Iterator
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
An Iterator
object is an object that conforms to the iterator protocol by providing a next()
method that returns an iterator result object. All built-in iterators inherit from the Iterator
class. The Iterator
class provides a [Symbol.iterator]()
method that returns the iterator object itself, making the iterator also iterable. It also provides some helper methods for working with iterators.
Description
The following are all built-in JavaScript iterators:
- The Array Iterator returned by
Array.prototype.values()
,Array.prototype.keys()
,Array.prototype.entries()
,Array.prototype[Symbol.iterator]()
,TypedArray.prototype.values()
,TypedArray.prototype.keys()
,TypedArray.prototype.entries()
,TypedArray.prototype[Symbol.iterator]()
, andarguments[Symbol.iterator]()
. - The String Iterator returned by
String.prototype[Symbol.iterator]()
. - The Map Iterator returned by
Map.prototype.values()
,Map.prototype.keys()
,Map.prototype.entries()
, andMap.prototype[Symbol.iterator]()
. - The Set Iterator returned by
Set.prototype.values()
,Set.prototype.keys()
,Set.prototype.entries()
, andSet.prototype[Symbol.iterator]()
. - The RegExp String Iterator returned by
RegExp.prototype[Symbol.matchAll]()
andString.prototype.matchAll()
. - The
Generator
object returned by generator functions. - The Segments Iterator returned by the
[Symbol.iterator]()
method of theSegments
object returned byIntl.Segmenter.prototype.segment()
. - The Iterator Helper returned by iterator helper methods such as
Iterator.prototype.filter()
andIterator.prototype.map()
.
Web APIs may return iterators too. Some reuse core JavaScript iterators while others define their own iterators. For example:
- Array-Like objects such as
NodeList
return an Array Iterator from their respective methodskeys()
,values()
,entries()
, and[Symbol.iterator]()
. - Map-Like objects from Web APIs such as
Headers
return their own iterator type like Headers Iterator from their respective methodskeys()
,values()
,entries()
, and[Symbol.iterator]()
. - Set-Like objects from Web APIs such as
FontFaceSet
return their own iterator type like FontFaceSet Iterator from their respective methodskeys()
,values()
,entries()
, and[Symbol.iterator]()
.
Note: NodeIterator
and other old interfaces are named as such but do not conform to the iterator protocol or iterable protocol.
Each of these iterators have a distinct prototype object, which defines the next()
method used by the particular iterator. For example, all string iterator objects inherit from a hidden object StringIteratorPrototype
, which has a next()
method that iterates this string by code points. StringIteratorPrototype
also has a [Symbol.toStringTag]
property whose initial value is the string "String Iterator"
. This property is used in Object.prototype.toString()
. Similarly, other iterator prototypes also have their own [Symbol.toStringTag]
values, which are the same as the names given above.
All of these prototype objects inherit from Iterator.prototype
, which provides a [Symbol.iterator]()
method that returns the iterator object itself, making the iterator also iterable.
Iterator helpers
Note:
These methods are iterator helpers, not iterable helpers, because the only requirement for an object to be iterable is just the presence of a [Symbol.iterator]()
method. There is no shared prototype to install these methods on.
The Iterator
class itself provides some helper methods for working with iterators. For example, you may be tempted to do the following:
const nameToDeposit = new Map([
["Anne", 1000],
["Bert", 1500],
["Carl", 2000],
]);
const totalDeposit = [...nameToDeposit.values()].reduce((a, b) => a + b);
This first converts the iterator returned by Map.prototype.values()
to an array, then uses the Array.prototype.reduce()
method to calculate the sum. However, this both creates an intermediate array and iterates the array twice. Instead, you can use the reduce()
method of the iterator itself:
const totalDeposit = nameToDeposit.values().reduce((a, b) => a + b);
This method may be more efficient, especially memory-wise, because it only iterates the iterator once, without memorizing any intermediate values. Iterator helper methods are necessary to work with infinite iterators:
function* fibonacci() {
let current = 1;
let next = 1;
while (true) {
yield current;
[current, next] = [next, current + next];
}
}
const seq = fibonacci();
const firstThreeDigitTerm = seq.find((n) => n >= 100);
You cannot convert seq
to an array, because it is infinite. Instead, you can use the find()
method of the iterator itself, which only iterates seq
as far as necessary to find the first value that satisfies the condition.
