Array.prototype.flat()
======================

Baseline Widely available
-------------------------

This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.

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 The **`flat()`** method of `Array` instances creates a new array with all sub-array
 elements concatenated into it recursively up to the specified depth.

Try it
------

Syntax
------


```js
flat()
flat(depth)
```

### Parameters

`depth` Optional

 The depth level specifying how deep a nested array structure should be flattened.
 Defaults to 1.


### Return value

A new array with the sub-array elements concatenated into it.

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

The `flat()` method is a copying method. It does not alter `this` but instead returns a shallow copy that contains the same elements as the ones from the original array.

The `flat()` method ignores empty slots if the array being flattened is sparse. For example, if `depth` is 1, both empty slots in the root array and in the first level of nested arrays are ignored, but empty slots in further nested arrays are preserved with the arrays themselves.

The `flat()` method is generic. It only expects the `this` value to have a `length` property and integer-keyed properties. However, its elements must be arrays if they are to be flattened.

Examples
--------

### Flattening nested arrays


```js
const arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]

const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]

const arr3 = [1, 2, [3, 4, [5, 6]]];
arr3.flat(2);
// [1, 2, 3, 4, 5, 6]

const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
```

### Using flat() on sparse arrays

The `flat()` method removes empty slots in arrays:


```js
const arr5 = [1, 2, , 4, 5];
console.log(arr5.flat()); // [1, 2, 4, 5]

const array = [1, , 3, ["a", , "c"]];
console.log(array.flat()); // [ 1, 3, "a", "c" ]

const array2 = [1, , 3, ["a", , ["d", , "e"]]];
console.log(array2.flat()); // [ 1, 3, "a", ["d", empty, "e"] ]
console.log(array2.flat(2)); // [ 1, 3, "a", "d", "e"]
```

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

The `flat()` method reads the `length` property of `this` and then accesses each property whose key is a nonnegative integer less than `length`. If the element is not an array, it's directly appended to the result. If the element is an array, it's flattened according to the `depth` parameter.


```js
const arrayLike = {
  length: 3,
  0: [1, 2],
  // Array-like objects aren't flattened
  1: { length: 2, 0: 3, 1: 4 },
  2: 5,
  3: 3, // ignored by flat() since length is 3
};
console.log(Array.prototype.flat.call(arrayLike));
// [ 1, 2, { '0': 3, '1': 4, length: 2 }, 5 ]
```

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

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

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

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See also
--------

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