Add async result to array in nodejs

Question

i'm fairly new to Node.js and by the life of it I can't seem to figure out the whole callback async thing.

what I want to achieve is "relatively" simple if it all where blocking calls.

1) request a list

2) iterate through the elements

3) create new elements for a new list with new element attributes being the result of another request

4) add new element to a new list

specifics:

var originalList = [{
    provider: "ABC",
    loc: [13.37224, 52.53862],
    state: 22
}, {
    provider: "CDE",
    loc: [13.37124, 52.53262],
    state: 33
}, {
    provider: "EFG",
    loc: [13.37214, 52.53662],
    state: 44
}];

var newElement;
var newList = [];
var vLat, vLng;

originalList.forEach(function(element, index, array) {

    newElement = {
        provider: element.provider,
        state: element.state
    };
    vLat = element.loc[1];
    vLng = element.loc[0];

    async.parallel([
        function(callback) {
            var location;
            googleMapsClient.reverseGeocode({
                latlng: [vLat, vLng],
                result_type: ['street_address'],
                location_type: ['ROOFTOP', 'APPROXIMATE']
            }, function(err, response) {
                if (!err) {
                    var result = response.json.results;
                    location = result[0].formatted_address;
                    newElement.location = location;
                    callback();
                } else {
                    callback(err);
                }
            });
        }
    ], function(err) {
        if (err) console.log(err);
        newList.push(newElement);
    });
});

I'm omitting the second async function which also makes a Google Maps request and should add that result also to newElement.

I really am struggling why the newElement doesn't get the new attribute (newElement.location). I know that it will be basic.

The json array seems to be wrong, There is an extra } after every object in the array — brk
– brk, Commented Oct 24, 2016 at 18:34
use async.map() and you can pass things from the iterator to the final callback — Plato
– Plato, Commented Oct 24, 2016 at 18:52

Ben · Accepted Answer · 2016-10-24 20:08:48Z

the loop needs to be async also to control the flow of processing all items. Use async.each() or async.eachSeries() (process the list in one after another):

var originalList = [{
    provider: "ABC",
    loc: [13.37224, 52.53862],
    state: 22
}, {
    provider: "CDE",
    loc: [13.37124, 52.53262],
    state: 33
}, {
    provider: "EFG",
    loc: [13.37214, 52.53662],
    state: 44
}];

var newList = [];

async.each(originalList, function (element, eachCb) {

    var newElement = {
        provider: element.provider,
        state: element.state
    };
    var vLat = element.loc[1];
    var vLng = element.loc[0];

    async.parallel([
        function(callback) {
            var location;
            googleMapsClient.reverseGeocode({
                latlng: [vLat, vLng],
                result_type: ['street_address'],
                location_type: ['ROOFTOP', 'APPROXIMATE']
            }, function(err, response) {
                if (!err) {
                    var result = response.json.results;
                    location = result[0].formatted_address;
                    newElement.location = location;
                    callback();
                } else {
                    callback(err);
                }
            });
        }
    ], function(err) {
        if (err) {
            console.log(err);
        } else {
            newList.push(newElement);
        }
        eachCb();   // <<<<< call to signal it's done!!
    });
}, function (err) {
    console.log('finish processing all items in originalList');
});

If there is only 1 task, you don't need async.parallel()

thelonglqd · Accepted Answer · 2016-10-25 03:57:16Z

I think your code should be like below

var originalList = [{
    provider: "ABC",
    loc: [13.37224, 52.53862],
    state: 22
}, {
    provider: "CDE",
    loc: [13.37124, 52.53262],
    state: 33
}, {
    provider: "EFG",
    loc: [13.37214, 52.53662],
    state: 44
}];

var newElement;
var newList = [];
var vLat, vLng;

originalList.forEach(function(element, index, array) {

    newElement = {
        provider: element.provider,
        state: element.state
    };
    vLat = element.loc[1];
    vLng = element.loc[0];

    async.parallel([
        function(callback) {
            var location;
            googleMapsClient.reverseGeocode({
                latlng: [vLat, vLng],
                result_type: ['street_address'],
                location_type: ['ROOFTOP', 'APPROXIMATE']
            }, function(err, response) {
                if (!err) {
                    var result = response.json.results;
                    location = result[0].formatted_address;
                    newElement.location = location;
                    callback(null, 'success');
                } else {
                    callback(err);
                }
            });
        }
    ], function(err, results) {
        if (err) console.log(err);
        newList.push(newElement);
    });
});

Following the async.parallel API document, the callback() in if (!err) must be callback(err, results)

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