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 by flatMap(), or a single non-iterator/iterable value to be yielded. 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.

A new iterator helper. The first time the iterator helper's next() method is called, it calls callbackFn 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 (like yield*). The next element is fetched from the underlying iterator when the previous one returned by callbackFn is completed. When the underlying iterator is completed, the iterator helper is also completed (the next() method produces { value: undefined, done: true }).

TypeError

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

flatMap accepts two kinds of return values from callbackFn: an iterator or iterable. They are handled in the same way as Iterator.from(): if the return value is iterable, the @@iterator method is called and the return value is used; otherwise, the return value is treated as an iterator and its next() 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("@@iterator called");
            return it;
          },
        };
    }
  })
  .toArray();
// Logs "@@iterator called"
// Returns [1, 2, 3]

The following example merges two Map 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")); // 1
console.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 (using Array.prototype.values()), because Array.prototype.flatMap() only flattens arrays, not iterables.

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

Strings are iterable, but flatMap() specifically rejects string primitives returned from callbackFn, 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 use Iterator.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']

• Polyfill of Iterator.prototype.flatMap in core-js