why express does not implement Node.js style callbacks?

[fluency03]

I have a question regarding the APIs. Why express does not implement Node.js style callbacks, i.e., Error-First Callbacks?

For example, router.METHOD:

router.get('/', function(req, res){
  res.send('hello world');
});

And furthermore, I also saw this Error handling:

app.use(function (err, req, res, next) {
  // logic
})

Then, my question is: after add the Error Handling middleware as above, can I regard my code as following?

router.get('/', function(err, req, res, next){
  res.send('hello world');
});

If so, I am able to use Q.denodeify converting the callback style to promise style right? Because Q.denodeify, as stated here, only deals with Node.js style (err, result) callback APIs

[riatzukiza]

Several of the node event based API's are not node style call backs either. For Example, all of the `http` namespace functions are not this way, it is because the callback style is not suited for dealing with stream like objects that can fail at multiple points, and have multiple locations which you would want to handle them with. Of course you can provide these abstractions your self, I personally have no idea if the above example of denodeify would work, but it wouldn't hurt to try and let the world know of your results.

[dougwilson]

Hi @fluency03 none of the functions you're showing are callbacks in the sense that Node.js defines them (they document this in https://nodejs.org/dist/latest-v6.x/docs/api/errors.html#errors_node_js_style_callbacks).

The function you are giving to app.use, router.get, etc. is not performing some sort of operation and expecting to get called back (i.e. is a callback). Instead it is more of an event listener, waiting for a request event to come in to handle. Just like event listeners which do not have an error object as the first argument, neither do the middleware in Express.js.

In essence, all your middleware functions are just request listeners (Node.js defines them here: https://nodejs.org/dist/latest-v6.x/docs/api/http.html#http_event_request) with the additional argument to allow you to chain them together instead of writing your entire server in a single function body.

You can think of your event listener as being the function that does the operation (responding to the request) and the next argument is like the callback to your work function.

I hope that helps!