js 将数字串转换成带逗号的显示方式

将数字转换成带逗号的显示方式，例： 123,456,789

formatNum(num){
      // 将数字串转换成带逗号的显示方式
      if(!/^([+\-])?(\d+)(\.\d+)?$/.test(num)){
        alert("wrong!");
        return num;
      }
      let a = RegExp.$1;
      let b = RegExp.$2;
      let c = RegExp.$3;
      let re = new RegExp().compile("(\\d)(\\d{3})(,|$)");
      while(re.test(b))
        b = b.replace(re,"$1,$2$3");
      return a +""+ b +""+ c;
    },

运行结果