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Promises and chaining 
Into callback hell … back again
About Hans 
@otype 
hans@otype.de
Intro: 
sync, async, callbacks
Humans are multi-threaded!
Javascript is single-threaded & 
synchronous. 
Browsers provide asynchrony*.
finish 
start 
a() 
b() 
c() 
a() 
b() 
c()
finish 
start 
a() 
b() 
c() 
a() 
b() 
c() 
b() 
a() 
c() 
c() 
b() 
a() 
c(),a() 
b()…
sync async
Why async?
Synchronous flow, here? Bad idea!
“Callbacks to the rescue!”
“Call me back!”
Use a callback! Way better!
Got it!
a() b() c() d() e() f() 
Callbacks in order?
“Pyramid of doom!”
“I am NOT going to debug that!”
“Free ticket into callback hell!”
Core: 
Promises to the rescue
“I hereby give you a promise that I 
will answer your question some time 
in the future … but not right now!” 
My definition of a promise
Promise 
fulfilled 
rejected 
pending 
settled
Still experimental 
kriskowal/q 
cujojs/when 
tildeio/rsvp.js 
WinJS.Promise
$q.defer()
.then()
.resolve() 
.reject()
.then() in .then() in .then() …
“I’ve seen that *$#%$* before …”
Chaining
“Wicked, Hans!”
“And what about collecting 
results from multiple promises?”
Combined 
results 
$q.all()
“OK! Enough theory!”
Get fancy: 
Demo Time!
Outro: 
Questions?
http://ejohn.org/blog/how-javascript-timers-work/ 
http://www.html5rocks.com/en/tutorials/async/deferred/ 
https://docs.angularjs.org/api/ng/service/$q 
https://github.com/kriskowal/q 
https://developer.mozilla.org/en-US/docs/Web/JavaScript/ 
Reference/Global_Objects/Promise 
https://www.promisejs.org/ 
https://github.com/promises-aplus/promises-spec
“Thank you! Over & out!”

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Promises And Chaining In AngularJS - Into Callback Hell And Back Again