How to write TypeScript definition for a middleware that adds property to the response?

Question

I'd like to use express-boom for express with TypeScript. It lacks of typings so I'd like to write my own. Just make it compile is trivial.

This middleware decorates the res object with a property boom (derived from the boom module):

var express = require('express');
var boom = require('express-boom');

var app = express();

app.use(boom());

app.use(function (req, res) {
  res.boom.notFound(); // Responsds with a 404 status code
});

But with typescript I need to cast it because neither http.ServerResponse and Express.Response have the boom property, of course:

return (<any>res).boom.badRequest('bla bla bla');

Which is the cleanest way to do it? Which are other typed middleware that are doing a similar thing?

Answer 1

There's quite a few other Express middlewares you can use as examples, e.g. Method-Override, and its type definitions.

As a more concrete example, if you want to add this .boom property to the response object, you should just need to create a type definition (express-boom.d.ts) containing:

declare namespace Express {
    interface Boom {
        // Add boom's properties in here
    }

    export interface Response {
        boom: Boom
    }
}