How do I convert this callback argument into a promise?

Question

I am using socket.io and I'd like to convert the callback function into a simple promise based function where I can return a value.

I'm trying to convert the acknowledgement found here: socket.io/docs/#sending-and-getting-data-(acknowledgements)

socket.on('getGames', (data, callback) => {
    Game.find(data)
        .then(data => callback(data)); //explicit call to callback
});

I'd like to call a function instead such as:

socketEvent(socket, 'getGames')
    .then((data) => {
        return Game.find(data); //returns a promise and can be chained
    });

I'm thinking something like:

//this doesn't work
const socketEvent = (socket, name) => {
    return new Promise(function (resolve, reject) {
        socket.on(name, (data, callback) => {
            resolve((data) => callback(data));
        });
    });
};

You probably shouldn't at all. A socket may fire the event multiple times, a promise is the wrong tool to represent that. — Bergi, Feb 17 '17 at 15:47
Well in any case it would fire a callback multiple times. The context isn't that relevant - I'd still like to know how to convert an explicit callback as above into a promise. — Nazzanuk, Feb 17 '17 at 15:50
A promise *cannot* fire multiple times, you're out of luck there. For the general case, see [How do I convert an existing callback API to promises?](http://stackoverflow.com/q/22519784/1048572) — Bergi, Feb 17 '17 at 15:57
You seem to be a bit confused about socket.io callback functions. You can register a callback function when you SEND a message that will give you confirmation the message was received by the other end. You don't use that callback upon receipt of an incoming message. — jfriend00, Feb 17 '17 at 16:56
@jfriend00 thank you, this helped me understand my mistake. What I am referring to as a callback, is in fact a send function exposed as an argument. I would still like to 'promisify' the action, but I'll leave it for now. — Nazzanuk, Feb 17 '17 at 17:26

score 1 · Answer 1 · answered Feb 17 '17 at 15:10

1

Just resolve(data). No need for an inner function. That will flow into the promise chain.

Also don't forget to call reject when there's a problem.

answered Feb 17 '17 at 15:10

Daniel A. White

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This doesn't work as I haven't then invoked `callback` to return `data`. I need to invoke `callback`, when the promise is resolved. – Nazzanuk Feb 17 '17 at 15:15
@Nazzanuk can you link to some documentation? – Daniel A. White Feb 17 '17 at 15:16
@Nazzanuk What data are you trying to send back? Where would it come from? – JLRishe Feb 17 '17 at 15:16
@DanielA.White So I'm trying to convert the acknowledgement found here: http://socket.io/docs/#sending-and-getting-data-(acknowledgements) – Nazzanuk Feb 17 '17 at 15:18
@JLRishe the information is just an example so in this case `Game.find(data)` is querying MongoDB with the `data` object. But any async Promise could be put in here. – Nazzanuk Feb 17 '17 at 15:44

JLRishe · Accepted Answer · 2017-02-18T03:36:15.033

Your second code snippet is not analogous to the first. In the first one, there is a mechanism to send an acknowledgement when Game.find() has retrieved its value, but in the second one, there's no way for it to send the acknowledgement back when its done.

I think this might be close to what you are trying to do:

const socketEvent = (socket, name) => {
    return new Promise(function (resolve, reject) {
        socket.on(name, (data, sendBack) => resolve({ data, sendBack }));
    });
};

You could then consume it like this:

socketEvent(socket, 'getGames')
    .then({ data, sendBack } => Game.find(data).then(sendBack))
    .then(... more stuff);

However, as others have pointed out, there is still a LARGE difference between this and the original code because the promise version will only capture the event once (that's how promises work). If you need to set up a mechanism to capture the same event multiple times, then promises are not the right tool for the job and you either need to use callbacks, or something like observables.

How do I convert this callback argument into a promise?

2 Answers2