Server shutting down itself using twitter api with nodejs and expressjs

Question

I have an web app with using Twitter API. Main focus is to get the users that are not following back based on given twitter user name. It works fine until this point but when I get an error because of the fact that user name does not exist on Twitter, Server shuts down itself. Using Nodejs, expressjs.

The Error Message :

App is running on port : 3000
[ { code: 34, message: 'Sorry, that page does not exist.' } ]
_http_outgoing.js:494
    throw new Error('Can\'t set headers after they are sent.');
    ^

Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:494:11)
    at ServerResponse.setHeader (_http_outgoing.js:501:3)
    at ServerResponse.header (/home/ugurcan/dev/Projects/Twitter-App/twitter-api-app/node_modules/express/lib/response.js:767:10)
    at ServerResponse.send (/home/ugurcan/dev/Projects/Twitter-App/twitter-api-app/node_modules/express/lib/response.js:170:12)
    at /home/ugurcan/dev/Projects/Twitter-App/twitter-api-app/twitter-api.js:30:21
    at Request._callback (/home/ugurcan/dev/Projects/Twitter-App/twitter-api-app/node_modules/twitter/lib/twitter.js:215:14)
    at Request.self.callback (/home/ugurcan/dev/Projects/Twitter-App/twitter-api-app/node_modules/request/request.js:186:22)
    at emitTwo (events.js:126:13)
    at Request.emit (events.js:214:7)
    at Request.<anonymous> (/home/ugurcan/dev/Projects/Twitter-App/twitter-api-app/node_modules/request/request.js:1163:10)

Problematic part of the code is below. The question is : How can I avoid this situation ? Or is it even possible ?

client.get('followers/ids', params, function(error, followers_results, response) {
        if (error) {
          res.send("error");
          console.log(error);
        }

         let followers = followers_results.ids;

         client.get('friends/ids', params, function(error, following_results, response ) {
            if (error) {
                res.send("error");
                console.log(error);
            }

            let following = following_results.ids;


            following.forEach(function(person){
                if(followers.indexOf(person) === -1){
                    one_way_following.push(person);
                }
            });

            // console.log(one_way_following);

            one_way_following = one_way_following.slice(0, 100);

            one_way_following_string = one_way_following.join();

           // console.log("----------------------------------------------------");
           // console.log(one_way_following_string);
           // console.log("----------------------------------------------------");

           client.get('users/lookup', {user_id: one_way_following_string}, function(error, user_results, response, next) {
               if (error) {
                   res.send("error");
                   console.log(error);
               }

               user_results.forEach(function(user){
                   let userObject = {
                       name : user.name,
                       screen_name : user.screen_name,
                       avatar: user.profile_image_url
                   }
               //   console.log(user.name);

               users.push(userObject);
               })
               res.render("results.ejs",{users:users});
            //    console.log(users);
           });
       });
   });

Answer 1

This error is actually happening because, you can't call res.render("results.ejs",{users:users}); or res.send("error"); multiple times, because headers can't be changed once set.

Easiest solution would be to break out of the function once you've caught an error (if (error) { on line 2), or handle it appropriately (render something blank, or render your friendly error view).

For more information, about this, check out the answer to this question, Error: Can't set headers after they are sent to the client

Hope that helps :)