四个使用promise的语句之间的不同点在哪儿？

http://jsbin.com/tuqukakawo/1/edit?js,console,output

#1

doSomething().then(function () {
  return doSomethingElse();
}).then(finalHandler);

答案：

doSomething
|-----------------|
                  doSomethingElse(undefined)
                  |------------------|
                                     finalHandler(resultOfDoSomethingElse)
                                     |------------------|

#2

doSomething().then(function () {
  doSomethingElse();
}).then(finalHandler);

答案：

doSomething
|-----------------|
                  doSomethingElse(undefined)
                  |------------------|
                  finalHandler(undefined)
                  |------------------|

#3

doSomething().then(doSomethingElse())
  .then(finalHandler);

答案

doSomething
|-----------------|
doSomethingElse(undefined)
|---------------------------------|
                  finalHandler(resultOfDoSomething)
                  |------------------|

#4

doSomething().then(doSomethingElse)
  .then(finalHandler);

答案

doSomething
|-----------------|
                  doSomethingElse(resultOfDoSomething)
                  |------------------|
                                     finalHandler(resultOfDoSomethingElse)
                                     |------------------|