JavaScript Iterators

The Iterator Object

An Iterator is an object that provides a standard way to access elements sequentially.

An Iterator must adheres to the Iterator Protocol: It must have a next() method.

The next() Method

The next() method returns an object with two properties:

• The value property holds the next value in the iteration sequence.
• The done property returns false if there are more elements to iterate over, otherwise it returns true.

The For Of Loop

The JavaScript for..of statement loops through the elements of an iterable object.

Syntax

for (variable of iterable) {

  // code block to be executed

}

Iterators provide a controlled way to work with data sequences, enabling custom iteration logic for various data structures.

They are particularly useful for handling streams of data, lazy computation of values, and building custom data structures with defined iteration behaviors.

Helper Functions

JavaScript 2025 (ECMAScript 2025) officially approved a set of new Iterator Helper methods that significantly enhance the functionality of iterators in JavaScript.

The methods provide a more functional and efficient way to work with iterable objects, including generators, by allowing direct manipulation and transformation without first converting them to arrays:

The Iterator.from() Method

The Iterator.from() creates an iterator object from an existing iterable or iterator object.

Example

// Create an iterator

const myIterator = Iterator.from([1, 2, 3]);

// Iterate over the elements

let text = "";

for (const x of myIterator) {

  text += x;

}

The drop() Method

The drop() method returns a new iterator that skips a specified number of elements before yielding the rest.

Example

// Create an iterator

const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);

// Remove the first five

const firstFive = myIterator.drop(5);

The every() Method

The every(fn) method returns true if all elements in the iterator satisfy the provided test function.

Example

// Create an Iterator

const myIterator = Iterator.from("123456789");

// Is every Element greater than 7?

let result = myIterator.every(x => x > 7);

The filter() Method

The filter() method returns a new iterator containing elements that satisfy a filter function.

Example

// Create an iterator

const myIterator = Iterator.from([32, 33, 16, 40]);

// Filter the iterator

const filteredIterator = myIterator.filter(x => x > 18);

The find() Method

The find(fn) method returns the first element that satisfies a test function.

Example

// Create an iterator

const myIterator = Iterator.from([3, 10, 18, 30, 20]);

// Find first greater than 18

let result = myIterator.find(x => x > 18);

The flatMap() Method

The flatMap() method returns a new iterator by mapping each element and then flattening the results into a single iterator.

Example

// Create an iterator

const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);

// Map the Iterator

const mappedIterator = myIterator.flatMap(x => [x, x * 10]);

The forEach() Method

The forEach() method executes a provided function once for each element in the iterator.

Example

// Create an iterator

const myIterator = Iterator.from("123456789");

// Iterate over all elements

let text = "";

myIterator.forEach (x => text += x);

The map() Method

The map() method returns a new iterator with all elements transformed by a map function.

Example

// Create an iterator

const myIterator = Iterator.from("123456789");

// Now you can use the map method

const mappedIterator = myIterator.map(x => x * 2);

The reduce() Method

The reduce() method applies a reducer function against an accumulator and each element to reduce it to a single value.

Example

// Create an Iterator

const myIterator = Iterator.from([175, 50, 25]);

// Reduce the Iterator

let result = myIterator.reduce(myFunc);

The some() Method

The some() method returns true if at least one element in the iterator satisfies the provided test function.

Example

// Create an Iterator

const myIterator = Iterator.from("123456789");

// Is some Element greater than 7?

let result = myIterator.some(x => x > 7);

The take() Method

The take() method returns a new iterator that yields at most a specified number of elements.

Example

const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);

// Take the first five elements

const firstFive = myIterator.take(5);