newId.js

//my-answer
function solution(new_id) {
  let answer = new_id
    .toLowerCase()
    .replace(/[^a-z0-9-_.]/gi, "")
    .replace(/[.]{2,}/gi, ".")
    .replace(/^[.]|[.]$/gi, "");
  if (answer === "") answer = "a";
  if (answer.length > 15) {
    answer = answer.substring(0, 15);
    answer = answer.replace(/[.]$/gi, "");
  }
  while (answer.length < 3) {
    answer += answer[answer.length - 1];
  }
  return answer;
}

//best-case
// const solution = (new_id) => {
//   const id = new_id
//       .toLowerCase()
//       .replace(/[^\w\d-_.]/g, '')
//       .replace(/\.{2,}/g, '.')
//       .replace(/^\.|\.$/g, '')
//       .padEnd(1, 'a')
//       .slice(0, 15)
//       .replace(/^\.|\.$/g, '')
//   return id.padEnd(3, id[id.length-1])
// }