Many developers new to Node.js struggle with writing asynchronous code in a clean, concise, easy to maintain manner. There are many strategies for managing and preventing 'callback hell' in Node.js. We'll walk through many of those strategies, from Promises to Generators to flow control libraries, and show that with a bit of forethought, writing asynchronous code in Javascript and Node.js can be easy and maintainable.

1. Promises,
Generators &
Callbacks!
Oh my!

2. Writing
asynchronous
code is hard

3. asynchronous
code is difficult to
read

4. asynchronous
code is difficult to
write

5. asynchronous
code is difficult to
maintain

6. Mike Frey

7. Why is writing
asynchronous
code hard?

8. Composition

9. Patterns

10. Callbacks

11. Promises

12. Generators

13. Now entering
Nerd War
territory

14. Goals

15. Callbacks
vs
Promises
vs
Generators

16. Callbacks
vs
Promises
vs
Generators
x
x

17. No
Winner

18. Unbiased

19. Informative

20. Callbacks

21. How do
they work?

22. Continuation
Passing

23. Pass a
function
to another
function

24. Ask for work now
Handle result later

25. askForWork(function(err, res) {
// handle result later
})

26. Where are
they used?

27. Everywhere

28. Node.js core

29. User-land

30. Even
Promises

31. Benefits

32. Simple.
Easy to use.
Prolific.
Fast.

33. Problems

34. Error
Handling

35. try{}
catch(){}

36. try{}
catch(){}

37. try {
doWork(function(res) {
// handle result
})
}
catch (err) {
// handle error
}

38. !
doWork(function(err, res) {
// handle error
// handle result
})
!
!

39. Homework!
read this:
!
joyent.com/developers/
node/design/errors

40. Z͈A͡ ̼͔̭͖͕̲
L͝ ͙͖̱
G̠͍̤̠͕
O͢ ̬̫
Z͗̒͊̅ͫ̎
̩̲̤͙̟
Ả͒͐̚
̥̞̥͜
L̀͊ͬ͡
̮̲Ğͥ̈ͩ̓͒
͕̘͉
O͍̼̘͇͔̠͐
!̓̾̆ͪ͆̚͞

41. asynchronous
or
synchronous
never both

42. Fix your API:
process.nextTick()
setImmediate()

43. Fix their API:
dezalgo

44. Callback
Hell

45. Composition
problem

46. Now
Later

47. Now
Later
Later-er

48. Now
Later
Later-er
Later-er-er

49. Now
Later
Later-er
Later-er-er
Later-er-er-er

50. There’s nothing forcing
you to write ten levels of
nested callbacks, but
the pattern does make
it easy for you to do so.
- Raymond Julin (paraphrased)

51. Now
Later
Later-er
Later-er-er
Later-er-er-er

52. Now
Later
Later-er
Later-er-er
Later-er-er-er

53. getA(function() {
getB(function() {
getC(function() {
// do something
})
})
})

54. function handleA() {
getB(handleB)
}
function handleB() {
getC(handleC)
}
function handleC() {
// do something
}
getA(handleA)

55. async
module

56. async.waterfall([
getA,
getB,
getC
],
function(err, result) {
// do something
})

57. Callbacks
!
Simple.
Everywhere.
Be careful.

58. Promises

59. How do
they work?

60. Eventual
Result

61. .then()

62. var promise = doSomething()
promise.then(
function (result) {
// success callback
},
function (error) {
// error callback
})

63. Now
Later success
Later failure

64. Where are
they used?

65. jQuery
AngularJS
Ember
User-land

66. Chrome 32
Firefox 29
Opera 19
Node.js 0.11.13

67. Benefits

68. Composition:
Chaining &
Error handling

69. .then()
.then()
.then()

70. Now
Later success
Later-er success
Later-er-er success
Later-er-er-er success

71. function addOne(num) {
return new Promise(
function(resolve, reject) {
resolve(num+1)
})
}
!
addOne(0)
.then(addOne)
.then(addOne)
.then(addOne)
.then(console.log)

72. function addOne(num) {
return num+1
}
!
var p = new Promise(
function(res, rej) { res(0) })
.then(addOne)
.then(addOne)
.then(addOne)
.then(console.log)

73. Rejections
bubble

74. Errors
bubble

75. Now
Later success
Later-er success
Later-er-er success
Any failure

76. getSpeakers('MidwestJS')
.then(getGithubUsers)
.then(getPublicRepos)
.then(listRepos,
handleError)

77. Problems

78. Slow

79. Slow

80. Incompatible
Proposals &
Implementations

81. jQuery
vs
everyone else

82. Promises
!
Composable.
Eventual Result.

83. Generators

84. What are
they?

85. How do
they work?

86. function*

87. function* tick() {
!
!
}
!
!
!
!

