CODEWITHSUNDEEP
javascriptnode.js
J. Plexor
J. Plexor

Reputation: 33

Trouble with Node JS promises

I cannot find a solution to why this function returns before my message array is updated with the necessary values.

var calculateDistance = function (message, cLongitude, cLatitude, cSessionID) {
  return new Promise(function (resolve, reject) {

    distance.key = options.apiKey;
    distance.units('metric');

    var origins = [];
    origins.push(cLatitude + ',' + cLongitude);

    message.forEach(function (obj) {
      obj.sessionId = cSessionID;
      var destinations = [];
      destinations.push(obj.geoLocation.latitude + ',' + obj.geoLocation.longitude);

      distance.matrix(origins, destinations, function (err, distances) {
        if (err) {
          return console.log(err);
        }
        if (!distances) {
          return console.log('no distances');
        }
        if (distances.status == 'OK') {
          for (var i = 0; i < origins.length; i++) {
            for (var j = 0; j < destinations.length; j++) {
              var origin = distances.origin_addresses[i];
              var destination = distances.destination_addresses[j];
              if (distances.rows[0].elements[j].status == 'OK') {
                var distance = distances.rows[i].elements[j].distance.text;
                console.log('Distance from ' + origin + ' to ' + destination + ' is ' + distance);
                obj.distance = distance;
              } else {
                console.log(destination + ' is not reachable by land from ' + origin);
                obj.distance = 'N/A';
              }
            }
          }
        }
      });

    });

    return resolve(message);
  });
}

Could someone point out to me what i am doing wrong here.

Regards Jimmy

Upvotes: 1

Views: 96

Answers (3)

Mustafa Mamun
Mustafa Mamun

Reputation: 2651

 var async = require('async');
 var calculateDistance = function (message, cLongitude, cLatitude, cSessionID) {
    return new Promise(function (resolve, reject) {

        distance.key = options.apiKey;
        distance.units('metric');

        var origins = [];
        origins.push(cLatitude + ',' + cLongitude);

        async.each(message, function(obj, callback) {
          obj.sessionId = cSessionID;
          var destinations = [];
          destinations.push(obj.geoLocation.latitude + ',' + obj.geoLocation.longitude);

          distance.matrix(origins, destinations, function (err, distances) {
            if (err) {
              callback(err);
            }
            if (!distances) {
              callback('no distances');
            }
            if (distances.status == 'OK') {
              for (var i = 0; i < origins.length; i++) {
                for (var j = 0; j < destinations.length; j++) {
                  var origin = distances.origin_addresses[i];
                  var destination = distances.destination_addresses[j];
                  if (distances.rows[0].elements[j].status == 'OK') {
                    var distance = distances.rows[i].elements[j].distance.text;
                    console.log('Distance from ' + origin + ' to ' + destination + ' is ' + distance);
                    obj.distance = distance;
                  } else {
                    console.log(destination + ' is not reachable by land from ' + origin);
                    obj.distance = 'N/A';
                  }
                }
              }
              callback(null);
            }
            });
          },function(err){
             if(err){
               return reject(err);
          }else{
              return resolve(message);
                }
         });
    });
};

Upvotes: 1

J Adrian Zimmer
J Adrian Zimmer

Reputation: 7

You need to read up on promises. It looks to me as if you are thinking of promises as magical way to set up a callback. "Magic" tends to mean "something I don't need to understand." In this case that's not true.

That executor function of yours (that is the function which begins with 'function(resolve,reject)') should set up one asynchronous request. If, as is normal, the request has a callback you put the 'resolve' and 'reject' in the callback. The result will be a promise object which has methods 'then' and 'catch' where your post-request processing goes.

Since you want to fill up a matrix with the results of a lot of async requests, you will need to read about 'Promise.all' so you can react when all of them have resolved.

Upvotes: 0

Sumeet Kumar Yadav
Sumeet Kumar Yadav

Reputation: 13015

This is happening because your distance.matrix(origins, destinations, callback )is asynchronous . In above code distance.matrix method is getting pushed to event loop and continues it's execution and before that method callbacks gets executed resolve(message) is returned .

Upvotes: 0

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