this in Js callbacks

app.js

var fn = function(a,b){
  console.log(this, a ,b);
}

setTimeout(fn, 100); // global, undefined , undefinded , assuming fn is called from the implimentation of 
                     // setTimeout like cb(); "this" in callbacks completely depends on the way they are called.

// as "this" in callbacks completely depends on the way they are called, still we can be sure about "this" in our callbacks.

var customThis = " this";
setTimeout(function(){
  fn.call(customThis,1,2);
}, 100);

// logs "this", 1,2