Refactoring nested callbacks, node.js, async - node.js

Question

function indexArticles(callback) {
fs.readdir("posts/", function(err, files) {
async.map(files, readPost, function(err, markdown) {
async.map(markdown, parse, function(err, results) {
async.sortBy(results, function(obj, callback) {
callback(err, obj.date);
}, function(err, sorted) {
callback( {"articles": sorted.reverse()} );
});
});
});
});
}
I'm trying to figure out how to make this prettier -- as you can tell I'm using caolan's async library, but I'm not sure which of the control flow structures to use. It seems like if I use async.waterfall, for example, that results in quite a bit more code, with each step having to be wrapped in an anonymous function. For example, this is just the first two lines of the nested version with waterfall:
function indexArticles(callback) {
async.waterfall([
function(callback) {
fs.readdir("posts/", function(err, files) {
callback(err, files)
})
},
function(files, callback) {
async.map(files, readPost, function(err, markdown) {
callback(err, markdown)
})
}])
}
How would you improve this?
If there were a way to partially apply arguments NOT only from the left, then I could see doing, for example,
function indexArticles(callback) {
async.waterfall([
async.apply(fs.readdir, "posts/"),
async.apply(async.map, __, readPost),
async.apply(async.map, __, parse),
// etc...
])
}

Answer 1

This is an interesting problem, as you need to bind arguments both to the left and to the right of your iterator functions, so neither bind/ nor bindRight (of which there are a few implementaions on StackOverflow) will work for you. There's a few options for you here:
(1) First, in your async.waterfall example, you have:
function(callback) {
fs.readdir("posts/", function(err, files) {
callback(err, files)
})
}
which is the same as:
function(callback) {
fs.readdir("posts/", callback)
}
Using Function.bind and this method, your entire function indexArticles could be written:
function indexArticles(callback) {
async.waterfall([
fs.readdir.bind(this, 'posts/'),
function(files, cb) { async.map(files, readPost, cb); },
function(text, cb) { async.map(text, parse, cb); },
function(results, cb) { async.sortBy(results, function(obj, callback) {
callback(null, obj.date);
}, cb) }
], function(err, sorted) {
callback( {"articles": sorted.reverse()} );
});
};
Which is a bit shorter.
(2) If you really want to avoid the wrapping functions, you can use a type of partial function application. First, at the top of your file (or in a module, etc), define a function called partial:
var partial = function(fn) {
var args = Array.prototype.slice.call(arguments, 1);
return function() {
var currentArg = 0;
for(var i = 0; i < args.length && currentArg < arguments.length; i++) {
if (args[i] === undefined)
args[i] = arguments[currentArg++];
}
return fn.apply(this, args);
};
}
This function takes a function and any number of arguments, and replaces undefined values in the arguments list with the actual arguments when the function is called. You would then use it like this:
function indexArticles(callback) {
async.waterfall([
fs.readdir.bind(this, 'posts/'),
partial(async.map, undefined, readPost, undefined),
partial(async.map, undefined, parse, undefined),
partial(async.sortBy, undefined, function(obj, callback) {
callback(null, obj.date);
}, undefined)
], function(err, sorted) {
callback( {"articles": sorted.reverse()} );
});
}
So, partial(async.map, undefined, readPost, undefined) returns a function that, when called by the Async library as fn(files, callback), it fills in files for the first undefined, and callback for the second undefined, ending in a call to async.map(files, readPost, callback).
(3) There is also a version of partial for Function.prototype at this StackOverflow answer, allowing you to use the syntax: async.map.partial(undefined, readPost, undefined); however, I would probably recommend against modifying Function.prototype in this way, and just use partial as a function.
In the end, it's up to you which method is the most readable and maintainable.

Answer 2

Looks like I have some overlap with Brandon's answer, but here's my take:
var async = require("async")
//dummy function
function passThrough(arg, callback){
callback(null, arg)
}
//your code rewritten to only call the dummy.
//same structure, didn't want to think about files and markdown
function indexArticles(callback) {
passThrough("posts/", function(err, files) {
async.map(files, passThrough, function(err, markdown) {
async.map(markdown, passThrough,
function(err, results) {
async.sortBy(results, function(obj, callback) {
callback(err, obj);
},
function(err, sorted) {
callback( {"articles": sorted.reverse()} );
});
});
});
});
}
indexArticles(console.log)
//version of apply that calls
//fn(arg, arg, appliedArg, apliedArg, callback)
function coolerApply(fn) {
var args = Array.prototype.slice.call(arguments, 1);
return function () {
var callback = Array.prototype.slice.call(arguments, -1)
var otherArgs = Array.prototype.slice.call(arguments, 0, -1)
return fn.apply(
null, otherArgs.concat(args).concat(callback)
);
};
};
//my version of your code that uses coolerAppl
function indexArticles2(callback){
async.waterfall([
async.apply(passThrough, "posts/"),
coolerApply(async.map, passThrough),
coolerApply(async.map, passThrough),
coolerApply(async.sortBy, function(obj, callback){callback(null,obj)})
],
function(err, sorted){
callback({"articles": sorted.reverse()})
})
}
//does the same thing as indexArticles!
indexArticles2(console.log)

Answer 3

Here's what I've ended up with so far.
function indexArticles(callback) {
var flow = [
async.apply(fs.readdir, "posts/"),
function(data, callback) { async.map(data, readPost, callback); },
function sortByDate(parsed, callback) {
var iterator = function(obj, callback) {
if (obj.date) { callback(null, obj.date); }
else { callback("Article has no date.") }
}
// Note that this sorts in reverse lexicographical order!
async.sortBy(parsed, iterator,
function(err, sorted) { callback(err, {"articles": sorted.reverse()} ); }
);
}
];
async.waterfall(flow, async.apply(callback))
}

Answer 4

I've recently created a simple abstraction named WaitFor to call async functions in sync mode (based on Fibers): https://github.com/luciotato/waitfor
I've not tested it with the async package, but it should work. If you run into problems, contact me.
Using wait.for and async your code will be:
var wait = require('waitfor');
var async = require('async');
function indexArticles(callback) {
var files = wait.for(fs.readdir,"posts/");
var markdown = wait.for(async.map, files, readPost);
var results = wait.for(async.map, markdown, parse);
var sorted = wait.for(async.sortBy, results, function(obj, callback) {
callback(null, obj.date);
});
callback( null, {"articles": sorted.reverse()} );
}
to call your fn (async-mode):
//execute in a fiber
wait.launchFiber(indexArticles,function(err,data){
// do something with err,data
});
to call your fn (sync-mode):
//execute in a fiber
function handleRequest(req,res){
try{
...
data = wait.for(indexArticles); //call indexArticles and wait for results
// do something with data
res.end(data.toString());
}
catch(err){
// handle errors
}
}
// express framework
app.get('/posts', function(req, res) {
// handle request in a Fiber, keep node spinning
wait.launchFiber(handleRequest,req,res);
});