How to *actually* use promises in JavaScript

Transcript

1. jakobm.com @jakobmattsson I’m a coder first and foremost. I also help companies recruit coders, train coders and architect software. Sometimes I do technical due diligence and speak at conferences. ! Want more? Read my story or blog.
2. From the Swedish depths
3. Social coding
4. FishBrain is the fastest, easiest way to log and share your fishing. ! Get instant updates from anglers on nearby lakes, rivers, and the coast.
5. Enough with the madness
6. Enough with the madness The lack and/or misuse of promises
7. Promises 101* *No, this is not a tutorial
8. getJSON({ url: '/somewhere/over/the/rainbow', success: function(result) { // deal with it } });
9. getJSON({ url: '/somewhere/over/the/rainbow', success: function(result) { // deal with it } }); rainbows.then(function(result) { // deal with it }); var rainbows = getJSON({ url: '/somewhere/over/the/rainbow' });
10. getJSON({ url: '/somewhere/over/the/rainbow', success: function(result) { // deal with it } }); rainbows.then(function(result) { // deal with it }); var rainbows = getJSON({ url: '/somewhere/over/the/rainbow' }); f(rainbows); // such preprocessing!
11. Why is that a good idea? • Loosen up the coupling • Superior error handling • Simplified async
12. That’s enough 101
13. Promises are not new
14. Promises are not new http://github.com/kriskowal/q http://www.html5rocks.com/en/tutorials/es6/promises http://domenic.me/2012/10/14/youre-missing-the-point-of-promises http://www.promisejs.org https://github.com/bellbind/using-promise-q https://github.com/tildeio/rsvp.js https://github.com/cujojs/when
15. They’ve even made it into ES6
16. They’ve even made it into ES6 Already implemented natively in Firefox 30 Chrome 33
17. So why are you not using them?
18. So why are you not using them?
19. How to draw an owl
20. 1. Draw some circles How to draw an owl
21. 1. Draw some circles 2. Draw the rest of the fucking owl How to draw an owl
22. We want to draw owls. ! Not circles.
23. getJSON('story.json').then(function(story) { addHtmlToPage(story.heading); ! // Map our array of chapter urls to // an array of chapter json promises. // This makes sture they all download parallel. return story.chapterUrls.map(getJSON) .reduce(function(sequence, chapterPromise) { // Use reduce to chain the promises together, // adding content to the page for each chapter return sequence.then(function() { // Wait for everything in the sequence so far, // then wait for this chapter to arrive. return chapterPromise; }).then(function(chapter) { addHtmlToPage(chapter.html); }); }, Promise.resolve()); }).then(function() { addTextToPage('All done'); }).catch(function(err) { // catch any error that happened along the way addTextToPage("Argh, broken: " + err.message); }).then(function() { document.querySelector('.spinner').style.display = 'none'; }); As announced for ES6
24. That’s circles. ! Nice circles! ! Still circles.
25. browser.init({browserName:'chrome'}, function() { browser.get("http://admc.io/wd/test-pages/guinea-pig.html", function() { browser.title(function(err, title) { title.should.include('WD'); browser.elementById('i am a link', function(err, el) { browser.clickElement(el, function() { browser.eval("window.location.href", function(err, href) { href.should.include('guinea-pig2'); browser.quit(); }); }); }); }); }); }); Node.js WebDriver Before promises
26. browser .init({ browserName: 'chrome' }) .then(function() { return browser.get("http://admc.io/wd/test-pages/guinea-pig.html"); }) .then(function() { return browser.title(); }) .then(function(title) { title.should.include('WD'); return browser.elementById('i am a link'); }) .then(function(el) { return browser.clickElement(el); }) .then(function() { return browser.eval("window.location.href"); }) .then(function(href) { href.should.include('guinea-pig2'); }) .fin(function() { return browser.quit(); }) .done(); After promises
27. Used to be callback-hell. ! Now it is then-hell.
30. These examples are just a different way of doing async. ! It’s still uncomfortable. It’s still circles!
31. The point of promises:
32. ! Make async code as easy to write as sync code The point of promises:
33. 1 Promises out: Always return promises - not callback 2 Promises in: Functions should accept promises as well as regular values 3 Promises between: Augment promises as you augment regular objects Three requirements
34. Let me elaborate
35. Example - Testing a blog API • Create a blog • Create two blog entries • Create some users • Create some comments from those users, on the two posts • Request the stats for the blog and check if the given number of entries and comments are correct
36. post('/blogs', { name: 'My blog' }, function(err, blog) { ! post(concatUrl('blogs', blog.id, 'entries'), { title: 'my first post’, body: 'Here is the text of my first post' }, function(err, entry1) { ! post(concatUrl('blogs', blog.id, 'entries'), { title: 'my second post’, body: 'I do not know what to write any more...' }, function(err, entry2) { ! post('/user', { name: 'john doe’ }, function(err, visitor1) { ! post('/user', { name: 'jane doe' }, function(err, visitor2) { ! post('/comments', { userId: visitor1.id, entryId: entry1.id, text: "well written dude" }, function(err, comment1) { ! post('/comments', { userId: visitor2.id, entryId: entry1.id, text: "like it!" }, function(err, comment2) { ! post('/comments', { userId: visitor2.id, entryId: entry2.id, text: "nah, crap" }, function(err, comment3) { ! get(concatUrl('blogs', blog.id), function(err, blogInfo) { assertEquals(blogInfo, { name: 'My blog', numberOfEntries: 2, numberOfComments: 3 }); }); }); }); }); }); }); }); }); }); https:// github.com/ jakobmattsson/ z-presentation/ blob/master/ promises-in-out/ 1-naive.js 1 Note: without narration, this slide lacks a lot of context. Open the file above and read the commented version for the full story.
