array.js

function withIndex(list){
  for(let i = 0; i < list.length; i++){
    if(i % 5 !== 0){
      list[i].value = list[i].value * 2;
    }
  }
}

function withForEach(list){
  list.forEach((item, index) => {
    if(index % 5 !== 0){
      item.value = item.value * 2;
    }
  });
}

function withFilter(list){
  list.filter((item, index) => {
    return index % 5 !== 0
  }).forEach(item => {
    item.value = item.value * 2;
  });
}

class ListItem{
  constructor(value){
    this.value = value;
  }
  valueOf(){
    return this.value;
  }
  toString(){
    return this + "";
  }
}

function create(n){
  return Array(n).fill(0).map((_, index) => new ListItem(index));
}

function main(){
  const listA = create(10);
  withIndex(listA);
  console.log(listA.join(","));

  const listB = create(10);
  withForEach(listB);
  console.log(listB.join(","));

  const listC = create(10);
  withFilter(listC);
  console.log(listC.join(","));
}

main();