Call a Q promise function after promise chain invoked

Question

I am working in a Node.js app with Q promise library. I have two set of promise chains and one is for controlling the flow and one for calling service methods where I retrieve data from, My question is, I need to get the return value of the promise chain to my other promise chain.

MyExample.js

bookService.getBookById(bookId)
  .then(bookDetals)
  .then(function(returnValue) { // <- (A)
    res.send(200, returnValue); // <- (C)
    return returnValue;
  }).catch(function(error) {
    logger.error('Error getting values');
    res.send(500, error);
  });


bookDetals = function(book) {
  myService.retrieveATypeData(book, bookId)
    .then(function(bookData) {
      myService.retrieveBTypeData(bookId)
        .then(function(bdata) {
          bookData[bTypeData] = bdata;
          myService.retrieveCTypeData(bookId)
            .then(function(cdata) {
              bookData[cTypeData] = cdata;
            }).done(function() {
              return bookData; // <- (B)
            })
        });
    });
};

In the above code, I am calling bookService.getBookById(bookId) and getting the book. Then I am calling bookDetals function which is a promise chain. But my problem is it returns the returnValue before the promise chains over. How can I get the return value of promise chain (in line (B)) to return in place (C). Currently it return before. so in place C it says undefined.

Answer 1

Since you are using Node, I would move towards ES6 Promises. If your current version does not yet support ES6 Promises, I would recommend you switch over to a library ( es6-promise ) that polyfills it for you. With ES6, you could do something like this:

// mock async promise
const getThing = id => (
  new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve({
        id
      });
    }, 250);
  })
);

// mock async promise
const getDetailsA = thing => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(Object.assign({}, thing, {
        a: 'purple'
      }));
    }, 250);
  })
};

// mock async promise
const getDetailsB = thing => (
  new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(Object.assign({}, thing, {
        b: 'monkey'
      }));
    }, 250);
  })
);

// mock async promise
const getDetailsC = thing => (
  new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve(Object.assign({}, thing, {
        c: 'dishwasher'
      }));
    }, 250);
  })
);

getThing('123')
  .then(getDetailsA)
  .then(getDetailsB)
  .then(getDetailsC)
  .then(console.log)
  .catch(console.error);

Answer 2

You need to return a promise :

bookDetals = function(book) {
  return Q.Promise(function(resolve, reject, notify) {
    myService.retrieveATypeData(book, bookId)
        .then(function(bookData) {
          myService.retrieveBTypeData(bookId)
            .then(function(bdata) {
              bookData[bTypeData] = bdata;
              myService.retrieveCTypeData(bookId)
                .then(function(cdata) {
                  bookData[cTypeData] = cdata;
                }).done(function() {
                  resolve(bookData); // <- (B)
                })
            });
        });
  }  
}

---

edit :

deferred is an anti-pattern discussed here . Honestly, it might be best to use a polyfill since Promise is in the es6 spec.