Recursion with promises.

I have a scenario where recursion occurs with promises, and I need some help on this.

I have an array that contains a sequence of messages. Each message can be either: 1 primitive, like a string; or 2 compound, which is just a pointer to a DB area that contains a sequence of messages. For example, the array can be: ['hello', (compound), 'there'], where (compound) can contain ['how', 'are', 'you'] when the compound message is fetched from DB. Therefore, the final 'expanded' array looks like: ['hello', 'how', 'are', 'you', 'there']. To completely convert an array of messages into an array of primitive messages is called 'expand'.

Note that a message in a compound message can be also a compound message, this is where recursion gets in. For instance, if (compound) in the above example is ['how', 'are', 'you', (lower level compound)], and (lower level compound) is ['Tom', 'Jerry'], then (compound) will be expanded into ['how', 'are', 'you', 'Tom', 'Jerry'], and the original array will be expanded into ['hello', 'how', 'are', 'you', 'Tom', 'Jerry', 'there'].

Here is what I think the code looks like, without promises:

function expandMessages(messages, outputMessages) {
    messages.forEach(function(message) {
        if (message.primitive) {
            outputMessages.push(message);
        }
        else {
            var fetchedMessages = fetchMessages(message);
            expandMessages(fetchedMessages, outputMessages);
        }
    });
}

In the above code, fetchMessages fetches messages from DB for a compound message.

How should I promisify the above code, so that after the outer promise returns: 1 outputMessages contains only primitive messages; and 2 correct message order is maintained, that is, any compound message's 'submessages' are inserted to where the compound messages were in the original array.

Thanks!

kriskowal / q

Recursion with promises. #826