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koa middleware framework for nodejs

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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.


Koa-2-Browser requires browserify for ES2017 support.

$ npm install koa@next

Hello koa

const Koa = require('koa');
const app = new Koa();
// response 
app.use(ctx => {
  ctx.body = 'Hello Koa';

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


Koa is a middleware framework that can take 3 different kinds of functions as middleware:

  • common function
  • async function
  • generatorFunction

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

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

async functions (Babel required)

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


To use generator functions, you must use a wrapper such as co that is no longer supplied with Koa.

app.use(co.wrap(function *(ctx, next) {
  const start = new Date();
  yield next();
  const ms = new Date() - start;
  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);

Old signature middleware (v1.x) - Deprecated

Old signature middleware (v1.x) support will be removed in v3

Koa v2.x will try to convert legacy signature, generator middleware on app.use, using koa-convert. It is however recommended that you choose to migrate all v1.x middleware as soon as possible.

// Koa will convert 
app.use(function *(next) {
  const start = new Date();
  yield next;
  const ms = new Date() - start;
  console.log(`${this.method} ${this.url} - ${ms}ms`);

You could do it manually as well, in which case Koa will not convert.

const convert = require('koa-convert');
app.use(convert(function *(next) {
  const start = new Date();
  yield next;
  const ms = new Date() - start;
  console.log(`${this.method} ${this.url} - ${ms}ms`);

Babel setup

For Node 4.0 and Babel 6.0 you can setup like this:

$ npm install babel-register babel-plugin-transform-async-to-generator --save
// set babel in entry file 
  plugins: ['transform-async-to-generator']

Check out an example in koa's test.

Running tests

$ make test