Node.JS - How to escape multiple nested callbacks?

Question

I've currently got a expressJS app and I'm trying to retrieve information from an API using the getStats function. However, the profile route I've written seems to be multiple nested callbacks. How would I prevent this? Is there a way to make it get all of the stats, then assign them to variables after they all of the stats are retrieved?

function getStats(access_token, time_range, x, callback) {
    var stats = [];

    var options = {
        url: 'www.example.com',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        },
        json: true
    }

    request(options, function(error, response, body) {
        if (!error && response.statusCode == 200) {
            for (i = 0; i < body.items.length; i++) {
                stats.push(body.items[i].name);
            }
            return callback(stats);
        }
    })

}

app.get('/profile', function(req, res) {
    var access_token = 1234;

    getStats(access_token, 's', 'x', function(a){
        console.log(a);

        getStats(access_token, 's', 'y', function(b){
            console.log(b);

            getStats(access_token, 'm', 'x', function(c){
                console.log(c);

                getStats(access_token, 'm', 'y', function(d){
                    console.log(d);

                    getStats(access_token, 'l', 'x', function(e){
                        console.log(e);

                        getStats(access_token, 'l', 'y', function(f){
                            console.log(f);

                            res.send(a + "\n" + b + "\n" + c + "\n" + d + "\n" + e + "\n" + f);
                        });
                    });
                });
            });
        });
    });
});

Answer 1

Promises allow you to avoid the type of callback nesting like you've just shown. See my example which illustrates your example in Promise form:

function getStats(access_token, time_range, x, prevResult) {
    return new Promise((resolve, reject) => {
        if (prevResult) {
            resolve([...prevResult, "test", "test", "test"]);
        } 
        return resolve(["test", "test", "test"]);

    });
}

app.get('/profile', (req, res) => {
    var access_token = 1234;

    getStats(access_token, 's', 'x')
        .then((a) => {
            console.log(a);
            return getStats(access_token, 's', 'y', a);
        })
        .then((b) => {
            console.log(b);
            return getStats(access_token, 'm', 'x', b);
        })
        .then((c) => {
            console.log(c);
            return getStats(access_token, 'm', 'y', c);
        })
        .then((d) => {
            console.log(d);
            return getStats(access_token, 'l', 'x', d);
        })
        .then((e) => {
            console.log(e);
            return getStats(access_token, 'l', 'y', e);
        })
        .then((f) => {
            console.log(f);
            res.send(f.join("\n"));
    });
});

As you can see the nested callback structure and replace with a much more readable format. Read more about promises here.

The above could even be rewritten using Promise.all, which looks even nicer:

function getStats(access_token, time_range, x) {
    return new Promise((resolve, reject) => {
        return resolve(["test", "test", "test"]);
    });
}

app.get('/profile', (req, res) => {
    var access_token = 1234;

    Promise.all([
        getStats(access_token, 's', 'x'),
        getStats(access_token, 's', 'y'),
        getStats(access_token, 'm', 'x'),
        getStats(access_token, 'm', 'y'),
        getStats(access_token, 'l', 'x'),
        getStats(access_token, 'l', 'y')
    ]).then((values) => {
        console.log(values);
        res.send(values.join("\n"));
    });
});