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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
}

Note

Technically, iterables must implement the Symbol.iterator method.

In JavaScript the following are iterables:

  • Strings
  • Arrays
  • Typed Arrays
  • Sets
  • Maps

Because their prototype objects have a Symbol.iterator method:

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:

FunctionDescription
filter()Returns an iterator containing elements that satisfy a filter function
map()Returns an iterator with all elements transformed by a map function
flatMap()Returns an iterator by mapping each element and then flattening the results
take()Returns an iterator that yields a specified number of elements
drop()Returns an iterator that skips a specified number of elements before yielding the rest
find()Returns the first element that satisfies a test function
reduce()Applies a reducer function against each element to reduce it to a single value
every()Returns true if all elements satisfy a test function
some()Returns true if at least one element satisfy a test function
forEach()Executes a function once for each element in the iterator.

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;
}
Try it Yourself »

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);
Try it Yourself »

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);
Try it Yourself »

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]);
Try it Yourself »

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);
Try it Yourself »

The drop() Method

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




The find() Method

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


The reduce() Method

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


The every() Method

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


The some() Method

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


The forEach() Method

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



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