Recursion, Node js and Asynchronous calls

Question

I am trying to get the savePlaces array as the final output so that I can return it in the response. But I am getting success as undefined in function2(). Why I am not getting the savePlaces as expected. Please provide solution....

var saveOverriddenPlaces = function(index, savePlaces) {
    var groupSettingsDAO = new GroupSettingsDAO();
    var deferred = q.defer();
    if (index == savePlaces.length) {
        console.log("-----------------------------deferred--------------------------------", deferred);
        console.log("LAST RETURN -----------------------------" + savePlaces + "----------------------------------------------------", index);
        deferred.resolve(savePlaces);
        return deferred.promise;
    }
    var placeInfo = {
        "id": savePlaces[index].id,
        "groupurl": savePlaces[index].groupurl,
        "sla": savePlaces[index].sla
    };
    if (savePlaces[index]._id) {
        placeInfo._id = savePlaces[index]._id;
        //update the overriden places in the database
        groupSettingsDAO.updateOverriddenPlace(placeInfo)
            //updates the overriden place and sends result in response when successful else logs error
            .then(
                //success
                function(success) {
                    console.log("recursion ============" + index + "=======================");
                    deferred.resolve(saveOverriddenPlaces(++index, savePlaces));

                    return deferred.promise;
                },
                //failure
                function(error) {
                    console.log("PLACES     ERROR ===================================", error);
                });
        // placesWithID.push(value);
    }
    //WITHOUT ids
    else {
        placeInfo._id = guid();
        savePlaces[index]._id = placeInfo._id;
        groupSettingsDAO.saveOverriddenPlace(placeInfo)
            // saves the overridden place and sends the results in response if successful else logs the error
            .then(
                //success
                function(success) {
                    console.log("recursion ============" + index + "=======================");
                    deferred.resolve(saveOverriddenPlaces(++index, savePlaces));
                },
                //failure
                function(error) {
                    console.log("PLACES     ERROR ===================================", error);

                });
    }
}
function2(req, res) {
    saveOverriddenPlaces(0, req.savePlaces).then(function(success) {
        //getting success as undefined 
        res.send({
            "result": success
        });
    });
}

I am using the Q library and groupSettingsDAO is a dao object which has method updateOverriddenPlace() which returns a promise. — Tahir Mirza, May 22 '15 at 22:35

score 1 · Answer 1 · answered May 22 '15 at 17:37

1

This is because saveOverriddenPlaces is not returning a value and therefore it is not passed to the .then statement in function two. The "solution" is to make sure that saveOverridenPlaces returns a Promise that calls resolve with a value that you want to pass to success.

answered May 22 '15 at 17:37

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Thanks for replying but I too understood that the saveOverriddenPlaces is not returning the promise. The problem is I am not able to figure out how I can do that. As I tried returning the array with updated values on the last index value. I read other articles on recursion with node.js using promises but was not able to solve this problem. – Tahir Mirza May 22 '15 at 18:39
Understood the problem. – Tahir Mirza Aug 09 '15 at 02:21

score 0 · Accepted Answer · edited May 23 '17 at 12:21

The problem is that in the line

deferred.resolve(saveOverriddenPlaces(++index, savePlaces));

your saveOverriddenPlaces function does not (always) return a promise or a value. If a function is asynchronous, it must always return a promise.

You can easily do that by adding a return before the groupSettingsDAO.…().then(…) calls in your conditional statements - as then does return exactly the promise that you want if you return a value/prmise from its callback.

However, you should try to avoid the deferred antipattern. For your base case, use the Q function to create a fulfilled promise, for everything else just chain then calls - no deferreds needed.

function saveOverriddenPlaces(index, savePlaces) {
    if (index == savePlaces.length) {
        console.log("LAST RETURN -" + savePlaces + "-", index);
        return Q(savePlaces);
    }
    var groupSettingsDAO = new GroupSettingsDAO();
    var placeInfo = {
        "id": savePlaces[index].id,
        "groupurl": savePlaces[index].groupurl,
        "sla": savePlaces[index].sla
    };
    var promise;
    if (savePlaces[index]._id) {
        placeInfo._id = savePlaces[index]._id;
        promise = groupSettingsDAO.updateOverriddenPlace(placeInfo); //update the overriden places in the database
        // placesWithID.push(value);
    } else { // WITHOUT ids
        placeInfo._id = guid();
        savePlaces[index]._id = placeInfo._id;
        promise = groupSettingsDAO.saveOverriddenPlace(placeInfo) // saves the overridden place
    }
    return promise.then(function(success) {
//  ^^^^^^
        console.log("recursion =" + index + "=" + success);
        return saveOverriddenPlaces(++index, savePlaces));
//      ^^^^^^
    });
}
function2(req, res) {
    saveOverriddenPlaces(0, req.savePlaces).then(function(success) {
        //getting success as undefined 
        res.send({
            "result": success
        });
    }, function(error) {
        console.log("PLACES ERROR =", error);
    });
}

Thank you for replying Bergi. I finally understood where I made my mistake.:-) — Tahir Mirza, May 24 '15 at 01:33

Recursion, Node js and Asynchronous calls

2 Answers2