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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);

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 an HTTP server, as 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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