req.locals vs. res.locals vs. res.data vs. req.data vs. app.locals in Express middleware

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As you mentioned, both req.locals, res.locals or even your own defined key res.userData can be used. However, when using a view engine with Express, you can set intermediate data on res.locals in your middleware, and that data will be available in your view (see this post). It is common practice to set intermediate data inside of middleware on req.locals to avoid overwriting view data in res.locals, though this is not officially documented.

res.locals
An object that contains response local variables scoped to the request, and therefore available only to the view(s) rendered
during that request / response cycle (if any). Otherwise, this
property is identical to app.locals.

This property is useful for exposing request-level information such as
the request path name, authenticated user, user settings, and so on.

Source: http://expressjs.com/en/api.html#res.locals

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