`[[1,2,[3]],4] => [1,2,3,4]` full example

list-spec.js

import { flatten } from './list'

test('Should return array if not an array', () => {
   assert(flatten(1)).toMatchObject([1])
})

test('Should return array if an array', () => {
   assert(flatten([1])).toMatchObject([1])
})

test('Should return array if an nested array', () => {
   assert(flatten([1, [2], [3, [4]]])).toMatchObject([1, 2, 3, 4])
})

list.js

export const flatten = (l) => l.reduce ? l.reduce((acc, x) => acc.concat(fn(x)), []) : [l]

recipient.js

import { flatten } from './list'

flatten([[1,2,[3]],4])