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Installation

Koa requires node v7.6.0 or higher for ES2015 and async function support.

You can quickly install a supported version of node with your favorite version manager:

$ nvm install 7
$ npm i koa
$ node my-koa-app.js

Async Functions with Babel

To use async functions in Koa in versions of node < 7.6, we recommend using babel's require hook.

require('babel-register');
// require the rest of the app that needs to be transpiled after the hook
const app = require('./app');

To parse and transpile async functions, you should at a minimum have the transform-async-to-generator or transform-async-to-module-method plugins. For example, in your .babelrc file, you should have:

{
  "plugins": ["transform-async-to-generator"]
}

You can also use the env preset with a target option "node": "current" instead.

Application

A Koa application is an object containing an array of middleware 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-negotiation, 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:

const Koa = require('koa');
const app = new Koa();

app.use(async ctx => {
  ctx.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 async functions we can achieve "true" middleware. Contrasting Connect's implementation which simply passes control through series of functions until one returns, Koa invoke "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 response middleware. When a middleware invokes 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.

const Koa = require('koa');
const app = new Koa();

// logger

app.use(async (ctx, next) => {
  await next();
  const rt = ctx.response.get('X-Response-Time');
  console.log(`${ctx.method} ${ctx.url} - ${rt}`);
});

// x-response-time

app.use(async (ctx, next) => {
  const start = Date.now();
  await next();
  const ms = Date.now() - start;
  ctx.set('X-Response-Time', `${ms}ms`);
});

// response

app.use(async ctx => {
  ctx.body = 'Hello World';
});

app.listen(3000);

Settings

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

  • 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.listen(...)

A Koa application is not a 1-to-1 representation of an 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:

const Koa = require('koa');
const app = new Koa();
app.listen(3000);

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

const http = require('http');
const Koa = require('koa');
const app = new 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:

const http = require('http');
const https = require('https');
const Koa = require('koa');
const app = new Koa();
http.createServer(app.callback()).listen(3000);
https.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:

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

app.context

app.context is the prototype from which ctx is created. You may add additional properties to ctx by editing app.context. This is useful for adding properties or methods to ctx to be used across your entire app, which may be more performant (no middleware) and/or easier (fewer require()s) at the expense of relying more on ctx, which could be considered an anti-pattern.

For example, to add a reference to your database from ctx:

app.context.db = db();

app.use(async ctx => {
  console.log(ctx.db);
});

Note:

  • Many properties on ctx are defined using getters, setters, and Object.defineProperty(). You can only edit these properties (not recommended) by using Object.defineProperty() on app.context. See https://github.com/koajs/koa/issues/652.
  • Mounted apps currently use their parent's ctx and settings. Thus, mounted apps are really just groups of middleware.

Error Handling

By default outputs all errors to stderr unless app.silent is true. The default error handler also won't output errors when err.status is 404 or err.expose is true. To perform custom error-handling logic such as centralized logging you can add an "error" event listener:

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

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

app.on('error', (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.