If I understand it right, you want to read some value stored in session (in php stored in $_SESSION) as a client from javascript.

First: session is server-side thing, clients don’t know that they are in session.

But there is a way, how to send value.
This is what came to my mind:

<h2 id="titlesystem"></h2>
<p id="bodysystem"></p>
<input type="hidden" value=<?php echo $_SESSION["value"]?> id="mySessionValue">
<script>
    var sessionValue = document.getElementById("mySessionValue").value;
    if (sessionValue == 1)
    {
        document.getElementById("titlesystem").innerHTML = "Your System";
        document.getElementById("bodysystem").innerHTML = "Amazing!"
    }
</script>

I want the script to read the global element, detect if its value equals 1 and if so, display a load of text but for some reason it doesn’t seem to work.

var z = sessionStorage["system"]
if (z == 1) {
  document.getElementById("titlesystem").innerHTML = "Your System";
  document.getElementById("bodysystem").innerHTML = "Amazing!
}

<h2 id="titlesystem"></h2>
<p id="bodysystem"></p>