flatten.js

function flatten (arr) {
  if (!Array.isArray(arr)) {
    return arr
  }
  
  return arr.reduce((memo, elem) => {
    if (Array.isArray(elem)) {
      return memo.concat(flatten(elem))
    } else {
      return memo.concat(elem)
    }
  }, [])
}

// tests
function isArrayEqual (arr1, arr2) {
  if (!Array.isArray(arr1) || !Array.isArray(arr2)) {
    return false
  }
  return arr1.length === arr2.length && arr1.every((elem, idx) => elem === arr2[idx])
}

function assertEqual (arr1, arr2) {
  var result = isArrayEqual(arr1, arr2)
  console.assert(result, "assertEqual test failed")
  return result
}

assertEqual(flatten([[1], 2, [3, 4, [5]]]), [1, 2, 3, 4, 5])
assertEqual(flatten([[1,2,[3]],4]), [1, 2, 3, 4])
assertEqual(flatten([1, [2, 3], [4]]), [1, 2, 3, 4])