How to create a global per session variable in Node JS

I know doing things such as

myvar=var Math.Sqrt(25);

will create a per user variable rather than doing something like

var myvar=Math.Sqrt(25);

My use case is having a variable stored with a user's session and only for that user. My app works correctly if I use app.locals.myVar, but that is per server rather than perclient. I've tried to use express-session, but for some reason after I initialize my session I cannot create variables.

inside app.js

var session = require('express-session');
app.use(session({secret: 'qwerty'}));

....

app.get('/search', function (req, res){
    var searchParameters = req.query.searchInput;
    var cook="";
    sess=req.session;
    sess.testVar=null;

my code will error on the last line where I try to assign my testVar.