Example callback function

callbackex1.js

function doSomething(p, callback){
   // does something using the p parameter
   // maybe call some other service 
   // that takes a long time to complete
   
   // when that is finally completed.
   // it calls back
   // by just calling the callback function
   callback();
}

// The above is invoked like this, for example
module.doSomething(123, function () {
	console.log("it's done!");
});