You will find many iterator methods analogous to array methods, such as:
Iterator.prototype.drop()
and Iterator.prototype.take()
combined are somewhat analogous to Array.prototype.slice()
.
Among these methods, filter()
, flatMap()
, map()
, drop()
, and take()
return a new Iterator Helper object. The iterator helper is also an Iterator
instance, making the helper methods chainable. All iterator helper objects inherit from a common prototype object, which implements the iterator protocol:
next()
-
Calls the
next()
method of the underlying iterator, applies the helper method to the result, and returns the result. return()
-
Calls the
return()
method of the underlying iterator, and returns the result.
The iterator helper shares the same data source as the underlying iterator, so iterating the iterator helper causes the underlying iterator to be iterated as well. There is no way to "fork" an iterator to allow it to be iterated multiple times.
const it = [1, 2, 3].values();
const it2 = it.drop(0); // Essentially a copy
console.log(it.next().value); // 1
console.log(it2.next().value); // 2
console.log(it.next().value); // 3
Proper iterators
There are two kinds of "iterators": objects that conform to the iterator protocol (which, at its minimum, only requires the presence of a next()
method), and objects that inherit from the Iterator
class, which enjoy the helper methods. They do not entail each other — objects that inherit from Iterator
do not automatically become iterators, because the Iterator
class does not define a next()
method. Instead, the object needs to define a next()
method itself. A proper iterator is one that both conforms to the iterator protocol and inherits from Iterator
, and most code expect iterators to be proper iterators and iterables to return proper iterators. To create proper iterators, define a class that extends Iterator
, or use the Iterator.from()
method.
class MyIterator extends Iterator {
next() {
// …
}
}
const myIterator = Iterator.from({
next() {
// …
},
});
Constructor
Iterator()
-
Intended to be extended by other classes that create iterators. Throws an error when constructed by itself.
Static methods
Iterator.from()
-
Creates a new
Iterator
object from an iterator or iterable object.
Instance properties
These properties are defined on Iterator.prototype
and shared by all Iterator
instances.
Iterator.prototype.constructor
-
The constructor function that created the instance object. For
Iterator
instances, the initial value is theIterator
constructor. Iterator.prototype[Symbol.toStringTag]
-
The initial value of the
[Symbol.toStringTag]
property is the string"Iterator"
. This property is used inObject.prototype.toString()
.Note: Unlike the
[Symbol.toStringTag]
on most built-in classes,Iterator.prototype[Symbol.toStringTag]
is writable for web compatibility reasons.
Instance methods
Iterator.prototype.drop()
-
Returns a new iterator helper that skips the given number of elements at the start of this iterator.
Iterator.prototype.every()
-
Tests whether all elements produced by the iterator pass the test implemented by the provided function.
Iterator.prototype.filter()
-
Returns a new iterator helper that yields only those elements of the iterator for which the provided callback function returns
true
. Iterator.prototype.find()
-
Returns the first element produced by the iterator that satisfies the provided testing function. If no values satisfy the testing function,
undefined
is returned. Iterator.prototype.flatMap()
-
Returns a new iterator helper that takes each element in the original iterator, runs it through a mapping function, and yields elements returned by the mapping function (which are contained in another iterator or iterable).
Iterator.prototype.forEach()
-
Executes a provided function once for each element produced by the iterator.
Iterator.prototype.map()
-
Returns a new iterator helper that yields elements of the iterator, each transformed by a mapping function.
Iterator.prototype.reduce()
-
Executes a user-supplied "reducer" callback function on each element produced by the iterator, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements is a single value.
Iterator.prototype.some()
-
Tests whether at least one element in the iterator passes the test implemented by the provided function. It returns a boolean value.
Iterator.prototype.take()
-
Returns a new iterator helper that yields the given number of elements in this iterator and then terminates.
Iterator.prototype.toArray()
-
Creates a new
Array
instance populated with the elements yielded from the iterator. Iterator.prototype[Symbol.iterator]()
-
Returns the iterator object itself. This allows iterator objects to also be iterable.
Examples
Using an iterator as an iterable
All built-in iterators are also iterable, so you can use them in a for...of
loop:
const arrIterator = [1, 2, 3].values();
for (const value of arrIterator) {
console.log(value);
}
// Logs: 1, 2, 3
Specifications
Specification |
---|
ECMAScript Language Specification # sec-%iteratorprototype%-object |
Browser compatibility
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