merge-lists.js

const firstList = [1, 2, 3, 5, 9, 10, 13, 16, 28];
const secondList = [5, 6, 7, 8, 11, 42, 11];

const maxLength = firstList.length > secondList.length ? firstList.length : secondList.length;

let finalList = []; 

for (let k = 0; k < maxLength; k++) {
  if (!firstList[k]) {
    finalList.push(secondList[k]);
  } else if (!secondList[k]) {
    finalList.push(firstList[k]);
  } else {
    finalList.push(firstList[k], secondList[k]);
  }
}

console.log(finalList);