Iterate over Async Function with For Loop (db query, Node.js)

Question

Im in the last part of my project and I'm trying to insert data from json in my MySQL db here are my sample data

{"data": 
[{"cpos": "g", "cfname": "e", "clname": "ejercito", "cvcount": "4"}, 
{"cpos": "g", "cfname": "j", "clname": "ejercito", "cvcount": "5"}]}

and that sample data is being parsed by my function (sorry for long function)

checkPositionCandidateInsertVote: function(ref, prid, json, callback){
    var len = json.data.length;
    for (var i = 0; i < len; i++) {
        var fn = (json.data[i].cfname).toLowerCase();
        var ln = (json.data[i].clname).toLowerCase();
        var c = json.data[i].cvcount;
        console.log(fn, ' ', c);
        switch((json.data[i].cpos).toLowerCase()){
            case "g":
                module.exports.getCandiName(fn, ln, "Governor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            case "vg":
                module.exports.getCandiName(fn, ln, "Vice Governor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            case "m":
                module.exports.getCandiName(fn, ln, "Mayor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            case "vm":
                module.exports.getCandiName(fn, ln,  "Vice Mayor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            case "bm":
                module.exports.getCandiName(fn, ln, "Board Member", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            case "cg":
            case "cm":
            case "cw":
                module.exports.getCandiName(fn, ln, "Congressman", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            case "c":
                module.exports.getCandiName(fn, ln, "Councilor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
        }

    }
}

My function/s are working fine but when I check my db the data are wrong in VoteCount part.

Expected

e ejercito 4
j ejercito 5

but in the db is

Result

e ejercito 5
j ejercito 5

How to stop my for loop if my query is not yet finished?

You can't "pause" a `for` loop to wait for an async operation. A `for` loop is synchronous. Instead, you will have to iterate manually, putting an iteration into a function and starting the next iteration manually only when your async operation completes. — jfriend00, Mar 03 '16 at 18:31
@jfriend00 can you give me some example? this is my first month in nodejs (javascript) — something, Mar 03 '16 at 18:35
Here's are some examples: http://stackoverflow.com/questions/29880715/how-to-synchronize-a-sequence-of-promises/29906506#29906506 and http://stackoverflow.com/questions/34191788/how-to-process-a-big-array-applying-a-async-function-for-each-element-in-nodejs/34191957#34191957 — jfriend00, Mar 03 '16 at 18:38

score 1 · Accepted Answer · answered Mar 03 '16 at 21:02

There is no need to stop for loop, there is a beauty of asynchronous nature of JS.

Thing is that by the time module.exports.insertVotes(prid, ref, c, dataa.id, function(res){}); is executed, for loop already went through, so you end up with las index for all cases.

To fix that you can use forEach loop . Each iteration will have its own scope.

    checkPositionCandidateInsertVote: function(ref, prid, json, callback){

    json.data.forEach(function(listItem, i){

        var fn = (json.data[i].cfname).toLowerCase();
        var ln = (json.data[i].clname).toLowerCase();
        var c = json.data[i].cvcount;
        console.log(fn, ' ', c);
        switch((json.data[i].cpos).toLowerCase()){
            case "g":
                module.exports.getCandiName(fn, ln, "Governor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
            ...
            ...
            ...

            case "c":
                module.exports.getCandiName(fn, ln, "Councilor", function(dataa){
                    //dataa.id
                    module.exports.insertVotes(prid, ref, c, dataa.id, function(res){
                    });
                });
                break;
        }

    });


}

thankyouu! can you explain the difference of foreach and for-loop? so that I can apply it of the same occur again — something, Mar 03 '16 at 23:28
Happy to help! To understand what going on here, you need to understand few basic JS concepts. I would start with JS scoping https://toddmotto.com/everything-you-wanted-to-know-about-javascript-scope/ >> callbacks — oKonyk, Mar 04 '16 at 01:02

Iterate over Async Function with For Loop (db query, Node.js)

1 Answers1