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

Event Loop Subtleties Async’s not sync var hi = null; $.get("/echo/hi", function (result) { hi = result; }); console.log(hi); // null

Event Loop Subtleties 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...");

Event Loop Subtleties It’s not magic function fib(n) { return n < 2 ? 1 : fib(n-2) + fib(n-1); } console.log("1"); setTimeout(function () { 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; somethingFinished(); }); fourSquare.getCheckinsBy("Domenic", function (result) { checkins = result; somethingFinished(); });

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] )

Promises are Awesome Cleaner method signatures Uniform return/error semantics $.ajax(url, settings) settings.success(data, status, xhr) settings.error(xhr, status, errorThrown) settings.complete(xhr, status)

Promises are Awesome Cleaner method signatures Uniform return/error semantics fs.open( path, flags, [mode], [callback(error, file)] )

Promises are Awesome Cleaner method signatures Uniform return/error semantics fs.write( file, buffer, offset, length, position, [callback(error, written, buffer)] )

Promises are Awesome Cleaner method signatures Uniform return/error semantics 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); } );

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

Promises are Awesome Easy composition function getUser(userName) { return getAsPromise("/user?" + userName); }

Promises are Awesome Easy composition function getFirstName(userName, onSuccess, onError) { $.ajax("/user?" + userName, { success: function successProxy(data) { onSuccess(data.firstName); }, error: onError }); }

Promises are Awesome Easy composition function getFirstName(userName) { return getAsPromise("/user?" + userName) .get("firstName"); }

Promises are Awesome Easy sequential join $("#button").click(function () { promptUserForTwitterHandle(function (handle) { twitter.getTweetsFor(handle, function (tweets) { ui.show(tweets); }); }); });

Promises are Awesome Easy sequential join $("#button").clickPromise() .then(promptUserForTwitterHandle) .then(twitter.getTweetsFor) .then(ui.show);

Promises are Awesome Easy parallel join var tweets, answers, checkins; twitter.getTweetsFor("domenicdenicola", function (result) { tweets = result; somethingFinished(); }); stackOverflow.getAnswersFor("Domenic", function (result) { answers = result; somethingFinished(); }); fourSquare.getCheckinsBy("Domenic", function (result) { checkins = result; somethingFinished(); });

Promises are Awesome 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]); });

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

Promises are Awesome Always async function getUser(userName, onSuccess, onError) { if (cache.has(userName)) { onSuccess(cache.get(userName)); } else { $.ajax("/user?" + userName, { success: onSuccess, error: onError }); } }

Promises are Awesome Always async console.log("1"); getUser("ddenicola", function (user) { console.log(user.firstName); }); console.log("2"); // 1, 2, Domenic

Promises are Awesome Always async console.log("1"); getUser("ddenicola", function (user) { console.log(user.firstName); }); console.log("2"); // 1, Domenic, 2

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

Promises are Awesome Always async console.log("1"); getUser("ddenicola").then(function (user) { console.log(user.firstName); }); console.log("2"); // 1, 2, Domenic (every time)

Promises are Awesome Exception-style error bubbling getUser("Domenic", function (user) { getBestFriend(user, function (friend) { ui.showBestFriend(friend); }); });

Promises are Awesome Exception-style error bubbling getUser("Domenic", function (err, user) { if (err) { ui.error(err); } else { getBestFriend(user, function (err, friend) { if (err) { ui.error(err); } else { ui.showBestFriend(friend); } }); } });

Promises are Awesome Exception-style error bubbling getUser("Domenic") .then(getBestFriend, ui.error) .then(ui.showBestFriend, ui.error);

Promises are Awesome Exception-style error bubbling getUser("Domenic") .then(getBestFriend) .then(ui.showBestFriend, ui.error);

Promises are Awesome Exception-style error bubbling ui.startSpinner(); getUser("Domenic") .then(getBestFriend) .then( function (friend) { ui.showBestFriend(friend); ui.stopSpinner(); }, function (error) { ui.error(error); ui.stopSpinner(); } );

Promises are Awesome Exception-style error bubbling ui.startSpinner(); getUser("Domenic") .then(getBestFriend) .then(ui.showBestFriend, ui.error) .fin(ui.stopSpinner);

Promises are Awesome Exception-style error bubbling function getBestFriendAndDontGiveUp(user) { return getUser(user).then( getBestFriend, function (error) { if (error instanceof TemporaryNetworkError) { console.log("Retrying after error", error); return getBestFriendAndDontGiveUp(user); } throw error; }); }

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

Creating promises with Q Fulfilling promises // We have: setTimeout(doSomething, 1000); // We want: delay(1000).then(doSomething);

Creating promises with Q Fulfilling promises function delay(ms) { var deferred = Q.defer(); setTimeout(deferred.resolve, ms); return deferred.promise; } delay(1000).then(doSomething);

Creating promises with Q Breaking promises function getWithTimeout(url, ms, onSuccess, onError) { var isTimedOut = false, isHttpErrored = false; setTimeout(function () { 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); } } }); }

Creating promises with Q Breaking promises function getWithTimeout(url, ms) { var deferred = Q.defer(); setTimeout(function () { deferred.reject(new Error("timed out")); }, ms); $.ajax(url, { success: deferred.resolve, error: deferred.reject }); return deferred.promise; }

Creating promises with Q Building abstractions function timeout(promise, ms) { var deferred = Q.defer(); promise.then(deferred.resolve, deferred.reject); setTimeout(function () { deferred.reject(new Error("timed out")); }, ms); return deferred.promise; } 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

ECMAScript 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

ECMAScript Harmony 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

ECMAScript Harmony generators // 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

ECMAScript Harmony generators 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

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

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