Its worth creating an Intl formatter outside of a loop, then applying it inside the loop - if you are formatting everything in the loop the same.

const formatter = Intl.NumberFormat(/* options */);
return items.map(item => {
   return {
      ...item,
      total: formatter.format(item.total)
   }
});

…Or however you choose to mutate an array.

Well, certainly it used to be. Not sure if that is still the case!