在字符串中每第n个字符后添加特殊字符

function addSpecialChar(str, n, specialChar) {
  let result = '';
  for (let i = 0; i < str.length; i++) {
    result += str[i];
    if ((i + 1) % n === 0) {
      result += specialChar;
    }
  }
  return result;
}

const inputString = 'This is a test string.';
const n = 3;
const specialChar = '#';

const outputString = addSpecialChar(inputString, n, specialChar);
console.log(outputString);