Question: I have many of these "controllers": 

```
app.get('/',function(req,res){
    var stuff = { 'title': 'blah' };
    res.render('mytemplate',stuff);
});    

```

Notice res.render? I want to add this header to every response header I make:

`X-XSS-Protection: 0`

How can I add that response header automatically?

Answer: 
For express 4.x, the idiomatic way is as follows:

### Implementation

```
// no mount path; executed for every request.
app.use(function (req, res, next) {
  res.set('X-XSS-Protection', 0);
  next();
});

```

### Test

```
describe('Response Headers', function () {
  it('responds with header X-XSS-Protection: 0', function (done) {
    hippie(app)
    .get('/any/route/you/can/think/of')
    .expectHeader('X-XSS-Protection', 0)
    .end(done);
  });
});

```

### Dev Dependencies (for tests to work)

```
% npm install --save-dev mocha hippie

```

### Relevant Documentation

* [Application Level Middleware](http://expressjs.com/guide/using-middleware.html#middleware.application)
* [res.set](http://expressjs.com/4x/api.html#res.set)
I'd like to point out that none of these answer actually answer the question; the question is specifically relating to render responses; e.g. for an app like:

```
const router = require('express').Router();
router.use('/test.json', (req, res) => res.json({ test: 'hi' });
router.use('/test.html', (req, res) => res.render('test'));

```

It's not clear how to add headers (e.g. CSP headers, which can be very verbose) *only* to your HTML responses. Express doesn't have a hook to specifically do that. The only option at the moment is to organize your code so you don't have to, e.g.

```
app.use(jsonRouter);
app.use(htmlRouter);

```

...which allows you to do as some of the other answers suggest, and add generic middleware for setting the headers.