Flatten Arrays

flatten.js

const flatten = (arr) => {
  const res = []

  const flattenRec = (elem) => {
    if (Array.isArray(elem) && elem.length > 0) {
      elem.forEach(e => flattenRec(e))
    } else {
      res.push(elem)
    }
  }

  if (Array.isArray(arr) && arr.length > 0) {
    flattenRec(arr, res)
    return res
  } else {
    return arr
  }
}

module.exports = flatten

package.json

{
  "name": "citrus-flatten",
  "version": "1.0.0",
  "description": "Flatten Arrays",
  "main": "flatten.js",
  "scripts": {
    "test": "node test.js"
  },
  "devDependencies": {
    "tape": "^4.9.0"
  }
}

test.js

const test = require('tape')
const flatten = require('./flatten')

test('Should only process Array elements', t => {
  t.plan(6)
  t.equal(flatten(undefined), undefined, 'Should not process undefined')
  t.equal(flatten(null), null, 'Should not process null')
  t.equal(flatten(1), 1, 'Should not process literals')
  t.equal(flatten('foo'), 'foo', 'Should not process literals')
  t.deepEqual(flatten({}), {}, 'Should not process objects')
  t.deepEqual(flatten([1,2,3]), [1,2,3], 'Should process Arrays')
})

test('Should flatten nested Arrays', t => {
  t.plan(1)
  t.deepEqual(
    flatten([[1,2], [3], 4, [[[6], 5], 20], [[[[null], undefined]]], null, undefined]),
    [1, 2, 3, 4, 6, 5, 20, null, undefined, null, undefined],
    'Should flatten Arrays'
  )
})