88. function* tick() {
!
!
}
!
var itr = tick()
!
!

89. function* tick() {
!
!
}
!
var itr = tick()
itr.next()
!

90. function* tick() {
yield 42
!
}
!
var itr = tick()
itr.next().value // 42
!

91. yield

92. function* tick() {
yield 42
yield 43
}
!
var itr = tick()
itr.next().value // 42
itr.next().value // 43
itr.next().done // true

93. function* tick() {
var x = yield
var y = yield
}
!
var itr = tick()
itr.next()
itr.next(42) // x becomes 42
itr.next(43) // y becomes 43

94. function* tick() {
var num = 0
while (!(yield num++));
}
!
var itr = tick()
itr.next().value // 0
itr.next().value // 1
itr.next().value // 2

95. function* tick() {
var num = 0
while (!(yield num++));
}
!
var itr = tick()
itr.next().value // 0
itr.next().value // 1
itr.next(true).done // true

96. Replacing
callbacks

97. function delay(time, callback) {
setTimeout(function() {
callback('Slept for ' + time)
}, time)
}

98. function delayThings() {
delay(1000, function(result1) {
console.log(result1)
delay(1200, function(result2) {
console.log(result2)
})
})
}
// Slept for 1000
// Slept for 1200

99. function* delayThings() {
var results1 = yield delay(1000)
console.log(results1)
!
var results2 = yield delay(1200)
console.log(results2)
}

100. function run(generator) {
function resume(value) {
itr.next(value)
}
var itr = generator(resume)
itr.next()
}

101. function* delayThings() {
var results1 = yield delay(1000)
console.log(results1)
!
var results2 = yield delay(1200)
console.log(results2)
}
!

102. function* delayThings(resume) {
var results1 = yield delay(1000, resume)
console.log(results1)
!
var results2 = yield delay(1200, resume)
console.log(results2)
}
!
run(delayThings)

103. More
callbacks?

104. thunkify

105. // simplified from
// https://github.com/visionmedia/node-thunkify
!
function thunkify(fn){
return function(){
var args = Array.prototype.slice.call(arguments)
return function(done){
args.push(function(){
done.apply(null, arguments)
})
fn.apply(null, args)
}
}
}

106. delay = thunkify(delay)
!
var thunk = delay(1000)
!
thunk(function(result) {
console.log(result)
})

107. function run(generator) {
function resume(value) {
itr.next(value)
}
var itr = generator(resume)
itr.next()
}
!

108. function run(generator) {
function resume(ret) {
var obj = itr.next(ret)
if (obj.done) return
obj.value(resume)
}
var itr = generator()
resume()
}

109. function* delayThings() {
var results1 = yield delay(1000)
console.log(results1)
!
var results2 = yield delay(1200)
console.log(results2)
}
!
run(delayThings)

110. yield Now
Later

111. yield Now
yield Later
Later-er

112. yield Now
yield Later
yield Later-er
Later-er-er

113. yield Now
yield Later
yield Later-er
yield Later-er-er
Later-er-er-er
…

114. co

115. var delayThings = co(function*() {
var results1 = yield delay(1000)
console.log(results1)
!
var results2 = yield delay(1200)
console.log(results2)
})
!
delayThings()

116. var delayThings = co(function*() {
var delays = [delay(1000), delay(1200)]
var results = yield delays
!
console.log(results[0])
console.log(results[1])
})
!
delayThings()

117. Where are
they used?

118. co
koa
User-land

119. Firefox 31
Chrome (flag)
Node.js v0.11.10 (flag)
Opera (flag)

120. Benefits

121. Feels
synchronous

122. try{}
catch(){}

123. New cool

124. Problems

125. Support

126. regenerator
facebook.github.io/regenerator/

127. traceur
github.com/google/traceur-compiler

128. Immature
implementations

129. Generators
!
yield statement.
Pausable function*.
Built for iterators.

130. Writing
asynchronous
code is hard

131. But it
doesn’t
have to be!

132. Call to
action

133. Explore

134. Streams
http://nodestreams.com
http://highlandjs.org

135. Thank you!

136. Questions?
!
References available here:
github.com/mikefrey/cpg-talk