Callbacks, Promises, and Coroutines (oh my!): Asynchronous Programming Patterns in JavaScript

Callbacks, Promises, and Coroutines
(oh my!)

Asynchronous Programming
Patterns in JavaScript

Domenic Denicola
http://domenicdenicola.com
@domenicdenicola

In non-web languages,
most of the code we write is synchronous.

aka blocking

Console.WriteLine("What is your name?");
string name = Console.ReadLine();
Console.WriteLine("Hello, " + name);

var fileNames = Directory.EnumerateFiles("C:");

foreach (var fileName in fileNames)
{
using (var f = File.Open(fileName, FileMode.Open))
{
Console.WriteLine(fileName + " " + f.Length);
}
}

using (var client = new WebClient())
{
string html =
client.DownloadString("http://news.ycombinator.com");

Console.WriteLine(html.Contains("Google"));
Console.WriteLine(html.Contains("Microsoft"));
Console.WriteLine(html.Contains("Apple"));
}

Thread.Start BackgroundWorker

Control.InvokeRequired

This often causes us some pain…

… but hey, there’s always threads!

Dispatcher.Invoke

ThreadPool

.AsParallel()

Q: What are these threads doing, most of the time?

A: waiting

In JavaScript, we do things differently.

There’s only one thread in JavaScript,
so we use that thread to get stuff done.

OK, let’s talk about…
• The event loop
• Callbacks
• Promises
• Coroutines

You’ve seen event loops before:

int WINAPI WinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPSTR lpCmdLine,
int nCmdShow)
{
MSG msg;
while (GetMessage(&msg, NULL, 0, 0) > 0)
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return msg.wParam;
}

this.btnOK.Click += this.btnOK_Click;

private void btnOK_Click(object sender,
EventArgs e)
{
// ...
}

$("#ok-button").click(function () {
// ...
});

setTimeout(function () {
// ...
}, 100);

$.get("http://example.com", function (result) {
// ...
});

Some event loop subtleties
• Yielding
• Async’s not sync
• Errors
• It’s not magic

Event Loop Subtleties

Yielding
console.log("1");

$.get("/echo/2", function (result) {
console.log(result);
});

console.log("3");

// 1, 3, 2


Async’s not sync
var hi = null;

$.get("/echo/hi", function (result) {
hi = result;
});

console.log(hi);

// null


Errors
console.log("About to get the website...");

$.ajax("http://sometimesdown.example.com", {
success: function (result) {
console.log(result);
},
error: function () {
throw new Error("Error getting the website");
}
});

console.log("Continuing about my business...");


It’s not magic
function fib(n) {
return n < 2 ? 1 : fib(n-2) + fib(n-1);
}

console.log("1");

console.log("2");
}, 100);

fib(40);

// 1 ... 15 seconds later ... 2

http://teddziuba.com/2011/10/node-js-is-cancer.html

The event loop is tricky… but powerful.

What we’ve seen so far has been doing
asynchronicity through callbacks.

Callbacks are OK for simple operations, but
force us into continuation passing style.

Recurring StackOverflow question:
function getY() {
var y;
$.get("/gety", function (jsonData) {
y = jsonData.y;
});
return y;
}
Why doesn’t it work???
var x = 5;
var y = getY();

console.log(x + y);

After getting our data, we have to do
everything else in a continuation:

function getY(continueWith) {
$.get("/gety", function (jsonData) {
continueWith(jsonData.y);
});
}

var x = 5;
getY(function (y) {
console.log(x + y);
});

CPS Headaches
• Doing things in sequence is hard
• Doing things in parallel is harder
• Errors get lost easily

CPS Headaches

Doing things in sequence is hard
$("#button").click(function () {
promptUserForTwitterHandle(function (handle) {
twitter.getTweetsFor(handle, function (tweets) {
ui.show(tweets);
});
});
});

CPS Headaches

Doing things in parallel is harder
var tweets, answers, checkins;

twitter.getTweetsFor("domenicdenicola", function (result) {
tweets = result;
somethingFinished();
});
stackOverflow.getAnswersFor("Domenic", function (result) {
answers = result;
});
fourSquare.getCheckinsBy("Domenic", function (result) {
checkins = result;
});

CPS Headaches

Doing things in parallel is harder
var finishedSoFar = 0;

function somethingFinished() {
if (++finishedSoFar === 3) {
ui.show(tweets, answers, checkins);
}
}

CPS Headaches

Errors get lost easily
function getTotalFileLengths(path, callback) {
fs.readdir(path, function (err, fileNames) {
var total = 0;

var finishedSoFar = 0;
function finished() {
if (++finishedSoFar === fileNames.length) {
callback(total);
}
}

fileNames.forEach(function (fileName) {
fs.readFile(fileName, function (err, file) {
total += file.length;
finished();
});
});
});
}

