of loop to iterate. We can use for .. in . (\for(key in object))*

const fruits = ["apple", "oranges", "grapes", "watermelon", "papaya"];
const evenIdxFruitsIterator = {
  [Symbol.iterator]: function () {
    let nextFruitIndex = 0;

    return {
      next: function () {
        if (nextFruitIndex < fruits.length) {
          let tempIdx = nextFruitIndex;
          nextFruitIndex += 2;
          return {
            value: fruits[tempIdx],
            done: false
          };
        } else {
          return {
            done: true
          };
        }
      }
    };
  }
};

const custIte = evenIdxFruitsIterator[Symbol.iterator]();

console.log(custIte.next()); //{value: "apple", done: false}
console.log(custIte.next()); //{value: "grapes", done: false}
console.log(custIte.next()); //{value: "papaya", done: false}
console.log(custIte.next()); //{done: true}
console.log(custIte.next()); //{done: true}