expressjs.com/_includes/api/en/4x/app-locals.md

app.locals

The app.locals object has properties that are local variables within the application, and will be available in templates rendered with res.render.

The `locals` object is used by view engines to render a response. The object keys may be particularly sensitive and should not contain user-controlled input, as it may affect the operation of the view engine or provide a path to cross-site scripting. Consult the documentation for the used view engine for additional considerations.

console.dir(app.locals.title)
// => 'My App'

console.dir(app.locals.email)
// => 'me@myapp.com'

Once set, the value of app.locals properties persist throughout the life of the application, in contrast with res.locals properties that are valid only for the lifetime of the request.

You can access local variables in templates rendered within the application. This is useful for providing helper functions to templates, as well as application-level data. Local variables are available in middleware via req.app.locals (see req.app)

app.locals.title = 'My App'
app.locals.strftime = require('strftime')
app.locals.email = 'me@myapp.com'