Array.nthItem.js

const
  arrPrototype = Object.getPrototypeOf([]);

const {
  from: arrayFrom,
  isArray,

} = Array;

function isFunction(type) {
  return (
       (typeof type === 'function')
    && (typeof type.call === 'function')
    && (typeof type.apply === 'function')
  );
}

function getSanitizedTarget(type) {
  return (type ?? null);
}

function getNotationGroup(notation) {
  // - inspired by `:nth-child()` CSS pseudo-class ... [https://developer.mozilla.org/en-US/docs/Web/CSS/:nth-child]
  //
  // - an array's `nth-item` notation ... [https://regex101.com/r/tZmeZS/1/]
  //
  const regXNthItemNotation = (/^(?<nthBase>[+-]?\d+)(?:\*?(?<series>n)(?<crementer>[+-]\d+)?)?$/);
  const regXSanitizeNotation = (/\s+/g);

  return String(notation)
    .replace(regXSanitizeNotation, '')
    .match(regXNthItemNotation)
    ?.groups;
}

function getNthIndexSeriesDescending(getNthIndex, maximumIndex) {
  const nthIndexList = [];
  let nthIndex;
  let n = -1;
  while ((nthIndex = getNthIndex(++n)) >= 0) {
    if (nthIndex <= maximumIndex) {

      nthIndexList.push(nthIndex);
    }
  }
  return nthIndexList;
}
function getNthIndexSeriesAscending(getNthIndex, maximumIndex) {
  const nthIndexList = [];
  let nthIndex;
  let n = -1;
  while ((nthIndex = getNthIndex(++n)) <= maximumIndex) {
    if (nthIndex >= 0) {

      nthIndexList.push(nthIndex);
    }
  }
  return nthIndexList;
}

function getNthIndexSeriesList(notationGroup, maximumIndex) {
  const nthIndexList = [];
  let { series, nthBase, crementer } = notationGroup;

  nthBase = Number(nthBase);
  crementer = Number(crementer ?? 0);

  const getNthIndex = n => ((nthBase * n) + crementer);

  if (!!series) {
    if (nthBase !== 0) {

      const isDescending = (getNthIndex(0) > getNthIndex(1));
      nthIndexList.push(

        ...isDescending
        ? getNthIndexSeriesDescending(getNthIndex, maximumIndex)
        : getNthIndexSeriesAscending(getNthIndex, maximumIndex)
      );
    } else if (crementer >= 0) {

      // `crementer` in this case equals the result of the first nth-index computation.
      nthIndexList.push(crementer);
    }
  } else if ((nthBase >= 0) && (nthBase <= maximumIndex)) {

    // just the `nthBase` as the sole nth-index item.
    nthIndexList.push(nthBase);
  }
  return nthIndexList;
}

function nthItem(notation, callback, target) {
  let result = [];

  const notationGroup = getNotationGroup(notation);
  const arr = notationGroup && ((isArray(this) && this) || arrayFrom(this ?? []));

  // fail silently.
  const nthIndexList = arr && isFunction(callback) && getNthIndexSeriesList(notationGroup, (arr.length - 1));
  if (nthIndexList && nthIndexList.length) {

    target = getSanitizedTarget(target);
    result = nthIndexList.map(nthIndex =>

      callback.call(target, arr[nthIndex], nthIndex, arr)
    );
  }
  return result;
}

Object.defineProperty(arrPrototype, 'nthItem', {
  configurable: true,
  writable: true,
  value: nthItem
});
Object.defineProperty(arrPrototype.nthItem, 'toString', {
  value: () => 'function nthItem() { [custom code] }'
});

// export default Array;

/**
 *  Get every nth element of an array
 *  [https://stackoverflow.com/questions/68588326/get-every-nth-element-of-an-array/68593956#68593956]
 */

/* const cells = [
  'texta1', 'texta2', 'texta3', 'texta4', 'texta5', 'texta6', 'buttona',
  'textb1', 'textb2', 'textb3', 'textb4', 'textb5', 'textb6', 'buttonb',
  'textc1', 'textc2', 'textc3', 'textc4', 'textc5', 'textc6', 'buttonc',
];

function processItem(nthItem, nthIndex, array) {
  console.log({ nthItem, nthIndex });
  
  // - any passed argument can be processed.
  // - this callback is aware of it's `this` context.
  // - the callback's return value will be mapped to the
  //   `nthItem`'s internal array of to be processed indices
  //   which also, entirely mapped, is the `nthItem`'s return value.

  return nthItem;
}
console.log(
  "cells.nthItem('7n-1', processItem) ...",
  cells.nthItem('7n-1', processItem)
);
console.log(
  "cells.nthItem('7n-5', processItem) ...",
  cells.nthItem('7n-5', processItem)
); */