node.js - Where do the parameters in a javascript callback function come from? -

i understand essence of callback functions in function executed again after being passed parameter function. however, i'm confused variables inside callback function come shown in following node.js example:

router.get('/', function(req, res){     res.render('index', {}); }); 

how variables req , res populated? example explaining how can call res.render(...) without declaring res myself appreciated.

they come same place come when normal non callback function invoked, @ invocation time.

if have function,

function add (a, b) {   return + b } 

you're fine knowing , b come when invoke add,

add(1,2) 

and it's same principle callbacks, don't let brain twisted because it's getting invoked later.

at point function pass router.get going invoked, , when does, receive req , res.

let's pretend definition router.get looks this

router.get = function(endpoint, cb){    //do    var request = {}    var response = {}    cb(request, response) // invocation time } 

in case of example, it's node pass function request , response whenever .get invoked.