小程序怎麼設置默認值爲0，使用if便可

/*
author:咔咔
wechat:fangkangfk
*/
<view class='money' wx:if="{
  {sum == ''}}">應付：0元</view>
<view class='money' wx:if="{
  {sum != ''}}">應付：{
  {sum}}元</view>