替换第N个分隔符之后的值

function replaceAfterNthSeparator(str, separator, replaceValue, n) {
  let count = 0;
  let index = -1;
  
  // 遍历字符串，记录分隔符的位置和出现次数
  while (count < n && (index = str.indexOf(separator, index + 1)) !== -1) {
    count++;
  }
  
  // 如果找到第N个分隔符
  if (count === n) {
    // 替换分隔符后的值
    const newValue = str.substring(0, index + 1) + replaceValue + str.substring(str.indexOf(separator, index + 1));
    
    return newValue;
  }
  
  // 如果没有找到第N个分隔符，则返回原始字符串
  return str;
}

const inputString = "a,b,c,d,e,f,g";
const separator = ",";
const replaceValue = "REPLACED";
const N = 3;

const outputString = replaceAfterNthSeparator(inputString, separator, replaceValue, N);
console.log(outputString); // 输出： "a,b,c,REPLACED,e,f,g"