javascript - How does error handling work in node js -

i got sample node js code using expressgenerator ,which default got error handling code follows

app.use(function (req, res, next) {       console.log("first callback 1");       var err = new error('not found');       err.status = 404;       next(err);       console.log("first callback 2"); });  app.use(function (err, req, res, next) {      // set locals, providing error in development      console.log("second callback");      res.locals.message = err.message;      res.locals.error = req.app.get('env') === 'development' ? err : {};       // render error page      res.status(err.status || 500);      res.render('error');      console.log("error send"); }); 

i've added few console.logs in error handling callbacks.what understood callbacks since didnt provided route here these 2 call backs called routes.

now in code i've added 1 route as

app.get('/login', function(req, res, next) {     res.sendfile(__dirname + '/public/views/login.html');  }); 

now when run app , go localhost:3000/home not there in app.js.so printing following

first callback 1 second callback error send first callback 2 first callback 1 second callback error send first callback 2 

question 1:why console log getting printed twice?it should not print after firstcallback 2.

now when go localhost:3000/login renders me login.html page route present there no console logs in server side.

question 2:why first console "first callback 1" not getting printed.since callback routes , in case since doesnt have error atleast should print first console..but not..why not doing callback??

can explain , how callbacks called here , use of next()