javascript typeof 在 IE 中返回“未知”
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typeof returning "unknown" in IE
提问by Brett Postin
I have a window, where before being closed I refresh the underlying page.
我有一个窗口，在关闭之前我刷新底层页面。
if(opener && typeof(opener.Refresh) != 'undefined')
{
opener.Refresh();
}
If I moved away from the original opening page, this code would throw a "Permission Denied"error.
如果我离开原始打开页面，此代码将引发“权限被拒绝”错误。
Debugging the code revealed that typeof(opener.Refresh)was equal to "unknown"instead of the expected "undefined".
调试代码发现它typeof(opener.Refresh)等于"unknown"而不是预期的"undefined"。
As far as I'm aware "unknown"is not one of the return values for typeof, so how and why would this value be returned?
据我所知，“未知”不是 的返回值之一typeof，那么如何以及为什么返回该值？
Further Information
更多信息
I avoided the error by changing the check to:
我通过将支票更改为：
if(opener && typeof(opener.Refresh) == 'function')
However examples like this (detecting-an-undefined-object-property-in-javascript) do not seem to factor "unknown" into the equation.
然而，像这样的例子（detection-an-undefined-object-property-in-javascript）似乎并没有将“未知”因素纳入等式。
采纳答案by Marcel Korpel
According to a duplicate question at Bytes, the typeof value unknownis added to JScript version 8, along with date.
根据Bytes上的一个重复问题， typeof 值unknown与date.
A commentto a blog by Robert Nyman can also be explanatory:
一个评论由罗伯特·尼曼到博客上，也可以解释：
Internet Explorer displays “unknown” when the object in question is on the other side of a COM+ bridge. You may not know this or realize this, but MS's XMLHTTP object is part of a different COM+ object that implements IUnknown; when you call methods on it, you're doing so over a COM bridge and not calling native JavaScript.
Basically that's MS's answer if you try to test or access something that's not a true part of the JScript engine.
当有问题的对象位于 COM+ 桥的另一侧时，Internet Explorer 会显示“未知”。您可能不知道或意识到这一点，但 MS 的 XMLHTTP 对象是实现 IUnknown 的不同 COM+ 对象的一部分；当你调用它的方法时，你是通过一个 COM 桥而不是调用原生 JavaScript。
如果您尝试测试或访问不是 JScript 引擎真正部分的内容，基本上这就是 MS 的答案。
回答by Tomasz Dzi?cielewski
Try inoperator. I had the same problem (with applet) and I solved it using in:
尝试in运营商。我遇到了同样的问题（使用小程序），我使用in以下方法解决了它：
if("Refresh" in opener) {
opener.Refresh();
}
回答by James Allardice
The ECMAScript specificationstates that for host objects the return value of the typeofoperator is:
在ECMAScript规范规定，对于主机对象的返回值的typeof操作是：
Implementation-defined except may not be "
undefined", "boolean", "number", or "string".
实现定义的 except 可能不是“
undefined”、“boolean”、“number”或“string”。
I believe the unknownvalue is only ever returned in Internet Explorer. Interestingly, MSDNdoes not mention it:
我相信该unknown值只会在 Internet Explorer 中返回。有趣的是，MSDN并没有提到它：
There are six possible values that typeof returns: "number," "string," "boolean," "object," "function," and "undefined."
typeof 返回六个可能的值：“number”、“string”、“boolean”、“object”、“function”和“undefined”。
