답안

function solution(n){
  let answer = '1';

  if(n === 1){
    return 1;
  }

  for (let i = 1; i < n; i++){
    answer = rule(answer);
  }

  return answer;
}

function rule(answer){
  let answerMax = 9;
  let result = '';

  for (let i = 1; i < answerMax; i++){
    let re = new RegExp(i, 'g');
    let count = (answer.match(re) || []).length;

    if(count >= 1){
      result = result + String(i) + String(count);
    } 
  }
  return result;
}

const user_input = 7;
console.log(solution(user_input));