Flash cards
Review the key moves
What is the main idea behind JavaScript Iterators?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ (variable of iterable) {Put the learning moves in the order that makes the concept easiest to apply.
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
}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
// 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);Iterators bring the iteration concept directly into the core JavaScript language and provide a mechanism for customizing the behavior of for...of.