How does this work? Optional first argument used in Express (err, req, res, next) or (req, res, next)

With Express / Connect I can set up an middleware function in either of the formats:

function(req, res, next) // first argument will be a request

Or

function(err, req, res, next) // first argument will be an error

Stepping back from Express, Connect, to basic JavaScript: I don't understand is how this is possible to have an optional first argument?

How can express know that my function will accept an err object first? I thought to make this possible the structure would have to be like the following:

function(req, res, next, err)

Am I missing something basic here? Is it possible to query how many arguments a function is expecting?

The middleware function is passed to express, so the arguments variable is not valid. although length is correct... I think I have figured it out, would be good to have confirmation to whether this is the case. Example below:

var fn;

fn = function (one, two) {};
console.log(fn.length); // 2

fn = function (one, two, three) {};
console.log(fn.length); // 3