app.locals and res.locals life cycle

Question

I'm 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?

Answer 1

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