Difference between Q.defer() & Promise()

Question

I'm trying to understand why following code is behaving differently with Q.defer() and Promise()

Case 1 : When I'm using Q.defer()

getDocument(id)
.then(function (response) {
   console.log('in first then')
   return 'from two';
}).then(function (response) {
   console.log(response)
});

var getDocument=function(){
  var b = Q.defer();
    b.resolve('from getDocument'); // here will do some async operation..this is just an example
  return b.promise;
}

Output:

in first then
undefined

Case 2: using Promise()

getDocument(id)
.then(function (response) {
   console.log('in first then')
   return 'from two';
}).then(function (response) {
   console.log(response)
});

var getDocument=function(){
  return Promise.resolve('from getDocument');
}

Output:

in first then        
from two

Question

1. Why there is a difference in output?
2. I know that Q.defer is an anti-pattern but then how to choose when to use what?

Answer 1

In fact, both examples are returning the same result (the same order). Check this codepen Example

var getDocument=function(){
  var b = Q.defer();
    b.resolve('Q from getDocument'); // here will do some async operation..this is just an example
  return b.promise;
}

getDocument(1)
.then(function (response) {
   console.log('Q in first then')
   return 'Q from two';
}).then(function (response) {
   console.log(response)
});

var getDocumentP=function(){
  return Promise.resolve('P from getDocument');
}

getDocumentP(1)
.then(function (response) {
   console.log('P in first then')
   return 'P from two';
}).then(function (response) {
   console.log(response)
});

2) You can see here some uses of Q.defer: Q.defer you´re doing it wrong