The JavaScript Array flat() method is used to flatten an array by reducing the nesting of sub-arrays into a single array. It returns a new array with all the sub-array elements concatenated into it recursively up to a specified depth. By default, the method flattens the array to a depth of 1.

For example, array.flat(depth) is useful when you need to convert a nested array structure into a single-level array.

Syntax

The syntax for the Array.flat() method is:

array.flat(depth)

Parameters

The Array.flat() method accepts one optional parameter:

• depth (Optional): The depth level specifying how deep the flattening should go. The default value is 1.

Return Value

A new array with the sub-array elements concatenated into it, flattened according to the specified depth.

Examples of JavaScript Array.flat() Method

Example 1: Flattening a Nested Array

The default behavior of flat() flattens the array by 1 level.

const arr = [1, 2, [3, 4]];
const flatArray = arr.flat();
console.log(flatArray);  // Output: [1, 2, 3, 4]

Explanation: The flat() method flattens the nested array [3, 4] by one level, returning a new array [1, 2, 3, 4].

Example 2: Specifying a Depth to Flatten

You can specify a depth to control how deeply the array should be flattened.

const arr = [1, 2, [3, 4, [5, 6]]];
const flatArray = arr.flat(2);
console.log(flatArray);  // Output: [1, 2, 3, 4, 5, 6]

Explanation: The flat(2) method flattens the array two levels deep, so the nested array [5, 6] is also flattened, resulting in a fully flattened array.

Example 3: Flattening an Array with Default Depth

When no depth is specified, the flat() method uses the default depth of 1.

const arr = [1, 2, [3, 4, [5, 6]]];
const flatArray = arr.flat();
console.log(flatArray);  // Output: [1, 2, 3, 4, [5, 6]]

Explanation: Since no depth is specified, the array is flattened by 1 level, leaving [5, 6] still nested.

Example 4: Flattening an Array with Infinite Depth

To flatten all levels of a deeply nested array, you can specify Infinity as the depth.

const arr = [1, 2, [3, 4, [5, 6, [7, 8]]]];
const flatArray = arr.flat(Infinity);
console.log(flatArray);  // Output: [1, 2, 3, 4, 5, 6, 7, 8]

Explanation: The flat(Infinity) method flattens the array to all levels, returning a fully flattened array [1, 2, 3, 4, 5, 6, 7, 8].

Example 5: Flattening an Array with Empty Slots

The flat() method removes empty slots in arrays, treating them as undefined.

const arr = [1, 2, , 4, [5, , 7]];
const flatArray = arr.flat();
console.log(flatArray);  // Output: [1, 2, 4, 5, 7]

Explanation: The flat() method removes empty slots (,) in both the main array and sub-arrays, resulting in a new array [1, 2, 4, 5, 7].