如何格式化数字以在千位数上添加空格，在万位数上不加空格，等等

function formatNumberWithSpace(number) {
  let str = number.toString();
  let length = str.length;
  
  if (length <= 3) {
    return str;
  }
  
  let formattedStr = '';
  let count = 0;
  
  for (let i = length - 1; i >= 0; i--) {
    formattedStr = str[i] + formattedStr;
    count++;
    
    if (count % 3 === 0 && count !== length) {
      formattedStr = ' ' + formattedStr;
    }
  }
  
  return formattedStr;
}

// 示例用法
let number = 12345678;
let formattedNumber = formatNumberWithSpace(number);
console.log(formattedNumber); // 输出 "12 345 678"