OdessaJs conference 2016, better quality presentation: https://goo.gl/SQjwod Speaker: Kozhukharenko Nikolay, K&C; Topic: "Koajs as an alternative to Express" Content: - Why Koajs is the better version of Express? - What is es6 js generators? - How to use generators to write errorless code in NodeJS - Asynchronous code in synchronous manner with tj/co library; - Comparison of KoaJs and Express, similarities and differences

1. Koajs as an alternative to Express

2. I am Mykola Kozhuharenko
front-end dev in Krusche & Company
I’m on github
Hello!

3. Nodejs/Express downsides:
◉Code readability
Callback hell
◉Error handling
Duplicate callbacks / cb call might be lost in third
party library
If you miss an error handler - you are in troubles
Can't throw an error or use try/catch

4. app.post('/',function (req, res, next) {
var body = req.body;
if (!body.name || !body.email || !body.password) {
return res.status(400).send("Missing username or email or password")
};
// case #1- if user was registered (socials platform) before -> just add a password;
User.findOneAndUpdate({
email: body.email
}, {
$set: {password: body.password}
}, (err, user) => {
if (!user) {
// case #2 - user has never been registering before -> create new one;
User.create(body, (err, user) => {
if (err && err.name === 'ValidationError') {
return res.status(400).send(err)
} else if (err) {
return res.status(500).send(err)
}
return res.send({ user: user })
});
} else {
return res.send({ user: body })
}
});
});
Callbacks

5. User.findOneAndUpdate({
email: body.email
}, {
$set: {password: body.password}
}, (err, user) => {
if (err) {
return res.status(500).send(err)
}
if (!user) {
// case #2 - user has never been registering before -> create new one;
User.create(body, (err, user) => {
if (err && err.name === 'ValidationError') {
return res.status(400).send(err)
} else if (err) {
return res.status(500).send(err)
}
return res.send({ user: user })
});
} else {
return res.send({ user: body })
}
});
Callbacks

6. ◉ If you miss an error handler -
you are in troubles
◉ Duplicate callbacks / cb call
might be lost in third party
library

7. // foo is a function that returns a generator
function *foo() {
console.log('start');
yield 1;
console.log('second');
yield 2;
yield 3;
}
var gen = foo(); // get a generator object
gen.next() // 'start', { value: 1, done:false }
gen.next() // 'second', { value: 2, done:false }
gen.next() // { value: 3, done:false }
gen.next() // { value: undefined, done:true }
Generators #1

8. const Message = require('./models/message');
function *foo() {
yield new Promise((resolve, reject) => {
resolve("Hello")
});
yield Message.create({text: 'some text');
}
var gen = foo();
var genVal = gen.next().value // { value: Promise, done:false }
genVal.then(res => console.log(res)) // Hello
gen.next() // { value: Promise, done:false }
Generators #2 - Promises

9. const Message = require('./models/message');
function *foo() {
var greeting = yield new Promise.resolve('Hello');
var result = yield Message.create({text: greeting});
console.log(result) // saved! {id: 'ID', text: 'Hello'}
}
var gen = foo();
gen.next().value // Promise
.then(greeting => {
console.log(greeting); // 'Hello'
gen.next(greeting).value // Promise
.then(createRes = gen.next(createRes))
})
Generators #3 - Two ways

10. var fetch = require('isomorphic-fetch');
function *foo() {
try {
var greeting = yield fetch('/api/get-greetings')[0];
} catch (e) {
console.log(e);
}
}
var gen = foo();
gen.next().value
.then(greeting => gen.next(greeting))
.catch(e => gen.throw(e))
Generators #4 - throw

11. function runner (gen) {
var gen = gen();
next(null, null);
function next(err, res) {
const res = err ? gen.throw(err) : gen.next(res);
if (res.done) return;
res.value
.then((res) => next(null, res))
.catch((error) => next(error));
}
};
runner(function *foo() {
var greeting = yield Promise.resolve("Hello")
var name = yield Promise.resolve("John")
console.log(greeting, name); // 'Hello John'
})
Generators #5 - Runner

12. var co = require('co')
app.post('/',function (req, res, next) {
var body = req.body;
co(function *() {
try {
var user = yield User.findOneAndUpdate({
email: body.email
}, { $set: {password: body.password} });
if (!user) {
user = yield User.create(body);
}
res.send({ user: user })
} catch (e) {
if (err && err.name === 'ValidationError') {
return res.status(400).send(err)
} else if (err) {
return res.status(500).send(err)
}
}
});
});
Co

13. Using Co you can yield:
Thunks
(will be deprecated)
Arrays
co(function* () {
// parallel
var res = yield [
Promise.resolve(1),
Promise.resolve(2),
Promise.resolve(3),
];
console.log(res); // => [1, 2, 3]
}).catch(onerror);
Generators
function* greet() {
return yield new Promise((resolve, rej) => {
resolve("Hello, Mary!")
});
}
co(function *(){
var greeting = yield greet
console.log(greeting)
}).catch((err) => {
console.log(err)
});
Promises

