JavaScript flat method

last modified April 4, 2025

In this article we show how to flatten arrays using the flat method in JavaScript.

Array flattening

Array flattening is the process of reducing the dimensionality of a nested array. The flat method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

This method is useful when working with nested arrays and you need a single-level array. The flat method does not modify the original array but returns a new flattened array.

By default, flat only flattens one level deep. You can specify a depth parameter to control how many levels to flatten. To flatten all levels, use Infinity as the depth parameter.

Basic flat example

The following example demonstrates the basic usage of the flat method.

const arr = [1, 2, [3, 4]];
const flattened = arr.flat();

console.log(arr);        // Original array unchanged
console.log(flattened);  // New flattened array

We create a nested array and flatten it. The original array remains unmodified. The flat() method returns a new array with one level of nesting removed.

$ node main.js
[ 1, 2, [ 3, 4 ] ]
[ 1, 2, 3, 4 ]

Flattening multiple levels

The flat method can take a depth parameter to control flattening levels.

const arr = [1, [2, [3, [4, 5]]];
const flattened = arr.flat(2);

console.log(flattened);

We flatten an array with multiple levels of nesting. The depth parameter of 2 means we flatten two levels deep. The result still contains one level of nesting.

$ node main.js
[ 1, 2, 3, [ 4, 5 ] ]

Fully flattening an array

To completely flatten an array regardless of nesting depth, use Infinity.

const deeplyNested = [1, [2, [3, [4, [5]]]]];
const fullyFlattened = deeplyNested.flat(Infinity);

console.log(fullyFlattened);

We use Infinity as the depth parameter to flatten all levels of nesting. This works regardless of how deeply nested the original array is.

$ node main.js
[ 1, 2, 3, 4, 5 ]

Flat with empty slots

The flat method removes empty slots in arrays when flattening.

const arr = [1, 2, , 4, 5];
const flattened = arr.flat();

console.log(flattened);

When flattening an array with empty slots, those empty slots are removed in the result. This behavior is different from simply copying the array.

$ node main.js
[ 1, 2, 4, 5 ]

Combining flat with map

The flat method is often combined with map for complex transformations.

const arr = [1, 2, 3];
const result = arr.map(x => [x * 2]).flat();

console.log(result);

We first map each element to a new array, then flatten the result. This pattern is so common that there's a separate flatMap method for it.

$ node main.js
[ 2, 4, 6 ]

Source

Array flat - language reference

In this article we have demonstrated how to use the flat() method to flatten arrays in JavaScript.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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