Array.prototype.flatMap()
=========================

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The **`flatMap()`** method of `Array` instances returns a new array formed by applying a given callback function to each element of the array, and then flattening the result by one level. It is identical to a `map()` followed by a `flat()` of depth 1 (`arr.map(...args).flat()`), but slightly more efficient than calling those two methods separately.

Try it
------

Syntax
------


```js
flatMap(callbackFn)
flatMap(callbackFn, thisArg)
```

### Parameters

`callbackFn`

A function to execute for each element in the array. It should return an array containing new elements of the new array, or a single non-array value to be added to the new array. 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.

`array`

The array `flatMap()` was called upon.


`thisArg` Optional

A value to use as `this` when executing `callbackFn`. See iterative methods.

### Return value

 A new array with each element being the result of the callback function and flattened
 by a depth of 1.

Description
-----------

The `flatMap()` method is an iterative method. See `Array.prototype.map()` for a detailed description of the callback function. The `flatMap()` method is identical to `map(callbackFn, thisArg)` followed by `flat(1)` — for each element, it produces an array of new elements, and concatenates the resulting arrays together to form a new array. Read the iterative methods section for more information about how these methods work in general.

The `flatMap()` method is generic. It only expects the `this` value to have a `length` property and integer-keyed properties. However, the value returned from `callbackFn` must be an array if it is to be flattened.

### Alternative

#### Pre-allocate and explicitly iterate


```js
const arr = [1, 2, 3, 4];

arr.flatMap((x) => [x, x \* 2]);
// is equivalent to
const n = arr.length;
const acc = new Array(n \* 2);
for (let i = 0; i < n; i++) {
  const x = arr[i];
  acc[i \* 2] = x;
  acc[i \* 2 + 1] = x \* 2;
}
// [1, 2, 2, 4, 3, 6, 4, 8]
```


 Note that in this particular case the `flatMap` approach is slower than the
 for-loop approach — due to the creation of temporary arrays that must be
 garbage collected, as well as the return array not needing to be frequently
 resized. However, `flatMap` may still be the correct solution in cases where
 its flexibility and readability are desired.

Examples
--------

### map() and flatMap()


```js
const arr1 = [1, 2, 3, 4];

arr1.map((x) => [x \* 2]);
// [[2], [4], [6], [8]]

arr1.flatMap((x) => [x \* 2]);
// [2, 4, 6, 8]

// only one level is flattened
arr1.flatMap((x) => [[x \* 2]]);
// [[2], [4], [6], [8]]
```


 While the above could have been achieved by using map itself, here is an example that
 better showcases the use of `flatMap()`.

Let's generate a list of words from a list of sentences.


```js
const arr1 = ["it's Sunny in", "", "California"];

arr1.map((x) => x.split(" "));
// [["it's","Sunny","in"],[""],["California"]]

arr1.flatMap((x) => x.split(" "));
// ["it's","Sunny","in", "", "California"]
```


Notice, the output list length can be different from the input list length.

### For adding and removing items during a map()

`flatMap` can be used as a way to add and remove items (modify the number of
 items) during a `map`. In other words, it allows you to map *many items to
many items* (by handling each input item separately), rather than always
 *one-to-one*. In this sense, it works like the opposite of filter.
 Return a 1-element array to keep the item, a multiple-element array to add items, or a
 0-element array to remove the item.


```js
// Let's say we want to remove all the negative numbers
// and split the odd numbers into an even number and a 1
const a = [5, 4, -3, 20, 17, -33, -4, 18];
// |\ \ x | | \ x x |
// [4,1, 4, 20, 16, 1, 18]

const result = a.flatMap((n) => {
  if (n < 0) {
    return [];
  }
  return n % 2 === 0 ? [n] : [n - 1, 1];
});
console.log(result); // [4, 1, 4, 20, 16, 1, 18]
```

### Using the third argument of callbackFn

The `array` argument is useful if you want to access another element in the array, especially when you don't have an existing variable that refers to the array. The following example first uses `filter()` to extract operational stations and then uses `flatMap()` to create a new array where each element contains a station and its next station. On the last station, it returns an empty array to exclude it from the final array.


```js
const stations = ["New Haven", "West Haven", "Milford (closed)", "Stratford"];
const line = stations
  .filter((name) => !name.endsWith("(closed)"))
  .flatMap((name, idx, arr) => {
    // Without the arr argument, there's no way to easily access the
    // intermediate array without saving it to a variable.
    if (idx === arr.length - 1) return []; // last station has no next station
    return [`${name} - ${arr[idx + 1]}`];
  });
console.log(line); // ['New Haven - West Haven', 'West Haven - Stratford']
```


The `array` argument is *not* the array that is being built — there is no way to access the array being built from the callback function.

### Using flatMap() on sparse arrays

The `callbackFn` won't be called for empty slots in the source array because `map()` doesn't, while `flat()` ignores empty slots in the returned arrays.


```js
console.log([1, 2, , 4, 5].flatMap((x) => [x, x \* 2])); // [1, 2, 2, 4, 4, 8, 5, 10]
console.log([1, 2, 3, 4].flatMap((x) => [, x \* 2])); // [2, 4, 6, 8]
```

### Calling flatMap() on non-array objects

The `flatMap()` method reads the `length` property of `this` and then accesses each property whose key is a nonnegative integer less than `length`. If the return value of the callback function is not an array, it is always directly appended to the result array.


```js
const arrayLike = {
  length: 3,
  0: 1,
  1: 2,
  2: 3,
  3: 4, // ignored by flatMap() since length is 3
};
console.log(Array.prototype.flatMap.call(arrayLike, (x) => [x, x \* 2]));
// [1, 2, 2, 4, 3, 6]

// Array-like objects returned from the callback won't be flattened
console.log(
  Array.prototype.flatMap.call(arrayLike, (x) => ({
    length: 1,
    0: x,
  })),
);
// [ { '0': 1, length: 1 }, { '0': 2, length: 1 }, { '0': 3, length: 1 } ]
```

Specifications
--------------

| Specification |
| --- |
| ECMAScript Language Specification # sec-array.prototype.flatmap |

Browser compatibility
---------------------

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See also
--------

* Polyfill of `Array.prototype.flatMap` in `core-js`
* Indexed collections guide
* `Array`
* `Array.prototype.concat()`
* `Array.prototype.flat()`
* `Array.prototype.map()`
* `Array.prototype.reduce()`