如何使用“Object.keys”函数在vue.js中列出firebase子节点的长度时防止出现错误？(How to prevent an error when listing the length of a firebase child node in vue.js using an “Object.keys” function?)

 想象一下Vue.js应用程序，其中firebase中的数据结构将发展为如下形式： 
 
item: {
  name: itemName,
  childOne: {
    subChildA: true,
    subChildB: true
  },
  childTwo: {
    subChildA: true,
    subChildB: true
  }
}
 
 现在我有一个组件，我需要列出子节点的长度（这些子节点只会在稍后添加）。 我的第一个想法是简单地做到这一点，但这种策略是行不通的： 
 
<tbody>
  <tr v-for="item in items" >
     <td>
        {{item.childOne.length - item.childTwo.length}}
     </td>
   </tr>
</tbody>
 
 所以或者我得到这样的长度： 
 
<tbody>
   <tr v-for="item in items" >
      <td>
         {{Object.keys(item.childOne).length - Object.keys(item.childTwo).length}}
      </td>
    </tr>
</tbody>
 
 正如我所说的，我需要在创建项目的子项之前渲染该组件......这些子项节点后来通过Google Cloud Function有条件创建，所以我不可避免地从这个简单的结构开始： 
 
item: {
  name: itemName,
}
 
 ...但在这个阶段，我收到一条错误消息： 
 
 
 
 TypeError：无法将未定义或null转换为Function.keys（）处的对象 
 
 
 我在想，如果我使用第一种策略，vue.js足够聪明，可以避免此错误呈现组件，并且如果没有孩子，则不显示任何内容，但它无法获得长度 。 
 
 第二种策略在我添加孩子时非常有效，但它无法处理数据的初始缺失，甚至无法呈现视图 。 
 
 无论如何，当孩子节点不在那里时，使用第二种策略来防止这种错误？ 

Imagine a Vue.js app where the data structure in firebase will develop into something like this:
 
item: {
  name: itemName,
  childOne: {
    subChildA: true,
    subChildB: true
  },
  childTwo: {
    subChildA: true,
    subChildB: true
  }
}
 
Now I have a component where I need to list the length of the child nodes (these child nodes will only be added later). My first thought was to simply do this, but this strategy doesn't work:
 
<tbody>
  <tr v-for="item in items" >
     <td>
        {{item.childOne.length - item.childTwo.length}}
     </td>
   </tr>
</tbody>
 
So alternatively I got the length like this:
 
<tbody>
   <tr v-for="item in items" >
      <td>
         {{Object.keys(item.childOne).length - Object.keys(item.childTwo).length}}
      </td>
    </tr>
</tbody>
 
As I said I need to render that component before the item's children are created... These child nodes are later conditionally created through a Google Cloud Function so I inevitably start with this simple structure:
 
item: {
  name: itemName,
}
 
...but at this stage I get an error message:
 
 
 
TypeError: Cannot convert undefined or null to object at Function.keys ()
 
 
I am thinking that if I use the first strategy, vue.js is smart enough to avoid this error renders the component, and displays nothing if there are no children, but it can't get the lengths. 
 
The second strategy works perfectly when I add the children but it can't deal with the initial absence of data and doesn't even render the view.
 
Is there anyway to prevent this error using the second strategy when the child nodes are not there yet?