How to Check Whether Session Is Null or Not in JavaScript

How to check whether session is null or not in javascript?

Here's a solution that will test every 500 milliseconds if the user session has expired.

 function CheckSession() {
            var session = '<%=Session["username"] != null%>';
            if (session == false) {
                alert("Your Session has expired");
                window.location = "login.aspx";
            }
        }

setInterval(CheckSession(),500);

How to check if session object contains any object at all?

You can use something like this if(Object.keys(req.session).length > 0)

Check in javascript code if object from session is exist

I suggest you move the if condition to the server side. If the reservation isn't present, just don't render the Javascript at all.

So, instead of

if ( @Session["Reservation"] ) 

Use

@if ( Session["Reservation"] != null )

In context:

@if ( Session["Reservation"] != null )
{
     $('.reservationDetails :input').attr("disabled", false));
     $('#termSection :input').attr("disabled", true);
}

If Reservation is not null, the output Javascript will be:

$(function () {
    $('.reservationDetails :input').attr("disabled", false));
    $('#termSection :input').attr("disabled", true);
}

If Reservation is null, the output Javascript will be:

$(function () {
}

In other words, it won't do anything, which seems to be the intention.

Get Current Session Value in JavaScript?

The session is a server side thing, you cannot access it using jQuery.
You can write an Http handler (that will share the sessionid if any) and return the value from there using $.ajax.

Checking session if empty or not

Use this if the session variable emp_num will store a string:

 if (!string.IsNullOrEmpty(Session["emp_num"] as string))
 {
                //The code
 }

If it doesn't store a string, but some other type, you should just check for null before accessing the value, as in your second example.

How to check if session value is null or session key does not exist in asp.net mvc - 5

if(Session["TenantSessionId"] != null)
{
  // cast it and use it
  // The code
}

laravel how to check if session variable is empty

You can use

@elseif (session()->has('package_id'))

to verify if it's in session or in case it might be in session but also set to null, you can use:

@elseif (session()->get('package_id'))

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