FE user session not available in developer tools device mode - typo3

Question

We have made an extension in typo3 (ver. 6.2).
At a point we make a ajax call, where we want to save some values.
In order to check if the user is logged in we use
$GLOBALS['TSFE']->fe_user->user['userid']
The userid for the two testusers are greater than 0. The (edited) php that is called looks like this:
<?php
$var = $GLOBALS['TSFE']->fe_user->user['userid'];
if($var > 0) {
echo "var = ".$var."\n";
echo "var > 0 - logged in!";
}else{
echo "var = ".$var."\n";
echo "var < 1 - not logged in.";
}
?>
Our problem is that when we look at the page-source, we can see the content from the if-condition:
var = 1 (or: var = 2)
var > 0 - logged in!
When we look at the console (F12), we can see the content from the else-condition:
var =
var < 1 - not logged in.
The variable is empty here?
The Apache error.log gives a notice:
Trying to get property of non-object in .....
Can anyone explain what is going on? If it is a non-object, how come the variable is set to 1 (or 2)?

Answer 1

The Frontend User Session in TYPO3 is (unless configured differently in the Install Tool) bound to the User-Agent. The browser console uses a different user agent if you set it to another kind of device (e.g. iPhone):
So the user session doesn't apply anymore.
For development purposes or if you're aware of the risks (session hi-jacking), you might change this in the install tool:

Answer 2

A) If you used the code just as you showed within raw PHP file it won't work as the $GLOBALS['TSFE'] object is NOT initialized - you didn't specify how do you handle AJAX request, but Trying to get property of non-object in ... shows that clearly. If you'll rise log level for development you'd see a notice
Notice: Undefined index: TSFE in /your/path/to/script.php
You should create a front-end plugin instead and handle it via custom typeNum (preferred) or use eID mechanismus. The eID may be faster in some cases (better performance), but you will have more work with it as you need to initialize TSFE parts still.
B) If user is not logged the var $GLOBALS['TSFE']->fe_user->user is false otherwise it's an Array, you don't need to check the user ID.
C) If you want to check integer value and you are not sure if index exists you should use
intval($GLOBALS['TSFE']->fe_user->user['uid']) instead
D) TSFE has a property which determines if FE user is logged in or not:
if ($GLOBALS['TSFE']->loginUser) {
echo "User IS logged";
} else {
echo "User is NOT logged";
}