How to flatten an array in JavaScript

In this Article we will go through how to flatten an array only using single line of code in JavaScript. This is a one-line JavaScript code snippet that uses one of the most popular ES6 features => Arrow Function.

Let's define this short function:

const flat = arr => [].concat.apply([], arr.map(a => Array.isArray(a) ? flat(a) : a));

const flat = arr => arr.reduce((a, b) => Array.isArray(b) ? [...a, ...flat(b)] : [...a, b], []);

// See the browser compatibility at https://caniuse.com/#feat=array-flat
const flat = arr => arr.flat();

flat(['cat', ['lion', 'tiger']]);   // ['cat', 'lion', 'tiger']