koa

What is koa?

Koa is a new web framework designed by the team behind Express, which aims to be a smaller, more expressive, and more robust foundation for web applications and APIs. Koa uses async functions to eliminate callback hell and simplify error handling. It does not bundle any middleware within its core, and it provides an elegant suite of methods that make writing servers fast and enjoyable.

What are koa's main functionalities?

HTTP Server

Koa can be used to create an HTTP server that listens on a given port. The example shows a basic server that responds with 'Hello World' to every request.

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

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

app.listen(3000);

Middleware

Koa is known for its middleware stack that allows for more control over the request/response cycle. The example demonstrates a simple timing middleware that records how long a request takes to process.

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

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

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

app.listen(3000);

Error Handling

Koa provides a structured way to handle errors. In this example, middleware is used to catch and handle errors that may occur during request processing.

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

app.use(async (ctx, next) => {
  try {
    await next();
  } catch (err) {
    ctx.status = err.status || 500;
    ctx.body = err.message;
    ctx.app.emit('error', err, ctx);
  }
});

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

app.listen(3000);

Context

Koa provides a context object encapsulating the Node's request and response objects into a single object which provides many helpful methods for writing web applications and APIs.

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

app.use(async ctx => {
  ctx.body = `Request Type: ${ctx.method}`;
});

app.listen(3000);

Other packages similar to koa

express

Express is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It is one of the most popular Node.js frameworks and has a large ecosystem of middleware available. Compared to Koa, Express is more established but relies on callback functions rather than async/await for handling asynchronous operations.

hapi

Hapi is a rich framework for building applications and services that allows developers to focus on writing reusable application logic instead of spending time building infrastructure. It is known for its powerful plugin system. Hapi is more configuration-driven compared to Koa's minimalistic and middleware-centric approach.

fastify

Fastify is a fast and low overhead web framework for Node.js. It is inspired by Hapi and Express and aspires to be faster than them. Fastify emphasizes performance and provides a robust plugin architecture. Unlike Koa, which is minimalist by design, Fastify comes with more built-in features.

sails

Sails.js is a MVC framework for Node.js that is built on top of Express. It is designed to emulate the familiar MVC pattern of frameworks like Ruby on Rails, but with support for the requirements of modern apps: data-driven APIs with scalable, service-oriented architecture. Sails is more opinionated and includes more built-in features compared to Koa's minimalistic approach.

README

Expressive HTTP middleware framework for node.js to make web applications and APIs more enjoyable to write. Koa's middleware stack flows in a stack-like manner, allowing you to perform actions downstream then filter and manipulate the response upstream.

Only methods that are common to nearly all HTTP servers are integrated directly into Koa's small ~570 SLOC codebase. This includes things like content negotiation, normalization of node inconsistencies, redirection, and a few others.

Koa is not bundled with any middleware.

Installation

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

$ npm install koa

Hello Koa

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

// response
app.use(ctx => {
  ctx.body = 'Hello Koa';
});

app.listen(3000);

Getting started

• Kick-Off-Koa - An intro to Koa via a set of self-guided workshops.
• Workshop - A workshop to learn the basics of Koa, Express' spiritual successor.
• Introduction Screencast - An introduction to installing and getting started with Koa

Middleware

Koa is a middleware framework that can take two different kinds of functions as middleware:

• async function
• common function

Here is an example of logger middleware with each of the different functions:

async functions (node v7.6+)

app.use(async (ctx, next) => {
  const start = Date.now();
  await next();
  const ms = Date.now() - start;
  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});

Common function

// Middleware normally takes two parameters (ctx, next), ctx is the context for one request,
// next is a function that is invoked to execute the downstream middleware. It returns a Promise with a then function for running code after completion.

app.use((ctx, next) => {
  const start = Date.now();
  return next().then(() => {
    const ms = Date.now() - start;
    console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
  });
});

Koa v1.x Middleware Signature

The middleware signature changed between v1.x and v2.x. The older signature is deprecated.

Old signature middleware support will be removed in v3

Please see the Migration Guide for more information on upgrading from v1.x and using v1.x middleware with v2.x.

Context, Request and Response

Each middleware receives a Koa Context object that encapsulates an incoming http message and the corresponding response to that message. ctx is often used as the parameter name for the context object.

app.use(async (ctx, next) => { await next(); });

Koa provides a Request object as the request property of the Context.
Koa's Request object provides helpful methods for working with http requests which delegate to an IncomingMessage from the node http module.

Here is an example of checking that a requesting client supports xml.

app.use(async (ctx, next) => {
  ctx.assert(ctx.request.accepts('xml'), 406);
  // equivalent to:
  // if (!ctx.request.accepts('xml')) ctx.throw(406);
  await next();
});

Koa provides a Response object as the response property of the Context.
Koa's Response object provides helpful methods for working with http responses which delegate to a ServerResponse .

Koa's pattern of delegating to Node's request and response objects rather than extending them provides a cleaner interface and reduces conflicts between different middleware and with Node itself as well as providing better support for stream handling. The IncomingMessage can still be directly accessed as the req property on the Context and ServerResponse can be directly accessed as the res property on the Context.

Here is an example using Koa's Response object to stream a file as the response body.

app.use(async (ctx, next) => {
  await next();
  ctx.response.type = 'xml';
  ctx.response.body = fs.createReadStream('really_large.xml');
});

The Context object also provides shortcuts for methods on its request and response. In the prior examples, ctx.type can be used instead of ctx.response.type and ctx.accepts can be used instead of ctx.request.accepts.

For more information on Request, Response and Context, see the Request API Reference, Response API Reference and Context API Reference.

Koa Application

The object created when executing new Koa() is known as the Koa application object.

The application object is Koa's interface with node's http server and handles the registration of middleware, dispatching to the middleware from http, default error handling, as well as configuration of the context, request and response objects.

Learn more about the application object in the Application API Reference.

Documentation

• Usage Guide
• Error Handling
• Koa for Express Users
• FAQ
• API documentation

Troubleshooting

Check the Troubleshooting Guide or Debugging Koa in the general Koa guide.

Running tests

$ npm test

Reporting vulnerabilities

To report a security vulnerability, please do not open an issue, as this notifies attackers of the vulnerability. Instead, please email dead_horse, jonathanong, and niftylettuce to disclose.

Authors

See AUTHORS.

Community

• Badgeboard and list of official modules
• Examples
• Middleware list
• Wiki
• Reddit Community
• Mailing list
• 中文文档 v1.x
• 中文文档 v2.x
• #koajs on freenode

Job Board

Looking for a career upgrade?

Backers

Support us with a monthly donation and help us continue our activities.

Sponsors

Become a sponsor and get your logo on our README on Github with a link to your site.

License

MIT