promise-then-return.js

var p1, p2, tp1;

// p1 is original promise
p1 = new Promise(function(resolve, reject){
  setTimeout(function(){
    console.log('+ resolve to hello');
    resolve("hello");
  }, 1000)
});

// p2 is a new promise after p1.then()
p2 = p1.then(function(value){
  console.log("expect p1.then() got 'hello':", value == "hello" || value);
  console.log("expect p1 and p2 are not the same instance:", p1 !== p2);

  // tp1 is internal promise, used for Promise.resolve
  tp1 = Promise.resolve("yep");
  console.log('+ resolve to yep, with tp1');
  return tp1;
});

p2.then(function(value){
  console.log("expect p2.then() got 'yep':", value == "yep" || value);
  console.log("expect p1 and p2 and tp1 are not the same instance:", !(p1 === p2 && tp1 === p2));

  console.log('+ reject with error');
  return Promise.reject("error");
}).catch(function(value){
  console.log("expect p2.catch got 'error':", value === 'error' || value);
});

p1.catch(function(value){
  console.log("this should not be shown");
});