app.locals and res.locals life cycle

I really confused by app.locals and res.locals because I don't know WHEN should I use them and how? And actually I want to know the app.locals and res.locals life cycle.

For example where should I save my user (authenticated user) details (username, roles etc.)? In app.locals or res.locals?

Thanks in advance geeks.

You can consider app.locals to be global. Some examples of things you might want to store in app.locals: URL helpers, application-level constants. You should put anything here that you want accessible in every single view.

res.locals stores data only for a particular response (which responds to a particular request). For example, GET /something will create a new res.locals that gets passed through all the middleware responding to '/something.' Appropriate information here is stuff like authenticated user details from your question.

The lifecycle looks like this, where your responsibilities are bold (everything else is done for you by express):

  1. app is created (var app = express();)
  2. app.locals is created
  3. request arrives
  4. res.locals is created for that request
  5. you add things to res.locals like user roles (res.locals.role = 'admin';)
  6. you serve a response to the request (res.render('some/view');)
  7. res.locals for that request is garbage collected, gone
  8. app.locals continues to exist as long as the app exists