format-with-thousands-separators.js

function formatWithThousandsSeparator(num) {
  let numAsString = num.toString();
  
  let characters = numAsString.split("").reverse();
  
  let parts = [];
  
  for (let i = 0; i < characters.length; i += 3) {
    let part = characters.slice(i, i + 3).reverse().join("");    
    
    parts.unshift(part);
  }
  
  return parts.join(",");
}