DEV Community

Katherine Kelly
Katherine Kelly

Posted on • Updated on

Celebrating JavaScript .flat()

I’m a little late to the party and this was so last year, but JavaScript finally has a built-in array flattening method so I’m dedicating this post to flat(), a kind of belated one-year birthday celebration if you will.

pokemon birthday GIF

flat() and another array flattening method flatMap() were introduced with ECMAScript 2019 / ES10. Before this, you would have to flatten your array the old fashioned way of doing it yourself using a combination of reduce() and concat().

Note-I'll just be covering .flat() in this post, so keep an eye out for my upcoming post that will go into flatMap().

Browser and Node.js Version Compatibility

Even with these latest features, it doesn’t mean you’ll be able to use these methods everywhere yet. flat() is supported in the majority of web browsers, but not all and I’m sure you know which one I’m talking about (it rhymes with pinternet texplorer).

To check browser compatibility for any features, I head to Can I use.

Another thing to look out for is if your computer’s version of Node.js is out of date (guilty), flat() or any newer features will not work in your terminal’s Node.js REPL. This is also a plug to use the terminal REPL, aka Read-Evaluate-Print-Loop, as it's an easy way to test out Node features when you’re unsure about something. All you have to do is type in node and you’re on way to NodeLand. Google Chrome's DevTools Console is an excellent REPL as well.


The flat() method creates and returns a new array with all the sub-array elements concatenated onto it recursively up to the depth specified.


// depth is an optional parameter that specifies how deep the 
// nested array structure should be flattened. It is 1 by default.

If you don't specify a depth, it is 1 by default. A good trick I utilize to quickly determine the depth is count the number of opening brackets - 1. Or in a pinch with an undetermined level amount, you can use Infinity to capture the deepest level possible.

const arr1 = ['birthday', 'celebration', ['ice cream', 'cake']];
arr1.flat(); // ['birthday', 'celebration', 'ice cream', 'cake']

const arr2 = ['birthday', 'celebration', ['ice cream', ['cake']]];
arr2.flat(); // ['birthday', 'celebration', 'ice cream', ['cake']];
arr2.flat(2); // ['birthday', 'celebration', 'ice cream', 'cake'];

const arr3 = [2, [3, 4, 6, [7, 8, [9, 10, [11]]]]];
arr3.flat(4); // [2, 3, 4, 6, 7, 8, 9, 10, 11]

// passing in Infinity as parameter will capture the deepest level
arr3.flat(Infinity); // [2, 3, 4, 6, 7, 8, 9, 10, 11]

Use to Remove Empty Items in Array

You can also use flat() to easily remove empty items in arrays. This includes empty arrays

const arr4 = ['party', 'soirée', , 'bonanza'];
arr4.flat(); // ['party', 'soirée', 'bonanza']

const arr5 = ['fete', 'gala', [], , 'shindig'];
arr5.flat(); // ["fete", "gala", "shindig"]


Somewhere that comes to mind where I can’t use flat() as of writing is Leetcode. But never fear, you can flatten your array as you did before mid-2019: writing your own function using a mix of reduce(), concat(), and recursion.

Single-level Flatten

const arr6 = ['carrot cake', 'birthday cake', ['cheesecake', 'chocolate cake']];

arr6.reduce((acc, val) => acc.concat(val), []); 
// [ 'carrot cake', 'birthday cake', 'cheesecake', 'chocolate cake' ]

Deeper Flatten using Recursion

To get deep level flattening, you can use recursion with reduce(), concat(), and isArray(). Your function can take an optional parameter of depth level, and you can set it to 1 as the default. Below is the solution from MDN web docs.

const arr7 = ['carrot cake', ['birthday cake', ['cheesecake', ['chocolate cake']]]];

function flatDeep(arr, d = 1) {
  return d > 0 ? arr7.reduce((acc, val) => acc.concat(Array.isArray(val) ?
     flatDeep(val, d - 1) : val), []) : arr.slice();

// deepFlatten(arr7, 3); // ["carrot cake", "birthday cake", "cheesecake", "chocolate cake"]

flat() is a great addition to the JavaScript Array toolbox and I’ll be using this method going forward when supported. Thanks for following along and helping to celebrate flat()'s year of existence. Come back next week when when I perform a proper ceremony for flatMap(). Happy coding!


Top comments (2)

devtrong profile image
Trong Nguyen

Nice read :) As an additional alternative, i've always used Set to flatten my arrays.

const arr = ['carrot cake', 'birthday cake', ['cheesecake', 'chocolate cake']];
console.log([].concat( Set(arr)));
katkelly profile image
Katherine Kelly

Using a Set is a great alternative!