Question

Lets say I wanna send an email then update the database, both actions are async. This is how I would normally write it.

send_email(function(err, id){
    if(err){
        console.log("error");
    }else{
        update_database(id,function(err, id){
            if(err){
                console.log("error");
            }else{
                console.log("success");
            }
        });
    }
});

I would like to do this instead with middleware.

var mid = {};

mid.send_email = function(){
    return function(next){
        send_email(function(err,id){
            if(err){
                console.log("error");
            }else{
                next(id);
            }
        });
    }
}

mid.update_database = function(){
    return function(id,next){
        update_database(id,function(err,id){
            if(err){
                console.log("error");
            }else{
                next(id);
            }
        });
    }
}

mid.success = function(){
    return function(id,next){
        console.log("success")
        next(id);
    }   
}

Stacking the middleware.

middleware.use(mid.send_email());
middleware.use(mid.update_database());
middleware.use(mid.success());

There are two main questions at hand.

• How can I use middleware in place of nested callbacks?
• Is it possible to pass variables to next()?

Answer 1

What you want is to be able to handle a async control flow. Alot of js library can help you to achieve this. You can try the Async library with the waterfall function since you want to be able to pass variables to the next function that will be executed :

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

“Runs an array of functions in series, each passing their results to the next in the array. However, if any of the functions pass an error to the callback, the next function is not executed and the main callback is immediately called with the error.”

Example :

async.waterfall([
    function(callback){
        callback(null, 'one', 'two');
    },
    function(arg1, arg2, callback){
        callback(null, 'three');
    },
    function(arg1, callback){
        // arg1 now equals 'three'
        callback(null, 'done');
    }
], function (err, result) {
   // result now equals 'done'    
});