@eggjs/koa

@eggjs/koa is forked from Koa v2.x for LTS and drop Node.js < 18.19.0 support.

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 codebase. This includes things like content negotiation, normalization of node inconsistencies, redirection, and a few others.

Koa is not bundled with any middleware.

Installation

@eggjs/koa requires node v18.19.0 or higher for Node.js LTS support.

npm install @eggjs/koa

Hello Koa

import { Application } from '@eggjs/koa';

const app = new Application();

// 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

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`);
  });
});

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

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 fengmk2 to disclose.