core-js polyfills the Iterator helpers proposal (finished, ES2025) and the AsyncIterator helpers proposal (stage 2), both of which add functional-style methods directly to iterator objects.
Iterator helpers import 'core-js/actual/iterator' ;
Or import individual entry points:
import 'core-js/actual/iterator/from' ; import 'core-js/actual/iterator/concat' ; import 'core-js/actual/iterator/drop' ; import 'core-js/actual/iterator/every' ; import 'core-js/actual/iterator/filter' ; import 'core-js/actual/iterator/find' ; import 'core-js/actual/iterator/flat-map' ; import 'core-js/actual/iterator/for-each' ; import 'core-js/actual/iterator/map' ; import 'core-js/actual/iterator/reduce' ; import 'core-js/actual/iterator/some' ; import 'core-js/actual/iterator/take' ; import 'core-js/actual/iterator/to-array' ;
Iterator.from Wraps any iterable or iterator object in an Iterator instance that has the helper methods.
import 'core-js/actual/iterator/from' ; // Wrap a custom iterator-like object: Iterator . from ({ next : () => ({ done: Math . random () > 0.9 , value: Math . random () * 10 | 0 }), }). toArray (); // => e.g. [7, 6, 3, 0, 2, 8] // Wrap a built-in iterable: Iterator . from ([ 1 , 2 , 3 ]). map ( x => x * 2 ). toArray (); // => [2, 4, 6]
Iterator.concat Lazily concatenates multiple iterables or iterators into a single iterator.
import 'core-js/actual/iterator/concat' ; Iterator . concat ( [ 0 , 1 ]. values (), [ 2 , 3 ], ( function* () { yield 4 ; yield 5 ; })() ). toArray (); // => [0, 1, 2, 3, 4, 5]
Prototype methods All iterator prototype methods are lazy — they return new iterators and do not consume the source until you pull values.
Transforms each value using a mapping function. [ 1 , 2 , 3 ]. values (). map ( x => x ** 2 ). toArray (); // => [1, 4, 9]
Keeps only values for which the predicate returns true. [ 1 , 2 , 3 , 4 , 5 ]. values (). filter ( x => x % 2 === 0 ). toArray (); // => [2, 4]
Limit or skip a given number of values. [ 1 , 2 , 3 , 4 , 5 ]. values (). take ( 3 ). toArray (); // => [1, 2, 3] [ 1 , 2 , 3 , 4 , 5 ]. values (). drop ( 2 ). toArray (); // => [3, 4, 5]
Maps each value to an iterable and flattens one level. [ 1 , 2 , 3 ]. values () . flatMap ( x => [ x , x * 10 ]) . toArray (); // => [1, 10, 2, 20, 3, 30]
Reduces the iterator to a single value. [ 1 , 2 , 3 , 4 ]. values (). reduce (( acc , x ) => acc + x , 0 ); // => 10
Consumes the iterator and collects all values into an array. [ 1 , 2 , 3 ]. values (). toArray (); // => [1, 2, 3]
Calls a function for each value, consuming the iterator. [ 1 , 2 , 3 ]. values (). forEach ( x => console . log ( x )); // logs 1, 2, 3
Short-circuit boolean queries. [ 1 , 2 , 3 ]. values (). some ( x => x > 2 ); // => true [ 1 , 2 , 3 ]. values (). every ( x => x > 0 ); // => true [ 1 , 2 , 3 ]. values (). find ( x => x > 1 ); // => 2
Chained example import 'core-js/actual/iterator' ; [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. values () . drop ( 1 ) . take ( 5 ) . filter ( it => it % 2 ) . map ( it => it ** 2 ) . toArray (); // => [9, 25]
In the pure core-js-pure variant, new %IteratorPrototype% methods are not added to the real prototype to avoid pollution. Use Iterator.from([]).map(fn) instead of [].values().map(fn).
AsyncIterator helpers import 'core-js/actual/async-iterator' ;
Or import individual entry points:
import 'core-js/actual/async-iterator/from' ; import 'core-js/actual/async-iterator/drop' ; import 'core-js/actual/async-iterator/every' ; import 'core-js/actual/async-iterator/filter' ; import 'core-js/actual/async-iterator/find' ; import 'core-js/actual/async-iterator/flat-map' ; import 'core-js/actual/async-iterator/for-each' ; import 'core-js/actual/async-iterator/map' ; import 'core-js/actual/async-iterator/reduce' ; import 'core-js/actual/async-iterator/some' ; import 'core-js/actual/async-iterator/take' ; import 'core-js/actual/async-iterator/to-array' ;
AsyncIterator helpers mirror the synchronous Iterator helpers but return Promise-based results and accept async callback functions.AsyncIterator.from Wraps an async iterable, sync iterable, or async iterator.
import 'core-js/actual/async-iterator/from' ; async function* generate () { yield 1 ; yield 2 ; yield 3 ; } await AsyncIterator . from ( generate ()). map ( x => x * 2 ). toArray (); // => [2, 4, 6]
toAsync Converts a synchronous iterator to an async one.
import 'core-js/actual/iterator/to-async' ; await [ 1 , 2 , 3 ]. values () . toAsync () . map ( async it => it ** 2 ) . toArray (); // => [1, 4, 9]
Chained async example import 'core-js/actual/async-iterator' ; await AsyncIterator . from ([ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]) . drop ( 1 ) . take ( 5 ) . filter ( it => it % 2 ) . map ( it => it ** 2 ) . toArray (); // => [9, 25]
Method signatures Method Return type Description AsyncIterator.from(iterable)AsyncIteratorWraps an iterable or async iterable .map(async fn)AsyncIteratorTransforms each value .filter(async fn)AsyncIteratorKeeps values matching predicate .take(n)AsyncIteratorYields at most n values .drop(n)AsyncIteratorSkips first n values .flatMap(async fn)AsyncIteratorMaps and flattens one level .reduce(async fn, init)Promise<any>Reduces to a single value .toArray()Promise<Array>Collects all values .forEach(async fn)Promise<void>Calls fn for each value .some(async fn)Promise<boolean>Short-circuit OR .every(async fn)Promise<boolean>Short-circuit AND .find(async fn)Promise<any>First matching value
In the pure variant, %AsyncIteratorPrototype% methods are available only on wrappers. Use AsyncIterator.from([]).map(fn) rather than [].values().toAsync().map(fn). To make async function* () instances instanceof AsyncIterator, configure the library: const configurator = require ( 'core-js/configurator' ); configurator ({ USE_FUNCTION_CONSTRUCTOR: true }); require ( 'core-js/actual/async-iterator' );
Iterator.range Iterator.range is a stage 2 proposal. It is available only via the full entry point.import 'core-js/full/iterator/range' ;
Generates a lazy numeric range iterator.
for ( const i of Iterator . range ( 1 , 10 )) { console . log ( i ); // => 1, 2, 3, 4, 5, 6, 7, 8, 9 } for ( const i of Iterator . range ( 1 , 10 , { step: 3 , inclusive: true })) { console . log ( i ); // => 1, 4, 7, 10 } // Collect to array: [ ... Iterator . range ( 0 , 5 )]; // => [0, 1, 2, 3, 4]
Iterator.range is not included in core-js/actual because it is still a stage 2 proposal. It requires core-js/full/iterator/range.