Waiting for multiple callbacks in Node.js

Question

I have a Node.js application where multiple funcions might be called, depending on several factors, but only one last function is called after the last callback.

This is a simplified version of what I got:

if(foo === bar){
    function1(arg1, function(val1){
        doWhatever(val1, function(){
            res.end("Finished");
        });
    });
}else if(foo === baz){
    function2(arg2, function(val2){ 
        doWhatever(val2, function(){
            res.end("Finished");
        });
    });
}else{
    function3(arg3, function(val3){
        doWhatever(val3, function(){
            res.end("Finished");
        });
    });
}

And this is what im doing:

var finished = false;

if(foo === bar){
    function1(arg1, function(val1){
        result = val1;
        finished = true;
    });
}else if(foo === baz){
    function2(arg2, function(val2){ 
        result = val2;
        finished = true;
    });
}else{
    function3(arg3, function(val3){
        result = val3;
        finished = true;
    });
}

var id = setInterval(function(){
    if(finished === true){
        clearInterval(id);
        doWhatever(result, function(){
            res.end("Finished");
        });
    }
}, 100);

I guess this can be simplified by using promises, however im not sure how should I implement them.

You could consider using the excellent async module from github.com/caolan/async — m.jansink
– m.jansink, Commented Mar 18, 2014 at 16:54
async is pretty good, then you can get rid of the setinterval, which is pretty icky. You would just wait for the async callback to finish to do whatever. Another way would be to do the first way you have, and remove the extra function wrapper around 'dowtvr': function1(arg1, dowtvr); should work fine and be prettier. You also wouldn't need the cb in the dowtvr function. — mshell_lauren
– mshell_lauren, Commented Mar 18, 2014 at 17:27

Grokling · Accepted Answer · 2014-03-20 23:51:47Z

8

You could also do it using when and promises, which IMHO is the easiest to read.

var promises = [];

if(x) {
    var deferred1 = when.defer();
    doSomethingAsync({ callback: deferred1.resolve });
    promises.push(deferred1.promise);
} else if(y) {
    var deferred2 = when.defer();
    doSomethingAsync({ callback: deferred2.resolve });
    promises.push(deferred2.promise);
} else if(z) {
    var deferred3 = when.defer();
    doSomethingAsync({ callback: deferred3.resolve });
    promises.push(deferred3.promise);
}

when.all(promises).then(function () {
    console.log('Finished Promises');
});

edited Mar 20, 2014 at 23:51

Grokling

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answered Mar 19, 2014 at 8:53

jgillich

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2 Comments

user435943 Over a year ago

Everything works great, except that promises should be pushed to the array: promises.push(deferred1.promise);

Jeff Over a year ago

Using when is useful in situations where you cannot collect all of the asynchronous function calls into a single location, which seems to be necessary in order to use async.

aembke · Accepted Answer · 2014-03-18 17:39:51Z

3

Here's one way with async series.

https://github.com/caolan/async#series

async.series([
    function(callback){
        if(foo === bar){
            function1(arg1, function(val1){
                callback(null, val1);
            });
        }else if(foo === baz){
            function2(arg2, function(val2){ 
                callback(null, val2);
            });
        }else{
            function3(arg3, function(val3){ 
                callback(null, val3);
            });
        }
    }  
], function(error, valArray){
       doWhatever(valArray[0], function(){
           res.end("Finished");
       });
});

answered Mar 18, 2014 at 17:39

aembke

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Comments

Lucio M. Tato · Accepted Answer · 2014-03-19 08:43:02Z

3

Here's using wait.for

https://github.com/luciotato/waitfor

//in a fiber
var result;
if(foo === bar){
    result = wait.for(function1,arg1);
}else if(foo === baz){
    result = wait.for(function2,arg2);
}else{
    result = wait.for(function3,arg3);
};

doWhatever(result, function(){
        res.end("Finished");
});

You need to be in a fiber (or generator) to use wait.for, but, if you have a lot of callback hell, wait.for is a good approach.

edited Mar 19, 2014 at 8:43

answered Mar 18, 2014 at 23:31

Lucio M. Tato

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