How to do things synchronously in node

Question

I know node is all about async stuff but I want to do things in serial mode as follows:

make api request > convert body xml to JSON.stringify > pass string to template.

request.get({url:url, oauth:oauth}, function(err, res, body){
    parseString(body, function(err, result){
        output = JSON.stringify(result);

        res.render('home', { title: 'Fantasy Home', 
                output: output });
    });
});

Now I want to do this in sequence, but with all the callbacks I'm confused.

res.render doesn't work nested inside callbacks because the res object doesn't exist. Having it outside won't work because it'll run before the callbacks execute so you'd get "undefined" for output.

There has to be a way to do things in sequential order. Why is everything a callback?? Why can't these functions just return a regular non-callback result?

How can I make this work?

As for the why: @tjameson explains it very well [here](https://stackoverflow.com/a/17608144/3378621) — Mike Bovenlander, May 26 '17 at 07:14
`res.render doesn't work nested inside callbacks because the res object doesn't exist` this is completely wrong. The res object exist. If you have errors then that's not the reason for the errors — slebetman, May 26 '17 at 07:45
Your code already does run sequentially just fine. Accessing `res` might not work because you shadow it with the `res` parameter of the `request.get` callback. — Bergi, May 26 '17 at 07:52
Do you understand @Bergi 's comment? That is the key to your problem — borislemke, May 26 '17 at 14:43

score 1 · Accepted Answer · answered May 26 '17 at 14:46

The others fail to mention why your res.render does not work. You probably have something like this:

app.get('/', function(req, res, next) { // You need the res here to .render

    request.get({url:url, oauth:oauth}, function(err, res, body){ // But you are using this res instead, which does not have the render method
        parseString(body, function(err, result){
            output = JSON.stringify(result);

            res.render('home', { title: 'Fantasy Home', 
                    output: output });
        });
    });
});

Read the comments in the code. So your solution is, use res.render from the request handler, rename res in the request.get callback to something else.

score 0 · Answer 2 · answered May 26 '17 at 07:09

You should use middlewares, also promises is better thing to work with async in node, but I'll show you with callbacks. It is strongly suggested to not to block your thread with synchronous calls! Since node.js is single threaded. next() is an callback here so middleware won't allow execution of main route function (with res.render) until next() is called. You can pass as many middlewares as you wish.

app.use('/user/:id', middleware, (req, res) => {
    //here you'll find your data
    console.log(req.callData);
    res.render(view, req.callData);
}
middleware(req, res, next) => {
    dotheCall(dataToPass, (err, cb) => {
        req.callData = cb;
        // remember about handling errors of course!
        return next();
    })
}

score 0 · Answer 3 · answered May 26 '17 at 07:42

JavaScript is single threaded if we use synchronous code then that will itself a big problem around response time (node.js) and all. Everything is implemented with callback fashion due to the benefit of event loop.

You can take a deep understanding of event loop : https://youtu.be/8aGhZQkoFbQ (Very good explaination)

You can use Promisification for the scenario you want to implement : http://bluebirdjs.com/docs/getting-started.html

request.get({url:url, oauth:oauth}, function(err, res, body) {
    // Note :: This is Promisified Function
    return parseString(body)
        .bind(this)
        .then(function(result) {
            output = JSON.stringify(result);

            res.render('home', {title: 'Fantasy Home', output: output });

            return true;
        })
        .catch(function(error)
        {
            // Error Handling Code Here
            // Then send response to client
        });
});

You can implement promisified function using the following approach

function parseString(body) {
    var Promise = require("bluebird");

    return new Promise(function(resolve,reject) {
        // Your Parsing Logic here

        if(is_parsed_successfully) {
            return resolve(parsed_data);
        }

        return reject(proper_err_data);
    })
}

How to do things synchronously in node

3 Answers3