Return Value from Node Redis Command: Hubot and Coffeescript

Question

TL;DR How can I get the return value out of the client.ttl callback for use outside of the getTTL function?

Learning Coffeescript with Hubot and Redis, here. I've got a function that's not returning the value I expect it to. The function here is designed to get the TTL for a Redis key and return the TTL value, eg 4000 (seconds). Here's my Coffeescript:

getTTL = (key) ->
    client.ttl key, (err, reply) ->
        if err
            throw err
        else if reply in [-1, -2]
            "No TTL or key doesn't exist."
        else
            reply
    return

Now here is it compiled in JS:

var getTTL;

getTTL = function(key) {
  client.ttl(key, function(err, reply) {
    if (err) {
      throw err;
    } else if (reply === (-1) || reply === (-2)) {
      return "No TTL or key doesn't exist.";
    } else {
      return reply;
    }
  });
};

From coffeescript return function callback acting strange , I understand the need to add the empty return , but I'm still not receiving the value in the callback reply. If I integrate the function with the Response object in Hubot, I can do msg.send reply and that works just fine to output the return value.

However, if I assign the return value of the function to a variable, eg ttl_val = getTTL "some-key" , then I only get a boolean value returned ( true ), which I'm assuming is the exit status of the getTTL function itself. So, my question is:

What am I doing wrong that's preventing me from receiving the reply value in the callback function? Do I need to do something like How do I wait for a callback in coffeescript (or javascript)? to ensure that my callback completes before trying to yank the value?

Answer 1

You need to setup getTTL to accept its own callback function:

getTTL = (key, done = ()->) ->
    client.ttl key, (err, reply) ->
        if err
            throw err
        else if reply in [-1, -2]
            done "No TTL or key doesn't exist."
        else
            done reply

Then in your hubot script

robot.respond /what is the TTL for (.*)/i, (msg) ->
    getTTL msg.match[1] msg.send

Edit : To answer you're tl;dr edit: you cannot get the return value out of the callback. That value only exists in the context of the callback function. You can always assign that value to some kind of global variable from inside the callback function, but then you'll never know when that global value got assigned its value.