Flatten array

flatten.js

const assert = require("assert");

export const flattenArray = arr =>
  arr.reduce((accumulator, current)=>(
    accumulator.concat(
      Array.isArray(current) ? flattenArray(current) : current
    )
  ), [])


assert.deepEqual(flattenArray([1, 2, [3, 4, 5], 6, 7, [8]]), [1, 2, 3, 4, 5, 6, 7, 8]);
assert.deepEqual(flattenArray([1, 2, [3, [4, 5]], 6, 7, [8]]), [1, 2, 3, 4, 5, 6, 7, 8]);
assert.deepEqual(flattenArray([1, 2, [3, [4, 5]], 6, 7, [8, [9, [10]]]]), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
assert.deepEqual(flattenArray([[1], [[[[2]]]], [3, [4, [[5]]]], [6], 7, [8, [9, [10]]]]), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);