most of the things came in my mind while working

1. How does File uploading works behind the scene. each and every step specially showing exact progess bar(we've used multer package but don't know what it does internally, similar for client side as well ngf-upload in angularjs)
2. momory allocation in javascript - how exactly a variable, const, let store in javascript, where and how does it store, both in case of nodejs and browser as well
3. OAuth full flow(including OPT based login, 2FA, and how does google authenticator app works)
4. in javascript does function, function parameter,callback also allocates memory in heap ?