Question

In this example, there is an fn function with null and false arguments:

io.configure(function () {
  function auth (data, fn) {
    fn(null, false);
  };

  io.set('authorization', auth);
});

https://github.com/LearnBoost/Socket.IO-node/blob/master/test/manager.test.js#L400-403

What is fn and what does it do?

Does it just mean, example function, stick your own function here, or does it mean something else?

Answer 1

In this example, fn is a function that is passed to auth() as a parameter, so yes, you supply auth() with a bit of functionality of your own choosing. This is called a ‘higher order function’, see here for a short introduction. It’s a technique mostly associated with functional programming, and since Javascript’s object orientation is generally considered to be a bit weak, it’s the style more advanced programs in Javascript tend to be in (Javascript have sometimes been called “Scheme in Java’s clothing“)

In this exact code: it’s part of the test suite, so it configures io to always fail authorization, in order to see that an authorization failure is handled gracefully (line 408-409 for the moment):

res.statusCode.should.eql(403);
data.should.match(/handshake unauthorized/);

These lines should be reached without an exception and the tests checks that the results contain meaningful values.