4. 实现千分位的format函数

[senfish]

   // 接收一个number，返回一个string
   function format(number) {
    
   }
  console.log(format(12345.7890)); // 12,345.789,0
  console.log(format(0.12345678));// 0.123,456,78
  console.log(format(123456)); // 123,456

[senfish]

  function format (number) {
     let str = number.toString();
     let [int, dec = ''] = str.split('.');
     let intStr = ''
     for (let i = int.length - 1; i >= 0; i--) {
        if ((int.length - i) % 3 === 0 && i !== 0) {
             intStr = ',' + int[i] + intStr;
        } else {
           intStr = int[i] + intStr;
        }
     }
    let decStr = '';
    if (dec.length > 0) {
       for (let i = 0; i < dec.length; i ++) {
          let sum = decStr + dec[i];
          if ((i + 1 )% 3 === 0) {
             decStr = sum + ',';
          } else {
            decStr = sum;
          }
       }
     }
   return decStr.length > 0 ? `${intStr}.${decStr}` : `${intStr}`;
};