我正在使用JavaScript开发Windows 8应用程序.我需要获取给定目录中的所有图片(每张图片的路径).我该怎么做？

我有以下内容：

(function () {
    "use strict";
    var page = WinJS.UI.Pages.define("/html/scenario3.html",{
        ready: function (element,options) {
            document.getElementById("folder").addEventListener("click",pickFolder,false);
        }
    });

    function pickFolder() {
        // Clean scenario output
        WinJS.log && WinJS.log("","sample","status");

        // Verify that we are currently not snapped,or that we can 
        // unsnap to open the picker
        var currentState = Windows.UI.ViewManagement.ApplicationView.value;
        if (currentState === Windows.UI.ViewManagement.ApplicationViewState.snapped && !Windows.UI.ViewManagement.ApplicationView.tryUnsnap()) {
            // Fail silently if we can't unsnap
            return;
        }

        // Create the picker object and set options
        var folderPicker = new Windows.Storage.Pickers.FolderPicker;
        folderPicker.suggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.desktop;
        // Users expect to have a filtered view of their folders depending on the scenario.
        // For example,when choosing a documents folder,restrict the filetypes to documents for your application.
        folderPicker.fileTypeFilter.replaceAll(["*"]);
        //folderPicker.fileTypeFilter.replaceAll([".png",".jpg",".jpeg"]);

        folderPicker.pickSingleFolderAsync().then(function (folder) {
            if (folder) {
                // Application now has read/write access to all contents in the picked folder (including sub-folder contents)
                // Cache folder so the contents can be accessed at a later time
                Windows.Storage.AccessCache.StorageApplicationPermissions.futureAccessList.addOrReplace("PickedFolderToken",folder);

                var pictures = Windows.Storage.KnownFolders.folder;
                pictures.getFilesAsync().done(function (images) {
                    console.log(images.length + " images found.");
                    WinJS.log && WinJS.log("Total Images: " + images.length,"status");
                });


            } else {
                // The picker was dismissed with no selected file
                WinJS.log && WinJS.log("Operation cancelled.","status");
            }

        });
    }
})();

如果我使用上面的代码,它会出现以下错误：

无法获取未定义或空引用的属性“getFilesAsync”