Node asynchronous route code

Question

I'm using node with express 4.0. I can't find anything on the internet (including the docs) about embedding asynchronous code in a route.

With a middleware it's quite simple:

app.use('/something', function (req, res, next)
{
  doSomethingAsync(function(err, probablySomethingElse)
  {
    // probably some error checking
    next();
  });
});

The problem with routes is that there is no next callback, so how does express know when to move to the next job?

app.get('/something', function (req, res)
{
  res.render('someTemplate');
  // no next() here, but it still works
});

If I had to guess, I'd say that express moves to the next task right after the above function exits. But out of curiosity I've launched the following code...

app.get('/something', function (req, res, next)
{
  console.log(next);
});

...and there actually is some next callback passed. So what's going on here? How does it work behind the scenes? And how can I put asynchronous code there?

Could you be a little clearer, what async code do you want to put in the route, and how is that related to the `next` callback, which all routes happen to have. — adeneo, Jul 27 '14 at 12:35
When you `res.render()` something, you don't need to call `next()` (even if it is passed so that you *could* if you wanted to) because it's the **end of the chain**. — Bergi, Jul 27 '14 at 12:42
Express somehow needs to know when my callback exits so it can close the socket and drop the data associated with this request. Do I understand correctly, that if I call `res.render` it will asume that there are no async tasks scheduled and it doesn't have to wait for `next` to be called? — Sebastian Nowak, Jul 27 '14 at 12:52

score 5 · Accepted Answer · edited May 23 '17 at 12:15

Express will wait until you call res.render to close the socket. Which means that you can pass res.render into a callback that takes X secs to execute and everything will still work.

The next allows you to go to the next route that maps your value, you can find a very good explanation here: What is the parameter "next" used for in Express?

But under what you are asking here. The moment render() is called on the res object, then data will be sent and the socket closed.

Node asynchronous route code

1 Answers1