How do you implement login / signup?

This is going to be an unpopular answer, but I usually roll my own, from scratch. I've used frameworks like Devise in the past. These tools are great for banging out a quick login/signup experience, but they get more cumbersome when you want to customize the process or the user interface. I've found that, on average, the implementation time ends up being similar.

The counterargument to this is that rolling your own is a security risk, but I've found that not to be the case. Login/signup isn't that complicated, even if you are mitigating things like timing attacks (which often aren't addressed).

The other counterargument would be for social sign-in or SSO scenarios. My apps tend to use email/password login strategies, so I don't need to deal with this often. I'll concede that I might reach for a framework in these scenarios.

I use passport . its super easy to use and can be installed simply through npm .Its nicely documented as well with elaborated examples.
further there are hundreds of strategies to select from for authentication ( such as google , Facebook ,twitter, etc ) and all the user data gets directly stored in your mongo Db connected database.

I use Firebase. I used to use AWS but when I tried to merge authentication paths (federated user paths on Cognito using Twitter, Email, Facebook, etc), it was very difficult so I stopped. I had a much easier time using Firebase on pretty much every front including authentication. They have some drag and drop forms you can use if you search GitHub, although I find the programming it yourself gives you more control and is a little faster for me. Good luck!

As my SaaS boilerplate is written in Python/Flask, I use JWT (Flask JWT Extended package).

For SPA's I always use firebase authentication as its really simple and great to use. With server side frameworks, I've used laravels built in authentication for php. Testing out Sails.js now and its authentication system.

I always wanted to give https://auth0.com a go for this, but haven't yet. Anyone else used it?