FileIO.WriteTextAsync（IStorageFile，String）错误：未定义Windows

Question

我的代码：

var x = [{" Id ": " 1 ", " UserName ": " Sam Smith "},
         {" Id ": " 2 ", " UserName ": " Fred Frankly "}]
Windows.Storage.FileIO.writeTextAsync("par2.txt", x)

产生错误：

Uncaught ReferenceError: Windows is not defined

我该怎么做才能解决这个问题？