// common functions
const plus = (a, b) => a + b
const minus = (a, b) => a - b
const len = array => array.length
const condNext = (start, end, list) => plus(start, end) <= len(list) - 1 ? plus(start, end) : len(list) - 1
const condPrev = (start, end) => minus(start, end) >= 0 ? minus(start, end) : 0

// closure main function
const get = list => {
  return {
    next: (start = 0, end = 1) =>
            list[condNext(start, end, list)],

    prev: (start = 0, end = 1) =>
            list[condPrev(start, end)]
  }
}

// Elements
const arr = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
const arrObj = [
  {
    'field': '123'
  },
  {
    'field': '456'
  },
  {
    'field': '789'
  },
  {
    'field': '000'
  }
]

/* Usage
get(arr).next(0) returns the next array element, starting at position 0 -- return element at position 1
get(arr).next(1, 3) returns the next array element, starting at position 1 and ending at position 3 -- return element at position 4 (1 + 3)

get(arr).prev(1) return the previous array element, starting at position 1 -- return element at position 0
get(arr).prev(3, 1) return the previous array element, starting at position 3 and ending at position 1 -- return element at position 2 (3 - 1)
*/

// Tests
console.log(get(arr).prev(4, 3))
console.log(get(arr).next(5))

console.log(get(arrObj).prev(2, 2))
console.log(get(arrObj).next(1))

console.log(arr.map((el, i, _arr) => get(_arr).next(i, 1)))
console.log(arr.map((el, i, _arr) => get(_arr).prev(i, 1)))