return value inside promise chain isn't getting called

Question

I'm using the promise library Bluebird and I'm currently running into the issue that everything inside the function runs great, but when I try to return a value, the function instead returns undefined.

This is the promise chain:

function foo() {
    createGroupMembers(parsedChat).then(function(val) {
        var members = val;

        createMessages(parsedChat, maxPages).then(function(val) {
            var messages = val;

            Promise.all([ createFrontCover(subject, firstdateOfMessages, lastDateOfMessages, isPreview), createStats(parsedChat), createBackCover(parsedChat)])
            .then(function (results) {
                var front = results[0];
                var stats = results[1];
                var backcover = results[2];

                var book = head + front + stats + members + messages + backcover;

                console.log('pages in this book: ', pages);
                console.log(book); // logs perfect values.

                return book; 
            }); 

       });

    });
}

The problem is simple: when calling foo(), it's value becomes undefined instead of book. Why am I experiencing this behaviour?

Answer 1

function foo() {
    return createGroupMembers(parsedChat).then(function(val) {
        var members = val;

        return createMessages(parsedChat, maxPages).then(function(val) {
            var messages = val;

            return Promise.all([createFrontCover(subject, firstdateOfMessages, lastDateOfMessages, isPreview), createStats(parsedChat), createBackCover(parsedChat)])
                .then(function(results) {
                    var front = results[0];
                    var stats = results[1];
                    var backcover = results[2];

                    var book = head + front + stats + members + messages + backcover;

                    console.log('pages in this book: ', pages);
                    console.log(book); // logs perfect values.

                    return book;
                });

        });

    });
}

Now foo will return a promise which can resolve to the value of book

foo().then(function(book) {
    console.log('huzzah I have book ' + book);
});

To be honest, foo could be rewritten, but that's a different question altogether

FYI: you could do something like this for foo

function foo() {
    return createGroupMembers(parsedChat)
    .then(function(members) { // members
        return Promise.all([members, createMessages(parsedChat, maxPages)]);
    })
    .then(function(members_messages) {  // membersMessages
        return Promise.all([createFrontCover(subject, firstdateOfMessages, lastDateOfMessages, isPreview), createStats(parsedChat)].concat(members_messages, [createBackCover(parsedChat)]));
    })
    .then(function(results) { // front, stats, members, messages, back
        var book = head + results.join('');

        console.log('pages in this book: ', pages);
        console.log(book); // logs perfect values.

        return book;
    });
}

Messed around with the order in the second (was your only) Promise.all, and added the previous Promise results in it to make the final conatenation of parts as simple as a .join - doing it this way will also propagate any erros correctly, so your usage of foo can be

foo().then(function(book) {
    console.log('huzzah I have book ' + book);
}).catch(function(err) {
    // handle any and all errors here
});