javascript - Callback() Node Js -

i'm confused program. bought book called "node js, mongodb, , angularjs web development" brad dayley. found program demonstrate called closure, , shows program example. first part of program.

function logcar(logmsg, callback){     process.nexttick(function(){         callback(logmsg);     }); }  var cars = ["ferrari", "porsche", "bugatti"];  for(var idx in cars){     var message = "saw " + cars[idx];     logcar(message, function(){         console.log("normal callback: " + message);     }) } 

i've been trying figure out how program functions entire hour, can't figure out function of callback(logmsg).

i know basic question, can't wrap head around it.

callback function pass logcar(). when logcar completes doing whatever supposed do, call callback function. inside loop, call logcar() this..

logcar(message, function(){     console.log("normal callback: " + message); }) 

here, function() {..} callback function , called once logcar done executing. in case, callback function you've provided console.log message you've passed first parameter. have passed function perform different callback too.