flattenArray.js

const assert = require('assert');

/**
 * flattenArray
 * 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 {array} arrayNested Array to be flattened
 * 
 * @return{result} returns the flattened array
 */

const flattenArray = arrayToBeFlattened => 
  arrayToBeFlattened.reduce((accFlattened, item) =>
    accFlattened.concat(Array.isArray(item) ? flattenArray(item) : item)
    ,[]
  )

/* ------------- TESTS -------------  */

assert(typeof flattenArray !== 'undefined', 'flattenArray should exist')
assert(typeof flattenArray == 'function', 'flattenArray should be a function')
assert((() => {
  const CASE1 = [[1,2,[3]],4]
  const CASE2 = [1,2,3,4]
  const CASE3 = [[[[[[1,2,3,4]]]]]]
  const EXPECTED_RESULT = JSON.stringify([ 1, 2, 3, 4 ])

  return [CASE1, CASE2, CASE3]
    .map(flattenArray)
    .map(JSON.stringify)
    .every(resultCase => resultCase === EXPECTED_RESULT)

})(), 'flattenArray should flatten an arbitrarily nested arrays of integers')