Adding persistent data to applications - Build Windows 8 Apps with HTML5 and JavaScript - page 245

Reading file content
To read the content of the selected file you use the readTextAsync method on the Windows.Stor-
age.FileIO object. The method returns the entire content of the file as a string of text. The Windows.
Storage.FileIO object, however, also provides two more methods. One is readLinesAsync, which reads
the entire content and returns it as an array of lines of text. Using readLinesAsync or readTextAsync
depends on what you plan to do with the read text. For the purpose of deserializing some JSON
content, this method is the perfect fit since you are not going to split the text in lines anyway.
The other method available on the Windows.Storage.FileIO object is readBufferAsync . This method
returns a buffer object, namely an array of bytes. This might be a good fit if you are working with
binary files such as images. For text-based applications, you may safely ignore this method.
Here's the code to add to the TodoList.invokeOpenPicker method to grab the JSON string stored in
the selected file.
openPicker.pickSingleFileAsync().then(function (file) {
if (file) {
var io = Windows.Storage.FileIO;
io.readTextAsync(file)
.done(function (json) {
// JSON deserialization takes place here
}
});
The final step is deserializing the JSON string into a newly created Task object.
Deserializing Task objects
To serialize a Task object to a string you used the JSON.stringify method. A reverse method also exists
to revive a JSON string back to a JavaScript object. This method is JSON.parse . Here's the code you
need to add to the TodoList.invokeOpenPicker method:
// Build a Task object
var task = JSON.parse(json);
// Display the Task object
TodoList.displayTask(task);
Unfortunately, during execution this code raises an exception. The exception is due to a failure in
the building of the date of the Task object. As weird as it may sound, what JSON.stringify serializes, its
counterpart JSON.parse can't properly deserialize.
To be precise, this conflict shows up only if the serialized object has Date properties. If dates
are not involved, then everything goes smoothly. The problem can be tracked back to the JSON
specification that does not officially include the Date type. The major issue here is not that JSON.parse
can't handle a date string. More subtly, the issue is that it deserializes it as a plain string. So you won't