Recursively flatten arrays in JavaScript

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Have you ever had to flatten an array? Did you run off to lodash or another functional library? When developing in JavaScript you’ll find having to flatten arrays is a common occurrence and not something an external library is needed for. In this post I’ll show you a couple methods to flatten arrays: starting with single level, then moving to multidimensional.

A quick side note: if your unaware of the term flattening an array, flattening an array will return a one dimensional array from a nested array.

Flatten a single level array using the spread operator

const flatten = a => [].concat(...a);

Flatten a single level array using apply

const flatten = a => [].concat.apply([], a);

Now we’ll head into flattening multidimensional arrays using recursion.

Using reduce

const flatten = a => a.reduce(
    (b, c) => b.concat(Array.isArray(c) ? flatten(c) : c), []

Using some

const flatten = a => a.some(Array.isArray)
  ? flatten([].concat(...a))
  : a;

-Happy coding