You could write your own library to make this nicer…

parallel([
function (cb) {
twitter.getTweetsFor("domenicdenicola", cb);
},
function (cb) {
stackOverflow.getAnswersFor("Domenic", cb);
},
function (cb) {
fourSquare.getCheckinsFor("Domenic", cb);
}
], function (results) {
console.log("tweets = ", results[0]);
console.log("answers = ", results[1]);
console.log("checkins = ", results[2]);
});

And in fact many people have:

https://github.com/joyent/node/wiki/modules#wiki-async-flow

The best of these (IMO) are based on
an abstraction called “promises”

Un-inverts the chain of responsibility:
instead of calling a passed callback, return a promise.

addWithCallback(a, b, function (result) {
assert.equal(result, a + b);
});

var promise = addWithPromise(a, b);

promise.then(function (result) {
assert.equal(result, a + b);
});

Why promises are awesome

• Cleaner method signatures
• Uniform return/error semantics
• Easy composition
• Easy sequential/parallel join
• Always async
• Exception-style error bubbling

Promises are Awesome

Cleaner method signatures
Uniform return/error semantics
$.get(
url,
[data],
[success(data, status, xhr)],
[dataType]
)


$.ajax(url, settings)

settings.success(data, status, xhr)
settings.error(xhr, status, errorThrown)
settings.complete(xhr, status)


fs.open(
path,
flags,
[mode],
[callback(error, file)]
)


fs.write(
file,
buffer,
offset,
length,
position,
[callback(error, written, buffer)]
)


getAsPromise(url, [data], [dataType]).then(
function onFulfilled(result) {
var data = result.data;
var status = result.status;
var xhr = result.xhr;
},
function onBroken(error) {
console.error("Couldn't get", error);
}
);


Easy composition

function getUser(userName, onSuccess, onError) {
$.ajax("/user?" + userName, {
success: onSuccess,
error: onError
});
}


Easy composition

function getUser(userName) {
return getAsPromise("/user?" + userName);
}


Easy composition

function getFirstName(userName, onSuccess, onError) {
success: function successProxy(data) {
onSuccess(data.firstName);
},
error: onError
});
}


Easy composition

function getFirstName(userName) {
return getAsPromise("/user?" + userName)
.get("firstName");
}


Easy sequential join

$("#button").click(function () {
promptUserForTwitterHandle(function (handle) {
twitter.getTweetsFor(handle, function (tweets) {
ui.show(tweets);
});
});
});


Easy sequential join

$("#button").clickPromise()
.then(promptUserForTwitterHandle)
.then(twitter.getTweetsFor)
.then(ui.show);


Easy parallel join

var tweets, answers, checkins;

twitter.getTweetsFor("domenicdenicola", function (result) {
tweets = result;
});
stackOverflow.getAnswersFor("Domenic", function (result) {
answers = result;
});
fourSquare.getCheckinsBy("Domenic", function (result) {
checkins = result;
});


Easy parallel join

Q.all([
twitter.getTweetsFor("domenicdenicola"),
stackOverflow.getAnswersFor("Domenic"),
fourSquare.getCheckinsBy("Domenic")
]).then(function (results) {
console.log(results[0], results[1], results[2]);
});


Easy parallel join

Q.all([
twitter.getTweetsFor("domenicdenicola"),
stackOverflow.getAnswersFor("Domenic"),
fourSquare.getCheckinsBy("Domenic")
]).spread(function (tweets, answers, checkins) {
console.log(tweets, answers, checkins);
});


Always async

function getUser(userName, onSuccess, onError) {
if (cache.has(userName)) {
onSuccess(cache.get(userName));
} else {
success: onSuccess,
error: onError
});
}
}


Always async

console.log("1");

getUser("ddenicola", function (user) {
console.log(user.firstName);
});

console.log("2");

// 1, 2, Domenic


Always async

console.log("1");

getUser("ddenicola", function (user) {
});

console.log("2");

// 1, Domenic, 2


Always async

function getUser(userName) {
if (cache.has(userName)) {
return Q.ref(cache.get(userName));
} else {
return getWithPromise("/user?" + userName);
}
}


Always async

console.log("1");

getUser("ddenicola").then(function (user) {
});

console.log("2");

// 1, 2, Domenic (every time)


Exception-style error bubbling

getUser("Domenic", function (user) {
getBestFriend(user, function (friend) {
ui.showBestFriend(friend);
});
});



getUser("Domenic", function (err, user) {
if (err) {
ui.error(err);
} else {
getBestFriend(user, function (err, friend) {
if (err) {
ui.error(err);
} else {
}
});
}
});



getUser("Domenic")
.then(getBestFriend, ui.error)
.then(ui.showBestFriend, ui.error);



getUser("Domenic")
.then(getBestFriend)
.then(ui.showBestFriend, ui.error);



ui.startSpinner();
getUser("Domenic")
.then(
function (friend) {
ui.stopSpinner();
},
function (error) {
ui.error(error);
ui.stopSpinner();
}
);



ui.startSpinner();
getUser("Domenic")
.then(ui.showBestFriend, ui.error)
.fin(ui.stopSpinner);

Sounds great. How do I get in on this action?