14. Co

15. Along with co will be useful:
◉mz - promisify NodeJS core modules
◉Thenify promisify any callback-based function
var fs = require('mz/fs')
fs.exists(__filename).then(function (exists) {
if (exists) // do something
})
var thenify = require('thenify');
var somethingAsync = thenify(function somethingAsync(a, b, c, callback) {
callback(null, a, b, c);
});

16. Adopted by community
Callbacks Async Promises Generators
(along with co)

17. Meet Koa!
Philosophically, Koa aims to "fix and replace node",
whereas Express "augments node"

18. Koa Hello world
var koa = require('koa');
var app = koa();
app.use(function *() {
this.body = 'Hello World';
});
app.listen(3000);

19. Koa API
koa Request
CONTEXT
this.request
this.response
koa Response
this.query
this.url
this.headers
this.get - get a header
this.body
this.type - get content-type
this.redirect
this.status
this.set -set a header

20. app.use(function *(next) {
var start = new Date; // <<<------ 1
yield next; // waits for downstream middlewares
var delta = Math.ceil(Date.now() - start); // <<<------ 3
console.info(`${this.method} ${this.url} - ${delta}ms`);
this.set('X-Response-Time', delta + 'ms'); // <<<------ 4
});
app.use(function *() {
this.body = yield new Promise.resolve([10000]); // <<<------ 2
});
Response time - Koa middleware
3
4
2
1

21. Custom error
app.use(function *(next) {
try {
yield next;
} catch (e) {
if (e instanceof AuthError) {
this.statusCode = e.status;
this.redirect('/not-authorized');
this.body = this.render('not-authorized');
} else if (e.status) {
this.statusCode = e.status;
this.body = this.render('error', e);
} else {
this.statusCode = 500;
this.body = this.render('error', e);
logger.error('Unexpected error: ', e); // winston
}
}
});
app.use(function *(next) {
throw new AuthError()
});

22. app.post('/',function (req, res, next) {
var body = req.body;
if (!body.name || !body.email || !body.password) {
return res.status(400).send("Missing username or email or password")
};
// case #1- if user was registered (socials platform) before -> just add a password;
User.findOneAndUpdate({
email: body.email
}, { $set: {password: body.password} }, (err, user) => {
if (err) return res.status(500).send(err)
if (!user) {
// case #2 - user has never been registering before -> create new one;
User.create(body, (err, user) => {
if (err && err.name === 'ValidationError') {
return res.status(400).send(err)
} else if (err) {
return res.status(500).send(err)
}
return res.send({ user: user })
});
} else {
return res.send({ user: body })
}
});
});
Do you remember this example with callbacks?

23. app.use(function *(next) {
try {
yield next;
} catch (e) {
if (e.status) { // app error
this.body = e.message;
this.statusCode = e.status;
} if else (e && e.name ===
'ValidationError') {
this.body = e;
this.statusCode = 400;
} else { // Internal server error
this.body = "Oops!";
this.status = 500;
}
}
});
Koa way
app.use(function *(next) {
var body = this.request.body;
if (!body.name || !body.email || !body.password) {
throw new Error("Missing username||email||password");
}
var user = yield User.findOneAndUpdate({email: body.email}, {
$set: {password: body.password}
});
if (!user) user = yield User.create(body);
this.body = { user: this.body };
});
2nd middleware
1st middleware

25. app.get('/', function (req, res, next) {
var readStream = fs.createReadStream('text.txt');
readStream.pipe(res);
readStream.on('error', () => {
res.statusCode = 500;
res.end('Server error has occurred!');
});
res.on('close', () => readStream.destroy());
});
Send a file
app.use(function* (next) {
this.body = fs.createReadStream('text.txt')
});
Express/node
Koa

26. “
“Koa and express uses the same modules.
We’ve refactored express into jshttp and
pillarjs so that anyone else can make their
own framework as well”

28. Express & Koa similarities
Same modules
Accepts, content-type,
cookie, fresh, content-
disposition, on-finished,
parseurl, type-is, vary,
escape-html
Modular
Each feature is moved to a
separate module.
Minimalistic
Contains only essential
(core) modules (e.g.
headers parsing).
Same developers
Koa was build by core
developers of Express:
TJ Holowaychuk and
others

29. Koa middlewares:
◉koa-router
◉koa-bodyparse
◉koa-static
◉koa-session
◉koa-compress
◉koa-cors
◉Koa-logger
more here

30. Koa
◉ router
◉ bigger community
◉ needs domains to catch async
errors
◉ close to node style
◉ a lot of outdated examples and
tutorials
Express
vs
◉ better error handling with try catch
◉ nice async flow control (no cb hell)
◉ api is more stable;
◉ no build-in router
◉ smaller community
◉ support stream out of the box
◉ easy to understand source code; no
monkey patching

31. Any questions ?
Thanks!