For loop not working properly in node js - node.js

Question

I want to insert multiple data by for loop , but this code add 2 or 3 data only , then its continue loading loading but nothing happen ...
router.post('/addclient' , function (req , res){
var finished = false;
var user = req.body.username;
for(var i = 0 ; i <30 ; i++){
req.body.username = user + i ;
objDB.insert("clientList" ,req.body, function(err, data){
//if(err) console.log(err);
if(i>20){
finished = true;
}
});
}
if(finished){
res.redirect('/client/client-list');
}
});

Answer 1

you are doing it wrong. insert is asynchronous, so when insert for i=1 completes and callback gets called, maybe i is equal to 20 and more and you can't guess it. you are missing synchronous and asynchronous concepts and differences.
here is two solutions for your problem :
first, adding 30 items at once :
var clientsList = [];
for (var i = 0 ; i < 30 ; i++) {
clientsList[i] = Ith client object; // create object you want to insert for client i
}
// for example insertAtOnce is a function to insert your list
objDB.insertAtOnce(clientsList,function(err,callback) {
if (err) console.log("error : ", err);
res.redirect('/client/client-list');
});
second, using bind
function insertCallback(itr, res, err,callback) {
if (itr == 29) {
res.redirect('/client/client-list');
}
}
----
for(var i = 0 ; i <30 ; i++){
req.body.username = user + i ;
objDB.insert("clientList" ,req.body,insertCallback.bind(null, i, res);
}

Answer 2

Would be best if you use the async node.js library which provides control flows like running each function in series. You could use the async.whilst() method and restructure your code as:
router.post('/addclient', function(req, res) {
var user = req.body.username,
count = 0,
result = { finished = false };
async.whilst(
function () { return count < 30; },
function (callback) {
count++;
req.body.username = user + count.toString();
objDB.insert("clientList", req.body, function(err, data){
if (err) { return callback(err); }
result["data"] = data;
if( count > 20 ) result.finished = true;
callback(null, result);
});
},
function (err, result) {
if (err) { /* handle err */};
if (result.finished) res.redirect('/client/client-list');
}
);
});

Answer 3

You have a callback called after running insert function. It runs not exactly after running insert, and not in loop(it asynchronous). So in your code res.redirrect will never be called, because finished will be true after all loop will run. If you don't need to have all this inserts finished before answer, you can go this way:
if(i == 29){
res.redirect('/client/client-list');
}
Otherwise you need to create some queue, for example, i'm using async library https://github.com/caolan/async to create queues and running callbacks after queue ends.

Answer 4

router.post('/addclient', function(req, res) {
var finished = false;
var user = req.body.username;
var i = 0;
while( true ) {
if ( i > 20 ) {
res.redirect('/client/client-list');
}
req.body.username = user + i;
var promise = new Promise();
objDB.insert("clientList", req.body, function(err, data) {
promise.resolve();
});
promise.then(function() {
i++;
continue;
});
}
});
I have written it from the top of my head. But the basic idea here is that you need a loop to run infinitely, then resolve a promise; the resolution triggers continuing the loop, and increment the counter.

Answer 5

Use Promise. I would have done something like below
router.post('/addclient' , function (req , res){
var finished = false;
var promise = [];
var user = req.body.username;
for(var i = 0 ; i <30 ; i++) {
req.body.username = user + i;
promise.push(new Promise(resolve, reject) {
objDB.insert("clientList" ,req.body, function(err, data) {
if(err) {
reject(err)
return
}
resolve(data)
})
})
}
Promise.all(promise).then(function() {
finished = true
})
if(finished){
res.redirect('/client/client-list');
}
});