Javascript 数组变得未定义

我现在有最奇怪的虫子；我构建此函数是为了从服务器的输出中删除xss漏洞，在解析MongoDB结果对象（模式中包含子文档）时，数组属性会立即从数组更改为未定义。请参阅下面的代码和跟踪：

代码：

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知道为什么会发生这种情况吗？

我知道这是一件如此愚蠢的事情。干杯

  for(let i = 0 ; i < object[prop].length ; i++){
    object[prop] = xss(object[prop][i]);
  }

xss Array
save: [null,null,null,null]
[ [Function: notify],
  [Function: notify],
  [Function: notify],
  [Function: notify] ]
object
[Function: Array]
Array
4
TypeError: Cannot read property 'length' of undefined
    at xssObjectEscape (/var/www/smq/services/secure/xss.js:30:39)
    at xssObjectEscape (/var/www/smq/services/secure/xss.js:34:7)
    at xssObjectEscape (/var/www/smq/services/secure/xss.js:34:7)
    at xssObjectEscape (/var/www/smq/services/secure/xss.js:34:7)
    at xss (/var/www/smq/services/secure/xss.js:5:12)
    at filter.user.then (/var/www/smq/handlers/session.js:29:21)
    at process._tickCallback (internal/process/next_tick.js:103:7)

  for(let i = 0 ; i < object[prop].length ; i++){
    object[prop] = xss(object[prop][i]);
  }

object[prop] = object[prop].map(xss)