TheflatMap() method ofIterator instances returns a newiterator helper object 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).

Syntax

flatMap(callbackFn)

Parameters

callbackFn

A function to execute for each element produced by the iterator. It should return an iterator or iterable that yields elements to be yielded byflatMap(). Note that unlikeArray.prototype.flatMap(), you cannot return single non-iterator/iterable values. The function is called with the following arguments:

element

The current element being processed in the array.

index

The index of the current element being processed in the array.

Return value

A newiterator helper object. The first time the iterator helper'snext() method is called, it callscallbackFn on the first element produced by the underlying iterator, and the return value, which should be an iterator or iterable, is yielded one-by-one by the iterator helper (likeyield*). The next element is fetched from the underlying iterator when the previous one returned bycallbackFn is completed. When the underlying iterator is completed, the iterator helper is also completed (thenext() method produces{ value: undefined, done: true }).

Exceptions

TypeError

Thrown ifcallbackFn returns a non-iterator/iterable value or a string primitive.

Description

flatMap accepts two kinds of return values fromcallbackFn: an iterator or iterable. They are handled in the same way asIterator.from(): if the return value is iterable, the[Symbol.iterator]() method is called and the return value is used; otherwise, the return value is treated as an iterator and itsnext() method is called.

[1, 2, 3]  .values()  .flatMap((x) => {    let itDone = false;    const it = {      next() {        if (itDone) {          return { value: undefined, done: true };        }        itDone = true;        return { value: x, done: false };      },    };    switch (x) {      case 1:        // An iterable that's not an iterator        return { [Symbol.iterator]: () => it };      case 2:        // An iterator that's not an iterable        return it;      case 3:        // An iterable iterator is treated as an iterable        return {          ...it,          [Symbol.iterator]() {            console.log("Symbol.iterator called");            return it;          },        };      default:        return undefined;    }  })  .toArray();// Logs "Symbol.iterator called"// Returns [1, 2, 3]

Examples

Merging maps

The following example merges twoMap objects into one:

const map1 = new Map([  ["a", 1],  ["b", 2],  ["c", 3],]);const map2 = new Map([  ["d", 4],  ["e", 5],  ["f", 6],]);const merged = new Map([map1, map2].values().flatMap((x) => x));console.log(merged.get("a")); // 1console.log(merged.get("e")); // 5

This avoids creating any temporary copies of the map's content. Note that the array[map1, map2] must first be converted to an iterator (usingArray.prototype.values()), becauseArray.prototype.flatMap() only flattens arrays, not iterables.

new Map([map1, map2].flatMap((x) => x)); // Map(1) {undefined => undefined}

Returning strings

Strings are iterable, butflatMap() specifically rejects string primitives returned fromcallbackFn, this is because the behavior of iterating by code points is often not what you want.

[1, 2, 3]  .values()  .flatMap((x) => String(x))  .toArray(); // TypeError: Iterator.prototype.flatMap called on non-object

You may want to wrap it in an array instead so the entire string is yielded as one:

[1, 2, 3]  .values()  .flatMap((x) => [String(x)])  .toArray(); // ['1', '2', '3']

Or, if the behavior of iterating by code points is intended, you can useIterator.from() to convert it to a proper iterator:

[1, 2, 3]  .values()  .flatMap((x) => Iterator.from(String(x * 10)))  .toArray();// ['1', '0', '2', '0', '3', '0']

Specifications

Browser compatibility

See also

• Polyfill ofIterator.prototype.flatMap incore-js
• es-shims polyfill ofIterator.prototype.flatMap