//使用在dom中使用$event传对象
<input v-if="!item.tags" v-model="item.balance" @keydown="cleanMoney($event)" @blur="changeMoney($event)" v-on:keyup="changeBalance(item)" type="text" class="el-input__inner moneyinput" style="width: 150px;">

//使用for循环的方法
function formatNum(event){
 let str = event.currentTarget.value
 let newStr = "";
 let count = 0;
 
 if(str.indexOf(".")==-1){
  for(var i=str.length-1;i>=0;i--){
   if(count % 3 == 0 && count != 0){
    newStr = str.charAt(i) + "," + newStr;
   }else{
    newStr = str.charAt(i) + newStr;
   }
   count++;
  }
  str = newStr + ".00"; //自动补小数点后两位
  console.log(str)
  event.currentTarget.value = str //赋值
 }
 else
 {
  for(var i = str.indexOf(".")-1;i>=0;i--){
   if(count % 3 == 0 && count != 0){
    newStr = str.charAt(i) + "," + newStr; //碰到3的倍数则加上“,”号
   }else{
    newStr = str.charAt(i) + newStr; //逐个字符相接起来
   }
   count++;
  }
  str = newStr + (str + "00").substr((str + "00").indexOf("."),3);
  console.log(str)
  event.currentTarget.value = str    //赋值
 }
}
 

//使用正则的方法
function regexNum(event){
 let str = event.currentTarget.value
 var regex = /(\d)(?=(\d\d\d)+(?!\d))/g;
 
 if(str.indexOf(".") == -1){
 
  str= str.replace(regex,',') + '.00';
  console.log(str)
  event.currentTarget.value = str   //赋值
 
 }else{
  var newStr = str.split('.');
  var str_2 = newStr[0].replace(regex,',');
 
  if(newStr[1].length <= 1){ 
   //小数点后只有一位时
   str_2 = str_2 + '.' + newStr[1] +'0';
   console.log(str_2)
   event.currentTarget.value = str_2   //赋值
 
  }else if(newStr[1].length > 1){ 
   //小数点后两位以上时
   var decimals = newStr[1].substr(0,2);
   var srt_3 = str_2 + '.' + decimals;
   console.log(srt_3)
   event.currentTarget.value = str_3    //赋值
  }
 }
};