Array

`Array.prototype.flat`

Creates a new array with all sub-array elements concatenated into it recursively up to the specified depth

Syntax

JavaScript

array.flat(depth?)

Parameters

Parameter	Type	Description
depth	number	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

Examples

Basic Usage

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

Practical Example

const deep = [1, [2, [3, [4]]]];
console.log(deep.flat(Infinity)); // [1, 2, 3, 4]

Advanced Usage

const withHoles = [1, , 3, , 5];
console.log(withHoles.flat()); // [1, 3, 5]

Understanding Array.prototype.flat

The Array.prototype.flat method in JavaScript creates a new array with all sub-array elements concatenated into it recursively up to the specified depth. It belongs to the Array object and is one of the most widely used methods for working with array values in modern JavaScript and TypeScript applications.

The method signature is array.flat(depth?). It accepts 1 parameter: depth. When called, it returns a new array with the sub-array elements concatenated. Understanding when and how to use flat() helps you write more expressive, readable code.

Common use cases for Array.prototype.flat include data transformation, input validation, API response processing, and building reusable utility functions. It works well alongside related methods like array-flatmap, array-concat, array-map, enabling you to chain operations together for complex data manipulation pipelines.

Browser support for Array.prototype.flat is excellent across all modern browsers including Chrome, Firefox, Safari, and Edge. It is also fully supported in Node.js and Deno. For older environments, transpilation with Babel or a polyfill may be needed.