128 lines
4 KiB
Markdown
128 lines
4 KiB
Markdown
# Application
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A Koa application is an object containing an array of middleware generator functions
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which are composed and executed in a stack-like manner upon request. Koa is similar to many
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other middleware systems that you may have encountered such as Ruby's Rack, Connect, and so on -
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however a key design decision was made to provide high level "sugar" at the otherwise low-level
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middleware layer. This improves interoperability, robustness, and makes writing middleware much
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more enjoyable.
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This includes methods for common tasks like content-negotation, cache freshness, proxy support, and redirection
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among others. Despite supplying a reasonably large number of helpful methods Koa maintains a small footprint, as
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no middleware are bundled.
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The obligatory hello world application:
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```js
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var koa = require('koa');
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var app = koa();
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app.use(function *(){
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this.body = 'Hello World';
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});
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app.listen(3000);
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```
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## Settings
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Application settings are properties on the `app` instance, currently
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the following are supported:
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- `app.name` optionally give your application a name
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- `app.env` defaulting to the __NODE_ENV__ or "development"
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- `app.proxy` when true proxy header fields will be trusted
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- `app.subdomainOffset` offset of `.subdomains` to ignore [2]
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- `app.jsonSpaces` default JSON response spaces [2]
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- `app.outputErrors` output err.stack to stderr [false in "test" environment]
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## app.listen(...)
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A Koa application is not a 1-to-1 representation of an HTTP server,
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as one or more Koa applications may be mounted together to form larger
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applications, with a single HTTP server.
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Create and return an HTTP server, passing the given arguments to
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`Server#listen()`. These arguments are documented on [nodejs.org](http://nodejs.org/api/http.html#http_server_listen_port_hostname_backlog_callback). The following is a useless Koa application bound to port `3000`:
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```js
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var koa = require('koa');
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var app = koa();
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app.listen(3000);
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```
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The `app.listen(...)` method is simply sugar for the following:
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```js
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var http = require('http');
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var koa = require('koa');
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var app = koa();
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http.createServer(app.callback()).listen(3000);
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```
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This means you can spin up the same application as both HTTP and HTTPS,
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or on multiple addresses:
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```js
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var http = require('http');
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var koa = require('koa');
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var app = koa();
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http.createServer(app.callback()).listen(3000);
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http.createServer(app.callback()).listen(3001);
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```
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## app.callback()
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Return a callback function suitable for the `http.createServer()`
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method to handle a request.
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You may also use this callback function to mount your koa app in a
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Connect/Express app.
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## app.use(function)
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Add the given middleware function to this application. See [Middleware](#middleware) for
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more information.
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## app.keys=
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Set signed cookie keys.
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These are passed to [KeyGrip](https://github.com/jed/keygrip),
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however you may also pass your own `KeyGrip` instance. For
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example the following are acceptable:
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```js
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app.keys = ['im a newer secret', 'i like turtle'];
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app.keys = new KeyGrip(['im a newer secret', 'i like turtle'], 'sha256');
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```
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These keys may be rotated and are used when signing cookies
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with the `{ signed: true }` option:
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```js
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this.cookies.set('name', 'tobi', { signed: true });
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```
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## Error Handling
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By default outputs all errors to stderr unless __NODE_ENV__ is "test". To perform custom error-handling logic such as centralized logging you
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can add an "error" event listener:
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```js
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app.on('error', function(err){
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log.error('server error', err);
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});
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```
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If an error in the req/res cycle and it is _not_ possible to respond to the client, the `Context` instance is also passed:
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```js
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app.on('error', function(err, ctx){
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log.error('server error', err, ctx);
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});
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```
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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
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appropriately with a 500 "Internal Server Error". In either case
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an app-level "error" is emitted for logging purposes.
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