Understanding Q-Deferred in NodeJS through example

 var Q = require('q');  
 var step1 = function() {  
     var deferred = Q.defer();  
     setTimeout(function() {  
         console.log('step1');  
         deferred.resolve();  
     }, 800);  
     return deferred.promise;  
 }  
 var step2 = function() {  
     var deferred = Q.defer();  
     setTimeout(function() {  
         console.log('step2');  
         deferred.resolve();  
     }, 600);  
     return deferred.promise;  
 }  
 var step3 = function() {  
     var deferred = Q.defer();  
     setTimeout(function() {  
         console.log('step3');  
         deferred.resolve();  
     }, 400);  
     return deferred.promise;  
 }  
 var step4 = function() {  
     var deferred = Q.defer();  
     setTimeout(function() {  
         console.log('step4');  
         deferred.resolve();  
     }, 200);  
     return deferred.promise;  
 }  
 // for the following calls
   step1();  
   step2();  
   step3();  
   step4();  
 // output  
 // step4  
 // step3  
 // step2  
 // step1  
 step1().then(step2).then(step3).then(step4).done();  
 // output  
 // step1  
 // step2  
 // step3  
 // step4