Redemption from Callback Hell

A strategy for avoiding messy callback APIs in JavaScript using Promises. Originally given on April 1, 2013 at HTML5 DevConf in San Francisco, CA.

Transcript

1. REDEMPTION FROM CALLBACK HELL
2. ASYNCHRONOUS LET’S TALK ABOUT JAVASCRIPT APIs
3. function getUser(name) { var sql = 'SELECT * FROM users WHERE name=?'; var user = query(sql, name); // <- blocking if (!user) throw new Error('no user!'); return user; }
4. function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { if (error) { callback(error); } else if (!user) { callback(new Error('no user!')); } else { callback(null, user); } }); }
5. function getUser(name) { var sql = 'SELECT * FROM users WHERE name=?'; var user = query(sql, name); // <- blocking if (!user) throw new Error('no user!'); return user; } function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { if (error) { callback(error); } else if (!user) { callback(new Error('no user!')); } else { callback(null, user); } }); }
6. NO RETURN IN A CALLBACK, THERE IS
7. function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { // Nobody gets this return value! return user; }); }
8. NO THROW IN A CALLBACK, THERE IS
9. function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { // Nobody can catch this error! if (error) throw error; }); }
10. function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { // Nobody can catch this error! if (error) throw error; // <- BAD }); }
11. • NO RETURN • NO THROW
12. function getUser(name) { var sql = 'SELECT * FROM users WHERE name=?'; var user = query(sql, name); // <- blocking if (!user) throw new Error('no user!'); return user; } function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { if (error) { callback(error); } else if (!user) { callback(new Error('no user!')); } else { callback(null, user); } }); }
13. NO STACK IN A CALLBACK, THERE IS
14. NO GUARANTEES USING CALLBACKS, THERE ARE
15. function getUsers(names, callback) { var sql = 'SELECT * FROM users WHERE name=?'; var users = []; names.forEach(function (name) { query(sql, name, function (error, user) { if (error) { callback(error); } else if (!user) { callback(new Error('no user!')); } else { users.push(user); if (users.length === names.length) { callback(null, users); } } }); }); }
16. • NO STACK (NO RETURN/THROW) • NO GUARANTEES • BUT AT LEAST WE DON’T BLOCK!?
17. function getUser(name) { var sql = 'SELECT * FROM users WHERE name=?'; var user = query(sql, name); // <- blocking if (!user) throw new Error('no user!'); return user; } function getUser(name, callback) { var sql = 'SELECT * FROM users WHERE name=?'; query(sql, name, function (error, user) { if (error) { callback(error); } else if (!user) { callback(new Error('no user!')); } else { callback(null, user); } }); }
18. ES6 GENERATORS THE FUTURE IS
19. function fibonacci() { var i = 0, j = 1; while (true) { yield i; var t = i; i = j; j += t; } } var generator = fibonacci(); for (var i = 0; i < 12; i++) { print(generator.next()); }
20. function fibonacci() { var i = 0, j = 1; while (true) { yield i; // <- awesome var t = i; i = j; j += t; } } var generator = fibonacci(); for (var i = 0; i < 12; i++) { print(generator.next()); }
21. • FIRST-CLASS COROUTINES • SUSPEND EXECUTION CONTEXT • SMALL API • NOT AVAILABLE YET
22. PROMISES/A+
23. • DISCOVERED CIRCA 1989 • INSPIRED BY E • WIDELY USED OUTSIDE JS
25. Horrible Lies!!!
30. https://github.com/promises-aplus/promises-spec
31. • Q • RSVP • when https://github.com/promises-aplus/promises-spec/blob/master/implementations.md
32. ASYNCHRONOUS A PROMISE IS AN VALUE
33. getUser('mjackson', function (error, user) { // ... }); // becomes getUser('mjackson').then(function (user) { // ... }, function (error) { // ... });
34. TRANSFORMS THERE ARE 4 BASIC SYNC TO ASYNC
35. var user = getUser('mjackson'); var name = user.name; // becomes getUser('mjackson').then(function (user) { return user.name; });
36. var user = getUser('mjackson'); if (!user) throw new Error('no user!'); var name = user.name; // becomes getUser('mjackson').then(function (user) { if (!timeline) throw new Error('no user!'); return user.name; });
37. try { deliverTweetTo(tweet, 'mjackson'); } catch (error) { handleError(error); } // becomes deliverTweetTo(tweet, 'mjackson') .then(undefined, handleError);
38. try { var user = getUser('mjackson'); } catch (error) { throw new Error('ERROR: ' + error.message); } // becomes getUser('mjackson').then(undefined, function (error) { throw new Error('ERROR: ' + error.message); });
39. try { var user = getUser('mjackson'); } catch (error) { throw new Error('ERROR: ' + error.message); } // becomes getUser('mjackson').then(undefined, function (error) { throw new Error('ERROR: ' + error.message); });
40. •STACK SEMANTICS •GUARANTEES
41. SEQUENCE OPERATIONS IN
42. var user = getUser('mjackson'); var tweets = getNewTweets(user); updateTimeline(tweets); // using callbacks getUser('mjackson', function (user) { getNewTweets(user, function (tweets) { updateTimeline(tweets); }); }); // using promises getUser('mjackson') .then(getNewTweets) .then(updateTimeline);
43. EXCEPTIONS HANDLING
44. try { var user = getUser('mjackson'); var tweets = getNewTweets(user); updateTimeline(tweets); } catch (error) { handleError(error); }
45. getUser('mjackson', function (error, user) { if (error) { handleError(error); } else { getNewTweets(user, function (error, tweets) { if (error) { handleError(error); } else { updateTimeline(tweets, function (error) { if (error) handleError(error); }); } }); } });
46. // Same example, complete with node.js time bombs! getUser('mjackson', function (error, user) { if (error) throw error; getNewTweets(user, function (error, tweets) { if (error) throw error; updateTimeline(tweets, function (error) { if (error) throw error; }); }); }); process.on('uncaughtException', handleError); // LOL
47. try { var user = getUser('mjackson'); var tweets = getNewTweets(user); updateTimeline(tweets); } catch (error) { handleError(error); } // becomes getUser('mjackson') .then(getNewTweets) .then(updateTimeline) .then(undefined, handleError);
48. FUD DISPEL THE
49. var fs = require('fs'); var q = require('q'); var readFile = q.denodeify(fs.readFile); var promise = readFile(__filename, 'utf8'); promise.then(console.log, console.error);
50. function createUser(userName, userData, callback) { return database.ensureUserNameNotTaken(userName) .then(function () { database.saveUserData(userName, userData); }) .nodeify(callback); }
51. •PROMISES COMING TO THE DOM •GENERATORS COMING TO V8 •PROMISES ARE FUTURE-PROOF
52. var q = require('q'); var getUser = q.async(function * (name) { var sql = 'SELECT * FROM users WHERE name=?'; var user = yield query(sql, name); if (!user) throw new Error('no user!'); return user; });
53. PARALLEL EXECUTE OPERATIONS IN
54. function getNewTweetsForUsers(users) { return users.map(getNewTweets); } // becomes var q = require('q'); function getNewTweetsForUsers(users) { var promises = users.map(getNewTweets); return q.all(promises); }
55. SERVERS BUILD FAULT-TOLERANT
56. var http = require('http'); var q = require('q'); var server = http.createServer(function (req, res) { handleRequest(req).then(function (response) { res.writeHead(response.status, response.headers); res.end(response.content); }, function (error) { console.error(error); res.writeHead(500, { 'Content-Type': 'text/plain' }); res.end('Internal Server Error'); }); }); server.listen(3000);
57. https://github.com/machjs/mach
58. THANKS!