Implementation of a prototypal `Array` method which, from both processed iterable structures, interlocks each item of same index, pairwise and repeatedly into a new array. Sparse locations on either side will be recognized and accordingly (re)assigned.

Array.interlock.js

/**
 *
 *  [Array.interlock.js]
 *
 *  Implementation of a prototypal `Array` method which,
 *  from both processed iterable structures, interlocks
 *  each item of same index, pairwise and repeatedly
 *  into a new array.
 *  Sparse locations on either side will be recognized
 *  and accordingly (re)assigned.
 *
 *  - `Array.prototype.interlock`
 *
 */

(function (Object, Array) {

  const arrPrototype = Array.prototype;
  const arrayFrom = Array.from;
  const { isArray } = Array;

  function interlockIterableItems(leftList, rightList) {
    const arr = [];

    let idx = -1;
    let leftIdx = -1;
    let rightIdx = -1;

    const leftCount = leftList.length;
    const rightCount = rightList.length;
    const overallCount = (leftCount + rightCount);

    arr.length = overallCount;

    /* eslint-disable no-plusplus */
    while ((idx + 1) < overallCount) {
      // console.log("within while :: 1st part :: idx, overallCount : ", idx, overallCount);

      if (++leftIdx < leftCount) {
        ++idx;

        // eslint-disable-next-line no-prototype-builtins
        if (leftList.hasOwnProperty(leftIdx)) { // take *sparse array* into account.

          arr[idx] = leftList[leftIdx];
        }
        // console.log("within A condition :: leftIdx, leftCount, arr : ", leftIdx, leftCount, arr);
      }

      // console.log("within while :: 2nd part :: idx, overallCount : ", idx, overallCount);

      if (++rightIdx < rightCount) {
        ++idx;

        // eslint-disable-next-line no-prototype-builtins
        if (rightList.hasOwnProperty(rightIdx)) { // take *sparse array* into account.

          arr[idx] = rightList[rightIdx];
        }
        // console.log("within B condition :: rightIdx, rightCount, arr : ", rightIdx, rightCount, arr);
      }
    }
    /* eslint-enable no-plusplus */

    return arr;
  }

  function interlock(iterable) {
    /*
     *  do not immediately "array from" at expected iterable structures
     *  since `Array.from` does not preserve empty positions of sparse
     *  arrays, but (re)assigns such positions the undefined value.
     */
    const leftList = (isArray(this) && this) || arrayFrom(this ?? []);
    const rightList = (isArray(iterable) && iterable) || arrayFrom(iterable ?? []);

    return interlockIterableItems(leftList, rightList);
  }

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

  // // provide static implementation as well.
  //
  // function staticInterlock(arr, condition, target) {
  //   return interlock.call(arr, condition, target);
  // }
  //
  // Object.defineProperty(Array, 'interlock', {
  //   configurable: true,
  //   writable: true,
  //   value: staticInterlock
  // });
  // Object.defineProperty(Array.interlock, 'toString', {
  //   value: () => 'function interlock() { [custom code] }'
  // });

  // return Array;

}(Object, Array));


console.log('[1,2,3].interlock("a");', [1,2,3].interlock("a"));
console.log('[1,2,3].interlock(["a"]);', [1,2,3].interlock(["a"]));
console.log('[1,2,3].interlock("a").join("");', [1,2,3].interlock("a").join(""));
console.log('[1,2,3].interlock(["a"]).join("");', [1,2,3].interlock(["a"]).join(""));

console.log('[1,2,3].interlock("ab");', [1,2,3].interlock("ab"));
console.log('[1,2,3].interlock(["a", "b"]);', [1,2,3].interlock(["a", "b"]));
console.log('[1,2,3].interlock("ab").join("");', [1,2,3].interlock("ab").join(""));
console.log('[1,2,3].interlock(["a", "b"]).join("");', [1,2,3].interlock(["a", "b"]).join(""));

console.log('[1,2,3].interlock("abc");', [1,2,3].interlock("abc"));
console.log('[1,2,3].interlock(["a", "b", "c"]);', [1,2,3].interlock(["a", "b", "c"]));
console.log('[1,2,3].interlock("abc").join("");', [1,2,3].interlock("abc").join(""));
console.log('[1,2,3].interlock(["a", "b", "c"]).join("");', [1,2,3].interlock(["a", "b", "c"]).join(""));


console.log("[1,3,5,7,9,11,13].interlock([2,4,6]);", `${[1,3,5,7,9,11,13].interlock([2,4,6])}`);
console.log("[2,4,6].interlock([1,3,5,7,9,11,13]);", `${[2,4,6].interlock([1,3,5,7,9,11,13])}`);

/* eslint-disable no-sparse-arrays */
console.log("[2,,,4,,,6].interlock([1,3,5,7,9,11,13,]);", `${[2,,,4,,,6].interlock([1,3,5,7,9,11,13,])}`);
console.log("[2,,,4,,,6].interlock([1,3,5,7,9,11,13,,]);", `${[2,,,4,,,6].interlock([1,3,5,7,9,11,13,,])}`);
console.log("[2,,,4,,,6].interlock([1,3,5,7,9,11,13,,,]);", `${[2,,,4,,,6].interlock([1,3,5,7,9,11,13,,,])}`);
console.log("[2,,,4,,,6].interlock([1,3,,5,7,,9,,11,13,,,]);", `${[2,,,4,,,6].interlock([1,3,,5,7,,9,,11,13,,,])}`);
console.log("[2,,,4,,,6].interlock([1,,3,,5,,7,,,9,,,11,13,,,]);", `${[2,,,4,,,6].interlock([1,,3,,5,,7,,,9,,,11,13,,,])}`);
/* eslint-enable no-sparse-arrays */