Flatten array without using native `flat()`.

flatten.js

`use strict`
const { isArray } = Array

const deepFlatten = (array) => {
  if (!isArray(array)) {
    throw new Error(`Expecting array to process, instead got ${typeof array}.`)
  }

  return array.reduce((accumulator, element) =>
    [
      ...accumulator,
      ...(isArray(element) ? deepFlatten(element) : [element])
    ], [])
}

module.exports = deepFlatten

flatten.test.js

`use strict`
const deepFlatten = require('./deepFlatten')

describe(`Array Deep Flatten`, () => {
  it(`should throw if argument isn't array`, () => {
    expect(() => deepFlatten(null)).toThrow()
    expect(() => deepFlatten(0)).toThrow()
    expect(() => deepFlatten(`zzz`)).toThrow()
    expect(() => deepFlatten(new Set())).toThrow()
  })

  it(`should return empty and shallow arrays as is`, () => {
    expect(deepFlatten([])).toEqual([])
    expect(deepFlatten([1, 2, 3])).toEqual([1, 2, 3])
  })

  it(`should handle 2-level deep nesting`, () => {
    expect(deepFlatten([[1, [2]], 3])).toEqual([1, 2, 3])
  })

  it(`should handle 4-level deep nesting`, () => {
    expect(deepFlatten([[1, [2, [{}, [4]]]], 5]))
      .toEqual([1, 2, {}, 4, 5])
  })

  it(`should handle 8-level deep nesting`, () => {
    expect(deepFlatten([[1, [2, [3, [4, 5, 6, [7, [`8`, [null, []]]]]]]], 10]))
      .toEqual([1, 2, 3, 4, 5, 6, 7, `8`, null, 10])
  })
})