koa

next generation web framework for node js

Simple. Social.

The world's simplest community building, campaigning and fundraising platform.

What is koa?

What THEY say ...

• designed by the team behind Express
• smaller, more expressive, and more robust foundation
• does not bundle any middleware within core
• leverages generators - allows you to ditch callbacks

Why not using koa?

• koa is barebone
• does not include any middleware
• routing is not provided
• many convenience utilities not provided (e.g. sending files)
• still not the same momentum as express

Why koa?

• Extremely minimal - no bundled middelware
• Freedom from callback hell - uses generators (ES6)
• Context - simplifies requests and responses in general
• Cascading - awesome middleware concept, using ES6 generators
• Simplifying error handling - through try/catch
• Speed - aprox 15% more requests handled
• koa is more modular

So lets see what koa is ...

Express

var express = require('express');
var app = express();

app.use(function(req, res, next){
  res.send('Hello World');
});

app.listen(3000);
					

01_express_helloworld.js

koa

var koa = require('koa');
var app = koa();

app.use(function *(){
  this.response.body = 'Hello World';
});

app.listen(3000);
					

02_koa_helloworld.js

koa context

app.use(function *(){  
  this; // is the Context
  this.request; // is a koa Request
  this.response; // is a koa Response
  this.req; // Node's request
  this.res; // Node's response

  this.response.body = ... ;
  this.body = ...
});
					

Let's write an REST-API ...

03_koa_router.js

... and call it in the browser with http://localhost:3000/v1/all

Generator funtions

The function* declaration defines a generator function, which returns a Generator object.

Generator funtions

• Generators are functions which can be exited and later re-entered
• Their context (variable bindings) will be saved across re-entrances.
• Calling a generator function does not execute its body immediately
• An iterator object for the function is returned instead
• When the iterator's next() method is called, the generator function's body is executed until the first yield expression
• So a generator function is controlled from "outside"

Generator example

function* idMaker(){
  var index = 0;
  while(index < 3)
    yield index++;
}

var gen = idMaker();

console.log(gen.next().value); // 0
console.log(gen.next().value); // 1
console.log(gen.next().value); // 2
console.log(gen.next().value); // undefined
// ...
					

04_generators.js

YIELD

• Yo
• I
• Expect a
• Little
• Delay

Express Way

koa Way

A koa application ...

... is an object containing an array of middleware generator functions which are composed and executed in a stack-like manner upon request

Let's write middleware ...

05_koa_middleware.js

Compare it to Express

06_express_middleware.js

... here you have to proxy the real end function.

Error handling ...

07_koa_errorhandling.js

koa

Frameworks & Middleware

https://github.com/koajs/koa/wiki

Summary

• koa is barebone
• Koa relies less on middleware
• koa is very modular
• koa simplifies error handling
• koa is fast
• koa is ES6

Thank you!

© 2015 Sebastian Hildebrandt