Q.all array of request promises, not sure how to get the results

Question

Probably an obvious answer to this but I'm not sure what way to take. request is a node module: https://github.com/request/request I fill an array of getHistory requests (with different parameters). p = [p1,p2...].

this.app.all('/api/heatmap', function(req,res) {
   // fill p here _.each(blabla, p.push(gethistory(someparams...)
   var result = [];
   function getHistory(params) {
        var options = { ...};
        var callback = function(error, response, body) {
            if(error) { //wtv 
            } else {
              // what to do with the body here ? return body ? result.push(body) ?
            }
        }

        request(options, callback);
    }

    Q.all(p).then(function() {
    });
}

So the problem here is that I when all of the request to be done , put everything in an array/object then send the whole thing to the client. How to have getHistory returning the fetched value (after the request is done ).

Hope it's clear.

Answer 1

The core problem here is that node.js-style callbacks and promises are not compatible. Promises emphasize on return values, node emphasizes on callbacks.

Therefore you need a sort of adapter that wraps node's callback convention properly, a process called Promisifcation . This can be done manually, but it's tedious at best and error-prone when you are not careful. Luckily, since node's conventions are well-established, it can be automated. Q has a few helpers for that, but Bluebird is quite a bit more convenient in this regard.

So the easy way to do it is to switch to Bluebird as the promise library and to use promisifyAll .

var Promise = require('bluebird');
var request = Promise.promisifyAll(require("request"));

this.app.all('/api/heatmap', function(req, res) {
    var requests = blabla.map(function (item) {
        return request.getAsync({ /* params */ });
    });

    Promise.all(requests).then(function (responses) {
        res.send( JSON.stringify(responses) ); // whatever
    }).catch(function (error) {
        res.send( "An error ocurred: " + error ); // handle error
    });
}

Answer 2

FWIW, here's another answer that shows how the same would look like when done properly with Q:

// promisified request
function requestAsync(options) {
    var result = Q.defer();
    request(options, function(error, response, body) {
        if (error) {
            result.reject(error);
        } else {
            result.resolve(body);
        }
    });
    return result.promise;
}

// returns promises for heatmapVolumes
function getHistory(params) {
    return requestAsync({
        method: 'GET',
        url: 'https://api.kaiko.com/v1/charts/' + 
            encodeURIComponent(params.exchange) + '/' + 
            encodeURIComponent(params.pair) +'/history',
        qs: params.qs,
        headers: {
            "Content-Type": "application/json",
            "Accept": "application/json"
        }
    }).then(function (body) {
        return heatmapVolume(body, params.exchange, params.pair);
    }).catch(function (error) {
        // log detailed error and send less revealing message downstream
        console.error('error fetching trades', error);
        throw new Error('Something went wrong while fetching trades');
    });
}

// usage
this.app.all('/api/heatmap', function(req, res) {
    getHistory({
        exchange: "foo", pair: "bar", qs: "qux"
    }).then(function (heatmap) {
        res.send(200, heatmap);
    }).catch(function (error) {
        res.send(500, error);
    });
});

Answer 3

Used Q.deferred and it worked as documented \\o/

function getHistory(params) {
                var deferred = Q.defer();
                var options = {
                    method: 'GET',
                    url: 'https://api.kaiko.com/v1/charts/' + params.exchange + '/' + params.pair +'/history',
                    qs:qs,
                    headers: {
                        "Content-Type": "application/json",
                        "Accept": "application/json"
                    }
                }

                var callback = function(error, response, body) {
                    if(error) {
                        console.log('error fetching trades', error);
                        res.send(500, 'Something went wrong while fetching trades');
                    } else {
                        var body = heatmapVolume(body, params.exchange, params.pair);
                        // console.log("result!", body.exchange, body.pair);
                        result.push(body);
                        // return body;
                        deferred.resolve();
                    }
                }

                request(options, callback);

                return deferred.promise;
            }