CODEWITHSUNDEEP
javascriptnode.jsasynchronous
user6002037
user6002037

Reputation:

Node - send response to browser when loop has finished

I am making a POST request in one of my routes add-users. I have created an array called success. In each loop provided the API POST runs successfully I am adding a string 'user added' to the array. Once the array has completed I want to send the response to the browser with the success array.

I have noticed something strange. When I type in the url the start of add-users it runs the loop before I hit enter to navigate to the page. This seems strange? Is node listening and predicting which url I am going to hit?

Here is my current attempt but its not working for some reason. 

app.get('/add-users', function (req, res) {
    var success = [];
    var count = 0;

    users.forEach(function(user, i){

        request({
            url: url,
            method: 'POST',
            json: true
        }, function(err, resp, body){
            if (!err && resp.statusCode === 200) {
               success.push('user added'); 
            }
        });

        if(count === users.length) {
          res.json(success);
        }
    });
});

Upvotes: 1

Views: 1426

Answers (3)

Paul
Paul

Reputation: 36349

@SantanuBiswas has one way you can solve your problem with the response returning before your requests are all complete, though depending on how many users you've got in your array and how slow the upstream service is, this is a potentially disastrous user experience as it will wait until all the requests are complete and only then fire back a response.

A better solution (in my opinion) would be to respond immediately with a 202 Accepted status code, and then update your DB with information about each user's status in the request handler for later reporting or debugging. Something like this (assuming you're using mongoose for your local user storage):

app.get('/add-users', function (req, res) {
   res.setStatus(202).send(); // you can put more info in the body if desired

users.forEach(function(user){

    request({
        url: url,
        method: 'POST',
        json: true
    }, function(err, resp, body){
        const status = err ? 'error' : 'complete';  // obviously you might want to put more info somewhere, eg. error log, if there is an error
        User.update({_id: user.id}, {status:status}, function(e) { if (e) console.error(err);});
    });
});

});

Still another way, though it adds complexity to your solution, is to implement websockets to have your client get updated each time a request is complete. That example is a bit longer than I have time to post here, but the docs on the site I linked are excellent.

Upvotes: 0

Santanu Biswas
Santanu Biswas

Reputation: 4787

Regarding browser fetching the response before hitting enter key on the url, it is very unusual behaviour. Maybe you should check your machine if it is infected by any malware!

Regarding the code used by you, count is not incremented anywhere in the forEach loop. So it remains 0 forever and never equals users.length. So the loop will end but it will never send a response.

Also, you are testing for equality between count and users.length at the wrong place in the code.

This code should work:

app.get('/add-users', function (req, res) {
    var success = [];
    var count = 0;

    users.forEach(function(user){

        request({
            url: url,
            method: 'POST',
            json: true
        }, function(err, resp, body){
            if (!err && resp.statusCode === 200) {
               success.push('user added'); 
            }

            count++;                        // <<=== increment count
                                            //
            if(count === users.length) {    // and then test if all done
               res.json(success);
            }
        });
    });
});

Upvotes: 1

user1481317
user1481317

Reputation:

The problem here is you are mixing synchronous and asynchronous code together in a wrong way. Please note that forEach is synchrounous and request is asynchronous. So, looping over users finishes faster than the first result you get from request method.

Upvotes: 1

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