flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4]

flatten.js

/**
 * flatten an array of arbitrarily nested arrays of integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].
 * @param {[number]} from array to be flatternd
 * @param {[number]} [to] array where integers are placed. Defaults to [], and it is used only for recursion.
 * @return {[number]} flattern array
 */
 
function flatten (from, to = []) {
  from.forEach(value => {
    if (Array.isArray(value)) {
      return flatten(value, to)
    }
    to.push(value)
  })
  return to
}

exports.flatten = flatten

flatten.test.js

// jest

const { flatten } = require('./flatten')

test('it returns a new array', () => {
  const from = [1, 2, 3, 4]
  const to = flatten(from)
  expect(to).not.toBe(from)
})

test('origin array is not modified', () => {
  const from = [1, 2, 3, 4, [[5, 6, 7], 8]]
  flatten(from)
  expect(from).toEqual([1, 2, 3, 4, [[5, 6, 7], 8]])
})

test('the array returned is flatten', () => {
  const to = flatten([1, 2, 3, 4, [[5, 6, 7], 8]])
  expect(to).toEqual([1, 2, 3, 4, 5, 6, 7, 8])
})

test('it does not crash with an empy array', () => {
  expect(() => flatten([])).not.toThrow()
})

test('it has no problems with deep empty arrays', () => {
  const to = flatten([1, 2, [], 3, 4, [[], 5]])
  expect(to).toEqual([1, 2, 3, 4, 5])
})

test('It flattern a very tough one', () => {
  const to = flatten([[[]], [[[1]]], [[2, [], 3]], [[4, [[], 5], [[[[],[[]]]]]]], [[[[[[[[[[6]]]]]]]]]]])
  expect(to).toEqual([1, 2, 3, 4, 5, 6])
})