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Application

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. Koa is similar to many other middleware systems that you may have encountered such as Ruby's Rack, Connect, and so on - however a key design decision was made to provide high level "sugar" at the otherwise low-level middleware layer. This improves interoperability, robustness, and makes writing middleware much more enjoyable.

This includes methods for common tasks like content-negotation, cache freshness, proxy support, and redirection among others. Despite supplying a reasonably large number of helpful methods Koa maintains a small footprint, as no middleware are bundled.

The obligatory hello world application:

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

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

app.listen(3000);

Cascading

Koa middleware cascade in a more traditional way as you may be used to with similar tools - this was previously difficult to make user friendly with node's use of callbacks. However with generators we can achieve "true" middleware. Contrasting Connect's implementation which simply passes control through series of functions until one returns, Koa yields "downstream", then control flows back "upstream".

The following example responds with "Hello World", however first the request flows through the x-response-time and logging middleware to mark when the request started, then continue to yield control through the to the response middleware. When a middleware invokes yield next the function suspends and passes control to the next middleware defined. After there are no more middleware to execute downstream, the stack will unwind and each middleware is resumed to perform its upstream behaviour.

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

// x-response-time

app.use(function *(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  this.set('X-Response-Time', ms + 'ms');
});

// logger

app.use(function *(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  console.log('%s %s - %s', this.method, this.url, ms);
});

// response

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

app.listen(3000);

Settings

Application settings are properties on the app instance, currently the following are supported:

  • app.name optionally give your application a name
  • app.env defaulting to the NODE_ENV or "development"
  • app.proxy when true proxy header fields will be trusted
  • app.subdomainOffset offset of .subdomains to ignore [2]
  • app.jsonSpaces default JSON response spaces [2]
  • app.outputErrors output err.stack to stderr [false in "test" environment]

app.listen(...)

A Koa application is not a 1-to-1 representation of a HTTP server. One or more Koa applications may be mounted together to form larger applications with a single HTTP server.

Create and return an HTTP server, passing the given arguments to Server#listen(). These arguments are documented on nodejs.org. The following is a useless Koa application bound to port 3000:

var koa = require('koa');
var app = koa();
app.listen(3000);

The app.listen(...) method is simply sugar for the following:

var http = require('http');
var koa = require('koa');
var app = koa();
http.createServer(app.callback()).listen(3000);

This means you can spin up the same application as both HTTP and HTTPS or on multiple addresses:

var http = require('http');
var koa = require('koa');
var app = koa();
http.createServer(app.callback()).listen(3000);
http.createServer(app.callback()).listen(3001);

app.callback()

Return a callback function suitable for the http.createServer() method to handle a request. You may also use this callback function to mount your koa app in a Connect/Express app.

app.use(function)

Add the given middleware function to this application. See Middleware for more information.

app.keys=

Set signed cookie keys.

These are passed to KeyGrip, however you may also pass your own KeyGrip instance. For example the following are acceptable:

app.keys = ['im a newer secret', 'i like turtle'];
app.keys = new KeyGrip(['im a newer secret', 'i like turtle'], 'sha256');

These keys may be rotated and are used when signing cookies with the { signed: true } option:

this.cookies.set('name', 'tobi', { signed: true });

Error Handling

By default outputs all errors to stderr unless NODE_ENV is "test". To perform custom error-handling logic such as centralized logging you can add an "error" event listener:

app.on('error', function(err){
  log.error('server error', err);
});

If an error in the req/res cycle and it is not possible to respond to the client, the Context instance is also passed:

app.on('error', function(err, ctx){
  log.error('server error', err, ctx);
});

When an error occurs and it is still possible to respond to the client, aka no data has been written to the socket, Koa will respond appropriately with a 500 "Internal Server Error". In either case an app-level "error" is emitted for logging purposes.

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