37. post('/blogs', { name: 'My blog' }, function(err, blog) { ! var entryData = [{ title: 'my first post', body: 'Here is the text of my first post' }, { title: 'my second post', body: 'I do not know what to write any more...' }] ! async.forEach(entryData, function(entry, callback), { post(concatUrl('blogs', blog.id, 'entries'), entry, callback); }, function(err, entries) { ! var usernames = ['john doe', 'jane doe']; ! async.forEach(usernames, function(user, callback) { post('/user', { name: user }, callback); }, function(err, visitors) { ! var commentsData = [{ userId: visitor[0].id, entryId: entries[0].id, text: "well written dude" }, { userId: visitor[1].id, entryId: entries[0].id, text: "like it!" }, { userId: visitor[1].id, entryId: entries[1].id, text: "nah, crap" }]; ! async.forEach(commentsData, function(comment, callback) { post('/comments', comment, callback); }, function(err, comments) { ! get(concatUrl('blogs', blog.id), function(err, blogInfo) { ! assertEquals(blogInfo, { name: 'My blog', numberOfEntries: 2, numberOfComments: 3 }); }); }); }); }); }); https:// github.com/ jakobmattsson/ z-presentation/ blob/master/ promises-in-out/ 2-async.js 2 Note: without narration, this slide lacks a lot of context. Open the file above and read the commented version for the full story.
38. https:// github.com/ jakobmattsson/ z-presentation/ blob/master/ promises-in-out/ 3-async-more-parallel.js 3 Note: without narration, this slide lacks a lot of context. Open the file above and read the commented version for the full story. post('/blogs', { name: 'My blog' }, function(err, blog) { ! async.parallel([ function(callback) { var entryData = [{ title: 'my first post', body: 'Here is the text of my first post' }, { title: 'my second post', body: 'I do not know what to write any more...' }]; async.forEach(entryData, function(entry, callback), { post(concatUrl('blogs', blog.id, 'entries'), entry, callback); }, callback); }, function(callback) { var usernames = ['john doe', 'jane doe’]; async.forEach(usernames, function(user, callback) { post('/user', { name: user }, callback); }, callback); } ], function(err, results) { ! var entries = results[0]; var visitors = results[1]; ! var commentsData = [{ userId: visitors[0].id, entryId: entries[0].id, text: "well written dude" }, { userId: visitors[1].id, entryId: entries[0].id, text: "like it!" }, { userId: visitors[1].id, entryId: entries[1].id, text: "nah, crap" }]; ! async.forEach(commentsData, function(comment, callback) { post('/comments', comment, callback); }, function(err, comments) { ! get(concatUrl('blogs', blog.id), function(err, blogInfo) { assertEquals(blogInfo, { name: 'My blog', numberOfEntries: 2, numberOfComments: 3 }); }); }); }); });
39. post('/blogs', { name: 'My blog' }).then(function(blog) { ! var visitor1 = post('/user', { name: 'john doe' }); ! var visitor2 = post('/user', { name: 'jane doe' }); ! var entry1 = post(concatUrl('blogs', blog.id, 'entries'), { title: 'my first post', body: 'Here is the text of my first post' }); ! var entry2 = post(concatUrl('blogs', blog.id, 'entries'), { title: 'my second post', body: 'I do not know what to write any more...' }); ! var comment1 = all(entry1, visitor1).then(function(e1, v1) { post('/comments', { userId: v1.id, entryId: e1.id, text: "well written dude" }); }); ! var comment2 = all(entry1, visitor2).then(function(e1, v2) { post('/comments', { userId: v2.id, entryId: e1.id, text: "like it!" }); }); ! var comment3 = all(entry2, visitor2).then(function(e2, v2) { post('/comments', { userId: v2.id, entryId: e2.id, text: "nah, crap" }); }); ! all(comment1, comment2, comment3).then(function() { get(concatUrl('blogs', blog.id)).then(function(blogInfo) { assertEquals(blogInfo, { name: 'My blog', numberOfEntries: 2, numberOfComments: 3 }); }); }); }); https:// github.com/ jakobmattsson/ z-presentation/ blob/master/ promises-in-out/ 4-promises-convoluted.js 4 Note: without narration, this slide lacks a lot of context. Open the file above and read the commented version for the full story.
