将短语中除第n个字符以外的所有字符替换为"_“

function replaceCharacters(phrase, n) {
  let result = "";
  for (let i = 0; i < phrase.length; i++) {
    if (i !== n - 1) {
      result += "_";
    } else {
      result += phrase[i];
    }
  }
  return result;
}

console.log(replaceCharacters("将短语中除第n个字符以外的所有字符替换为_", 5));