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
falseif there are more elements to iterate over, otherwise it returnstrue.
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:
| Function | Description |
|---|---|
| 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.