40. var blog = post('/blogs', { name: 'My blog' }); ! var entry1 = post(concatUrl('blogs', blog.get('id'), 'entries'), { title: 'my first post', body: 'Here is the text of my first post' }); ! var entry2 = post(concatUrl('blogs', blog.get('id'), 'entries'), { title: 'my second post', body: 'I do not know what to write any more...' }); ! var visitor1 = post('/user', { name: 'john doe' }); ! var visitor2 = post('/user', { name: 'jane doe' }); ! var comment1 = post('/comments', { userId: visitor1.get('id'), entryId: entry1.get('id'), text: "well written dude" }); ! var comment2 = post('/comments', { userId: visitor2.get('id'), entryId: entry1.get('id'), text: "like it!" }); ! var comment3 = post('/comments', { userId: visitor2.get('id'), entryId: entry2.get('id'), text: "nah, crap" }); ! var allComments = [comment1, comment2, comment2]; ! var blogInfoUrl = concatUrl('blogs', blog.get('id')); ! var blogInfo = getAfter(blogInfoUrl, allComments); ! assertEquals(blogInfo, { name: 'My blog', numberOfEntries: 2, numberOfComments: 3 }); https:// github.com/ jakobmattsson/ z-presentation/ blob/master/ promises-in-out/ 5-promises-nice.js 5 Note: without narration, this slide lacks a lot of context. Open the file above and read the commented version for the full story.
42. 1 Promises out: Always return promises - not callback 2 Promises in: Functions should accept promises as well as regular values 3 Promises between: Augment promises as you augment regular objects Three requirements
43. Augmenting objects is a sensitive topic
44. Extending prototypes ! vs ! wrapping objects
45. You’re writing Course of action An app Do whatever you want A library Do not fucking modify the
 god damn prototypes Complimentary decision matrix
47. Promises are already wrapped objects
48. _(names) .chain() .unique() .shuffle() .first(3) .value() Chaining usually requires a method to ”unwrap” or repeated wrapping u = _(names).unique() s = _(u).shuffle() f = _(s).first(3)
49. Promises already have a well-defined way of unwrapping.
50. _(names) .unique() .shuffle() .first(3) .then(function(values) { // do stuff with values... }) If underscore/lodash wrapped promises
51. But they don’t
52. Enter ! Z
53. 1 Deep resolution: Resolve any kind of object/array/promise/values/whatever 2 Make functions promise-friendly: Sync or async doesn’t matter; will accept promises 3 Augmentation for promises: jQuery/ underscore/lodash-like extensions What is Z?
54. Deep resolution var data = { userId: visitor1.get('id'), entryId: entry1.get('id'), text: 'well written dude' }; ! Z(data).then(function(result) { ! // result is now: { // userId: 123, // entryId: 456, // text: 'well written dude' // } ! }); Takes any object and resolves all promises in it. ! Like Q and Q.all, but deep 1
55. Promise-friendly functions var post = function(url, data, callback) { // POSTs `data` to `url` and // then invokes `callback` }; ! post = Z.bindAsync(post); ! var comment1 = post('/comments', { userId: visitor1.get('id'), entryId: entry1.get('id'), text: 'well written dude' }); bindAsync creates a function that takes promises as arguments and returns a promise. 2
56. Promise-friendly functions var add = function(x, y) { return x + y; }; ! add = Z.bindSync(add); ! var sum = add(v1.get('id'), e1.get('id')); ! var comment1 = post('/comments', { userId: visitor1.get('id'), entryId: entry1.get('id'), text: 'well written dude', likes: sum }); 2 bindSync does that same, for functions that are not async
57. Augmentation for promises var commentData = get('/comments/42'); ! var text = commentData.get('text'); ! var lowerCased = text.then(function(text) { return text.toLowerCase(); }); 3 Without augmentation every operation has to be wrapped in ”then”
59. Augmentation for promises Z.mixin({ toLowerCase: function() { return this.value.toLowerCase(); } }); ! var commentData = get('/comments/42'); ! commentData.get('text').toLowerCase(); 3 Z has mixin to solve this ! Note that Z is not explicitly applied to the promise
60. Augmentation for promises Z.mixin(zUnderscore); Z.mixin(zBuiltins); ! var comment = get('/comments/42'); ! comment.get('text').toLowerCase().first(5); 3 There are prepared packages to mixin entire libraries
61. 1 Promises out: Always return promises - not callback 2 Promises in: Functions should accept promises as well as regular values 3 Promises between: Augment promises as you augment regular objects Three requirements
62. Make async code as easy to write as sync code Enough with the madness
63. You don’t have to use Z to do these things
64. But FFS
66. #noandthen www.jakobm.com @jakobmattsson ! github.com/jakobmattsson/z-core bit.ly/sthlmjs + - Enough with the madness