Use Q
• By Kris Kowal, @kriskowal
• https://github.com/kriskowal/q
• Can consume promises from jQuery etc.
• Implements various CommonJS standards

If you’re already using jQuery’s promises, switch to Q:

https://github.com/kriskowal/q/wiki/jQuery

Callbacks, Promises, and Coroutines (oh my!): Asynchronous Programming Patterns in JavaScript

Creating promises with Q

Fulfilling promises
// We have:
setTimeout(doSomething, 1000);

// We want:
delay(1000).then(doSomething);


Fulfilling promises
function delay(ms) {
var deferred = Q.defer();
setTimeout(deferred.resolve, ms);
return deferred.promise;
}

delay(1000).then(doSomething);


Breaking promises
function getWithTimeout(url, ms, onSuccess, onError) {
var isTimedOut = false, isHttpErrored = false;

if (!isHttpErrored) {
isTimedOut = true;
onError(new Error("timed out"));
}
}, ms);

$.ajax(url, {
success: function (result) {
if (!isTimedOut) { onSuccess(result); }
},
error: function (xhr, status, error) {
if (!isTimedOut) {
isHttpErrored = true;
onError(error);
}
}
});
}


Breaking promises
function getWithTimeout(url, ms) {

deferred.reject(new Error("timed out"));
}, ms);

$.ajax(url, {
success: deferred.resolve,
error: deferred.reject
});

}


Building abstractions
function timeout(promise, ms) {
promise.then(deferred.resolve, deferred.reject);

deferred.reject(new Error("timed out"));
}, ms);

}

function getWithTimeout(url, ms) {
return timeout(getAsPromise(url), ms);
}

They clean up our method signatures.

They’re composable, they’re joinable,
and they’re dependably async.

They unify various callback conventions
into something very much like return
values and exceptions.

But… we still have to write in CPS.

“Coroutines are computer program
components that generalize subroutines
to allow multiple entry points for
suspending and resuming execution at
certain locations.”

http://en.wikipedia.org/wiki/Coroutine

Nice:
var xP = getX();
var yP = getY();
var zP = getZ();

Q.all([xP, yP, zP]).spread(function (x, y, z) {
console.log(x + y + z);
});

Nicer:
var [x, y, z] = await Q.all([getX(), getY(), getZ()]);

console.log(x + y + z);

Nice:
$("#button").clickPromise()
.then(promptUserForTwitterHandle)
.then(twitter.getTweetsFor)
.then(ui.show);

Nicer:
await $("#button").clickPromise();

var handle = await promptUserForTwitterHandle();
var tweets = await twitter.getTweetsFor(handle);

ui.show(tweets);

Q: Can’t the compiler do this for me?

A: yes… if you are willing to introduce a compiler.

Several options, none perfect
• Kaffeine: http://weepy.github.com/kaffeine/
• Traceur: http://tinyurl.com/traceur-js
• TameJS: http://tamejs.org/
• Node fork: http://tinyurl.com/node-async

Q: OK well… can’t the interpreter do this for me?

A: yes… if you’re willing to wait for the next version of JS.

The next version of JavaScript (“ECMAScript
Harmony”) has a limited form of coroutines that can
be twisted to do something like what we want.

ECMAScript Harmony generators
function* fibonacci() {
var [prev, curr] = [0, 1];
for (;;) {
[prev, curr] = [curr, prev + curr];
yield curr;
}
}

for (n of fibonnaci()) {
console.log(n);
}

http://wiki.ecmascript.org/doku.php?id=harmony:generators

var eventualAdd = Q.async(function* (pA, pB) {
var a = yield pA;
var b = yield pB;
return a + b;
});

https://github.com/kriskowal/q/tree/master/examples/async-generators

// Can only use yield as we want to within
// Q.async'ed generator functions

Q.async(function* () {
// Talk to the server to get one and two.
var three = yield eventualAdd(getOne(), getTwo());

assert.equal(three, 3);
})();

https://groups.google.com/d/topic/q-continuum/7PWKbgeFA48/discussion

// Given promise-returning delay(ms) as before:

var animateAsync = Q.async(function* (el) {
for (var i = 0; i < 100; ++i) {
element.style.left = i;
yield delay(20);
}
});

http://wiki.ecmascript.org/doku.php?id=strawman:async_functions

Q.async(function* () {
var el = document.getElementById("my-element");

yield animateAsync(el);

console.log("it's done animating");
})();

https://groups.google.com/d/topic/q-continuum/7PWKbgeFA48/discussion

So coroutines are a bit of a mess, but
we’ll see how things shape up.

Recap
• Async is here to stay
• But you don’t have to dive into callback hell
• Use promises
• Use Q
• Maybe use coroutines if you’re feeling brave

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