• The Array Iterator returned by Array.prototype[Symbol.iterator](), TypedArray.prototype[Symbol.iterator](), and arguments[Symbol.iterator]().
• The String Iterator returned by String.prototype[Symbol.iterator]().
• The Map Iterator returned by Map.prototype[Symbol.iterator]().
• The Set Iterator returned by Set.prototype[Symbol.iterator]().
• The RegExp String Iterator returned by String.prototype.matchAll().
• The generator functions.
• The Segments Iterator returned by the Intl.Segmenter.prototype.segment().
• The Iterator Helper returned by iterator helper methods such as Iterator.prototype.map().

Web APIs may return iterators too. Some reuse core JavaScript iterators while others define their own iterators. For example:

• NodeList return an Array Iterator from their respective methods keys(), values(), entries(), and [Symbol.iterator]().
• Headers return their own iterator type like Headers Iterator from their respective methods keys(), values(), entries(), and [Symbol.iterator]().
• FontFaceSet return their own iterator type like FontFaceSet Iterator from their respective methods keys(), values(), entries(), and [Symbol.iterator]().

Each of these iterators have a distinct prototype object, which defines the next() method used by the particular iterator. For example, all string iterator objects inherit from a hidden object StringIteratorPrototype, which has a next() method that iterates this string by code points. StringIteratorPrototype also has a Object.prototype.toString(). Similarly, other iterator prototypes also have their own [Symbol.toStringTag] values, which are the same as the names given above.

All of these prototype objects inherit from Iterator.prototype, which provides a iterable.

The Iterator class itself provides some helper methods for working with iterators. For example, you may be tempted to do the following:

const nameToDeposit = new Map([
  ["Anne", 1000],
  ["Bert", 1500],
  ["Carl", 2000],
]);

const totalDeposit = [...nameToDeposit.values()].reduce((a, b) => a + b);

This first converts the iterator returned by Array.prototype.reduce() method to calculate the sum. However, this both creates an intermediate array and iterates the array twice. Instead, you can use the reduce() method of the iterator itself:

const totalDeposit = nameToDeposit.values().reduce((a, b) => a + b);

This method may be more efficient, especially memory-wise, because it only iterates the iterator once, without memorizing any intermediate values. Iterator helper methods are necessary to work with infinite iterators:

function* fibonacci() {
  let current = 1;
  let next = 1;
  while (true) {
    yield current;
    [current, next] = [next, current + next];
  }
}

const seq = fibonacci();
const firstThreeDigitTerm = seq.find((n) => n >= 100);

You cannot convert seq to an array, because it is infinite. Instead, you can use the find() method of the iterator itself, which only iterates seq as far as necessary to find the first value that satisfies the condition.

You will find many iterator methods analogous to array methods, such as:

Array.prototype.slice().

Among the iterator helper methods, drop(), and take() return a new Iterator Helper object. The iterator helper is also an Iterator instance, making these helper methods chainable. All iterator helper objects inherit from a common prototype object, which implements the iterator protocol:

next()

Calls the next() method of the underlying iterator, applies the helper method to the result, and returns the result.

return()

Calls the return() method of the underlying iterator, and returns the result.

The iterator helper shares the same data source as the underlying iterator, so iterating the iterator helper causes the underlying iterator to be iterated as well. There is no way to "fork" an iterator to allow it to be iterated multiple times.

const it = [1, 2, 3].values();
const it2 = it.drop(0); / Essentially a copy
console.log(it.next().value); / 1
console.log(it2.next().value); / 2
console.log(it.next().value); / 3

Iterator.prototype.constructor

The constructor function that created the instance object. For Iterator instances, the initial value is the Iterator constructor.

Iterator.prototype[Symbol.toStringTag]

The initial value of the Object.prototype.toString().

Note: Unlike the [Symbol.toStringTag] on most built-in classes, Iterator.prototype[Symbol.toStringTag] is writable for web compatibility reasons.