ES6 non-mutating array manipulation helpers

arrutils.js

export const replace = (arr, index, value) => [
  ...arr.slice(0, index),
  value,
  ...arr.slice(index + 1)
]

export const insert = (arr, index, value) => [
  ...arr.slice(0, index),
  value,
  ...arr.slice(index)
]

export const remove = (arr, index) => [
  ...arr.slice(0, index),
  ...arr.slice(index + 1)
]

export const shiftUp = (arr, index) => [
  ...arr.slice(0, index - 1),
  arr[index],
  ...arr.slice(index - 1, index),
  ...arr.slice(index + 1)
]

export const shiftDown = (arr, index) => [
  ...arr.slice(0, index),
  ...arr.slice(index + 1, index + 2),
  arr[index],
  ...arr.slice(index + 2)
]

export const interleave = (arr, sep) => arr.reduce((arr, value) => {
  if (arr.length) arr.push(sep)
  arr.push(value)
  return arr
}, [])

Bonus:

export const head = (arr) => [arr[0], arr.slice(1)]

export const tail = (arr) => [arr[arr.length - 1], arr.slice(0, arr.length - 1)]