Express handler type constraints.

api.ts

import { Request, NextFunction } from 'express'

interface JsonResponse<T> {
  status(code: number): JsonResponse<T>
  json(value: T): JsonResponse<T>
}

type JsonHandler<T> = (
  req: Request,
  res: JsonResponse<T>,
  next: NextFunction
) => void

export const handleFoo: JsonHandler<Foo> = async (req, res, next) => {
  const foo = await getFoo()
  res.json(foo) // Error if `foo` is not of type Foo
}