How to write Express.js middleware with TypeScript

You can extend your Express.js server easily with custom middleware. All you have to do is to write a function that accepts three parameters (req, res, next).

Writing custom Express middleware

By writing a function that follows the ApplicationRequestHandler type of Express.js, you can extend your app server with custom functionality.

A “middleware” in Express is just a function that intercepts every request so that you can return a custom response or apply a custom filter when a request reaches your server. You can also call the next function to pass on the request for processing (to another registered middleware).

Example

import
 {Express
, Request
, Response
, NextFunction
} from
 'express'
;


function
 preventCrossSiteScripting
(req: Request, res: Response, next: NextFunction
): void
 {
  res.setHeader
('X-XSS-Protection'
, '1; mode=block'
);
  next
();
}

export
 function
 applyServerHardening
(app: Express
): void
 {
  app.disable
('x-powered-by'
);
  
  app.use
(preventCrossSiteScripting);
}

Table of contents

Writing custom Express middleware