Reading / writing file on local machine - firefox-addon

Question

I pretty much copied this code right out of the MDN File I/O page.. except I added an if statement to check if the file exists already and if it does, read it instead.
Components.utils.import("resource://gre/modules/NetUtil.jsm");
Components.utils.import("resource://gre/modules/FileUtils.jsm");
var file = Components.classes["#mozilla.org/file/directory_service;1"].
getService(Components.interfaces.nsIProperties).
get("Desk", Components.interfaces.nsIFile);
file.append("test.txt");
if (!file.exists()) {
this.user_id = Math.floor(Math.random()*10001) +'-'+ Math.floor(Math.random()*10001) +'-'+ Math.floor(Math.random()*10001) +'-'+ Math.floor(Math.random()*10001);
var ostream = FileUtils.openSafeFileOutputStream(file)
var converter = Components.classes["#mozilla.org/intl/scriptableunicodeconverter"].
createInstance(Components.interfaces.nsIScriptableUnicodeConverter);
converter.charset = "UTF-8";
var istream = converter.convertToInputStream(this.user_id);
// The last argument (the callback) is optional.
NetUtil.asyncCopy(istream, ostream, function(status) {
if (!Components.isSuccessCode(status)) {
alert('Error '+ status);
return;
}
alert('File created');
});
} else
{
NetUtil.asyncFetch(file, function(inputStream, status) {
if (!Components.isSuccessCode(status)) {
alert('error '+ status);
return;
}
// The file data is contained within inputStream.
// You can read it into a string with
this.user_id = NetUtil.readInputStreamToString(inputStream, inputStream.available());
});
alert('File exists already, do not create');
}
alert(this.user_id);
It creates the file just fine, I can open it and read it. If the file already exists however, it does not populate this.user_id.. just equals null. So my issue is specifically with reading the file.

Answer 1

File reading in your code works asynchronously - meaning that your code completes (including the alert() call which will show that this.user_id is null), then at some point the callback from NetUtil.asyncFetch() gets called with the data. Until that happens this.user_id won't be set of course. If you move alert(this.user_id) into the callback function it should show the correct value.
Note that it is highly recommended to keep file I/O operations asynchronous because they might take significant time depending on the current state of the file system. But you have to structure your code in such a way that it doesn't assume that file operations happen immediately.