Why & how to manage asynchronous functions.

1. Hao-kang Den <github @hden>
Harnessing asynchronicity - callbacks, promises,
generators, and coroutines

3. Agenda
● Why asynchronous?
● Callback
● Promise
● Generator [--harmony]
● Coroutines [--harmony]

4. Why asynchronous?
Callbacks

5. Original slides introducing node.js by
Ryan Dahl @jsconf 2009
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19. Pyramid of Doom
step1(function (value1) {
step2(value1, function(value2) {
step3(value2, function(value3) {
step4(value3, function(value4) {
// Do something with value4
});
});
});
});
Hairball

20. Promise/A+
Q.fcall(promisedStep1)
.then(promisedStep2)
.then(promisedStep3)
.then(promisedStep4)
.then(function (value4) {
// Do something with value4
})
.catch(function (error) {
// Handle any error from all above steps
})
.done(); https://github.com/kriskowal/q

21. ECMAScript a.k.a. JavaScript
● version 1 - June 1997
● version 2 - June 1998
● version 3 - December 1999
● version 4 - Abandoned
● version 5 - December 2009
● version 5.1 - June 2011
● version 6 - [Harmony] Work in progress

22. Generators
function* fibonacci() {
let [prev, curr] = [0, 1];
for (;;) {
[prev, curr] = [curr, prev + curr];
yield curr;
}
}
let seq = fibonacci();
print(seq.next()); // 1
print(seq.next()); // 2
print(seq.next()); // 3
print(seq.next()); // 5
print(seq.next()); // 8
ECMAScript 6 a.k.a Harmony

23. Coroutines
var fs = require(‘fs’);
function read(path) {
return function(done){
fs.readFile(path, ‘utf8', done);
}
}
makeCoroutine(function *(){
var a = yield read(‘Readme.md’);
var b = yield read(‘package.json’);
console.log(a);
console.log(b);
});
generator

24. Koa.js
Expressive middleware for
node.js using generators
-- by the core team of expres
app.use(function *(next){
var start = new Date;
yield next;
var ms = new Date - start;
this.set('X-Response-Time', ms + 'ms');
});

25. generator

26. Further Reading
● http://latentflip.com/imperative-vs-declarative/
● https://medium.com/code-adventures/174f1fe66127
● https://github.com/kriskowal/q
● http://taoofcode.net/promise-anti-patterns/
● http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html
read!