chain-able/src/deps/array/insert-at-index.js

interfaces that describe their intentions.

// http://stackoverflow.com/questions/7032550/javascript-insert-an-array-inside-another-array
// http://stackoverflow.com/questions/1348178/a-better-way-to-splice-an-array-into-an-array-in-javascript/41465578#41465578
// http://stackoverflow.com/questions/38060705/replace-element-at-specific-position-in-an-array-without-mutating-it
// function insertArrAt(array, index, arrayToInsert) {
//   // Array.prototype.splice.apply(array, [index, 0].concat(arrayToInsert))
//   // return array.slice.apply([index, 0].concat(arrayToInsert))
//   return array.slice(index, 0).apply([index, 0].concat(arrayToInsert))
//   return array
// }

module.exports = function insertAtIndex(arr, index, val) {
  if (index < arr.length) {
    return [...arr.slice(0, index), ...val, ...arr.slice(index + 1)]
  }
  else {
    return [...arr, ...Array(index - arr.length), ...val]
  }
}