As presented in CodeMotion Tel Aviv: http://telaviv.codemotionworld.com

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 30Chrome 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