node.js - turn an async function into a sync function

Question

node.js - turn an async function into a sync function

like many other peoples, I want to turn a async function of a third party module (Patio) into a sync function.

function get_user_message_list(parameters, array, response)
{
var new_array = [];
for (var i in array) {

    var json = array[i];
    json['content']['users_seen'] = ["1757842565"];
    json['content']['users_not_seen'] = [];

    new_array.push(json);
}

console.log("NEW ARRAY :");
console.log(new_array);

response.writeHeader(200, {'Content-Type':'application/json'});
response.end(JSON.stringify(new_array));
}

function get_json_message(parameters, json)
{
console.log("JSON OBJECT :");
console.log(json);
var dataset = db.from(TABLES.USER).join(TABLES.MOVIE_LIST, {MLUserId: sql.URId}).join(TABLES.MOVIE, {MVId: sql.MLMovieId});

dataset.where('MLSeen={seen} AND MVSourceId={movie} AND MVSource={source} AND URId!={user}', {seen: 1, movie: json['content']['movie_id'], source: json['content']['movie_source'], user:parameters.FACEBOOK_ID}).all().then(function(users){
    if (users) {
        for (var j in users) {
            json['content']['users_seen'].push(users[j].URId);
        }
    }

    //console.log(json['content']['users_seen']);

    dataset.where('MLSeen={seen} AND MVSourceId={movie} AND MVSource={source} AND URId!={user}', {seen: 0, movie: json['content']['movie_id'], source: json['content']['movie_source'], user:parameters.FACEBOOK_ID}).all().then(function(users){
        if (users) {
            for (var j in users) {
                json['content']['users_not_seen'].push(users[j].URId);
            }
        }

        console.log(json);
    }, errorHandler);
}, errorHandler);
}

In the get_user_message_list function I iterate into an array and for each iteration I calling the async function. In this async function I'm using Patio module to make request to MySQL database. But like you can see, I must wait for the query result to be get after sending a result to the previous function.

How can I turn get_json_message into sync_get_json_message ? Thank you.

javascript
node.js
asynchronous
sync
synchronous

Answer 1

You CAN and you SHOULD turn async functions into something that behaves like sync functions when a problem needs to be fixed. You can't is never the correct answer, the shouldn't is for the programmer to answer.

So, I recently found some code in the nodeunit module which may help you. It fires the async functions, keep track of which are ready. After all requests are in, fires the callback. This could be the idea behind the solution to your problem (so no, this is not the final solution).

async.forEachSeries = function (arr, iterator, callback) {
    if (!arr.length) {
        return callback();
    }
    var completed = 0;
    var iterate = function () {
        iterator(arr[completed], function (err) {
            if (err) {
                callback(err);
                callback = function () {};
            }
            else {
                completed += 1;
                if (completed === arr.length) {
                    callback();
                }
                else {
                    iterate();
                }
            }
        });
    };
    iterate();
};

This test triggered me to see how it was done:

exports['series'] = function (test) {
    var call_order = [];
    async.series([
        function (callback) {
            setTimeout(function () {
                call_order.push(1);
                callback(null, 1);
            }, 25);
        },
        function (callback) {
            setTimeout(function () {
                call_order.push(2);
                callback(null, 2);
            }, 50);
        },
        function (callback) {
            setTimeout(function () {
                call_order.push(3);
                callback(null, 3, 3);
            }, 15);
        }
    ],
    function (err, results) {
        test.equals(err, null);
        test.same(results, [1, 2, [3, 3]]);
        test.same(call_order, [1, 2, 3]);
        test.done();
    });
};

Happy programming!

Answer 2

You can't and you shouldn't. This would effectively block your Node.JS server and you would loose every advantage Node.JS provides. Also it is against the whole asynchronous idea behind Node.JS.

Just pass callback parameter to your function:

function get_json_message(parameters, json, callback){ // <---- note the callback
    // some other stuff
    dataset.where( ...
        // some other stuff
        dataset.where( ...
            // some other stuff
            // I've finished the job, call the callback
            callback(); // <--- you can pass additional params here
        });
    });
}

and call it like this:

get_json_message( params, json, function() {
    console.log('Hello world!');
    // do whatever you like inside callback
});

Answer 3

I've been using syncrhonize.js with great success. There's even a pending pull request (which works quite well) to support async functions which have multiple parameters. Far better and easier to use than node-sync imho. Added bonus that it has easy-to-understand and thorough documentation, whereas node-sync does not.