JS: Immutably insert value to an array

index.js

/**
 * Immutably insert attachments
 *
 * @example
 * insert([null, null, null, null, 5, null], 2, 9)
 * [null, null, 9, null, null, 5, null]
 *
 * @param {array} array
 * @param {number} index
 * @param {mixed} value
 * @return {array}
 */
export default function insert(array, index, value) {
  const left = array.slice(0, index);
  const right = array.slice(index);
  return [...left, value, ...right];
}