Implementing Login Retention

I'm developing a web app with node.js + express.js.

The login/logout function will work normally by implementing the login function with passport.

Now, I'd like to implement the login retention feature, and I'd like to ask for your help.

Please give me a simple principle example code or a site advice that I can refer to.

ps. I asked this question because it was hard to understand even if I googled. Thank you.