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
// 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 |
|---|---|
| drop() | Returns an iterator that skips a specified number of elements before yielding the rest |
| every() | Returns true if all elements satisfy a test function |
| filter() | Returns an iterator containing elements that satisfy a filter function |
| find() | Returns the first element that satisfies a test function |
| flatMap() | Returns an iterator by mapping each element and then flattening the results |
| forEach() | Executes a function once for each element in the iterator. |
| from() | creates an iterator object from an iterable |
| map() | Returns an iterator with all elements transformed by a map function |
| reduce() | Applies a reducer function against each element to reduce it to a single value |
| some() | Returns true if at least one element satisfy a test function |
| take() | Returns an iterator that yields a specified number of elements |
The Iterator.from() Method
The Iterator.from()
creates an iterator object from an existing iterable or iterator object.
Example
const myIterator = Iterator.from([1, 2, 3]);
// Iterate over the elements
let text = "";
for (const x of myIterator) {
text += x;
}
The filter() Method
The filter() method
returns a new iterator containing elements that satisfy a filter function.
Example
const myIterator = Iterator.from([32, 33, 16, 40]);
// Filter the iterator
const filteredIterator = myIterator.filter(x => x > 18);
The map() Method
The map() method
returns a new iterator with all elements transformed by a map function.
Example
const myIterator = Iterator.from("123456789");
// Now you can use the map method
const mappedIterator = myIterator.map(x => x * 2);
The flatMap() Method
The flatMap() method
returns a new iterator by mapping each element and then flattening
the results into a single iterator.
Example
const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);
// Map the Iterator
const mappedIterator = myIterator.flatMap(x => [x, x * 10]);
The take() Method
The take() method
returns a new iterator that yields at most a specified number of elements.
Example
// Take the first five elements
const firstFive = myIterator.take(5);
The drop() Method
The drop() method
returns a new iterator that skips a specified number of elements before yielding the rest.
Example
const myIterator = Iterator.from([1, 2, 3, 4, 5, 6]);
// Remove the first five
const firstFive = myIterator.drop(5);
The find() Method
The find(fn) method
returns the first element that satisfies a test function.
Example
const myIterator = Iterator.from([3, 10, 18, 30, 20]);
// Find first greater than 18
let result = myIterator.find(x => x > 18);
The reduce() Method
The reduce() method
applies a reducer function against an accumulator and each element to reduce it to a single value.
Example
const myIterator = Iterator.from([175, 50, 25]);
// Reduce the Iterator
let result = myIterator.reduce(myFunc);
The every() Method
The every(fn) method
returns true if all elements in the iterator satisfy the provided test function.
Example
const myIterator = Iterator.from("123456789");
// Is every Element greater than 7?
let result = myIterator.every(x => x > 7);
The some() Method
The some() method
returns true if at least one element in the iterator satisfies the provided test function.
Example
const myIterator = Iterator.from("123456789");
// Is some Element greater than 7?
let result = myIterator.some(x => x > 7);
The forEach() Method
The forEach() method
executes a provided function once for each element in the iterator.
Example
const myIterator = Iterator.from("123456789");
// Iterate over all elements
let text = "";
myIterator.forEach (x => text += x);
Note
Iterators bring the iteration concept directly into the core JavaScript language and provide a mechanism for customizing the behavior of for...of.