I'd like to expose a value to all request handlers using express & typescript. I'd like to be able to "inject" this value from a middleware (or some other way), the point is that it should be easy to mock it in case I need to.

I came up with this solution:

// The context type, I'd like to be able to inject this using the middleware below. 
// In a real scenario think of this like a database connection, etc.
type RequestContext = {
  foo: string
}

// The type enriching the Request type with the context field
type HasContext = {
  context: RequestContext
}

// Middleware attaching the context to the request
const contextMiddleware =
  (context: RequestContext) => 
  (req: Request & Partial<HasContext>, _res: Response, next: NextFunction) => {
    req.context = context
    next()
  }


// Now an actual route using the extra type
const mainRoute = express.Router().get('/test', (req: Request & HasContext, res) => {
  res.json({ context: req.context })
})

// Adding the middlewares and listen
app.use(contextMiddleware({ foo: 'bar' }))
app.use(mainRoute)

app.listen(8000)

My questions:

• Is this the intended way of doing this using express? I scouted the API but couldn't find a better solution
• The extra data is attached to the request. Is there any other way that does this without mutating the request or response itself?
• The type Request & HasContext has to be defined in each request that uses this context. Is there a better way?