WinJS StorageFolder file exists

StorageFolderExtensions.js

    Windows.Storage.StorageFolder.prototype.fileExistsAsync = function(fileName) {
        var folder = this;
        return WinJS.Promise(function (complete, error) {
            folder.getFileAsync(fileName).then(function() {
                complete();
            }, function() {
                error();
            });
        });
    };