How to make a cookie, that is random, and won’t change once the user comes back (can’t be PHPSESSID= because im running node.js)

I’m trying to create a cookie that acts like a PHPSESSID but is JavaScript, I would just use PHP but i’m running node so that is impossible. The hope is that you get a 7 long character id that stays even when you reload the page and can’t be replicated. This is what I have tryed but it creates a new one instead of keeping it every time.

 function randomString() {  
            //define a variable consisting alphabets in small and capital letter  
    var characters = "ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";  
              
            //specify the length for the new string  
    var lenString = 7;  
    var randomstring = '';  
  
            //loop to select a new character in each iteration  
    for (var i=0; i<lenString; i++) {  
        var rnum = Math.floor(Math.random() * characters.length);  
        randomstring += characters.substring(rnum, rnum+1);  
    }  
  var thing = randomstring;
    document.cookie = `${thing};expires=4000;`
}