callback vs promise basic example

gistfile1.js

function a(text) {
	console.log(text);
}

/********************************* Callback style ****************************/
setTimeout(function() {
	var text = "Hello World";
	// Calling the next function using its reference // cb style
	a(text);
	// call next function in the chain
}, 2000);

/********************************* Promise style  *****************************/
var q = require('q'); // External package

// delayed function
function delay() {
	var d = q.defer();
	
	setTimeout(function() {
		var text = "Hello World";
		return d.resolve(text);
	}, 2000);
	
	return d.promise;
}

// Calling deferred style
delay()
.then(function(text) {
	return a(text);
})
.then(....) // next function in the chain