Expressive HTTP middleware framework for Jsre and 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.
$ npm install @edgeros/koa
const Koa = require('@edgeros/koa');
const app = new Koa();
// response
app.use(ctx => {
ctx.body = 'Hello Koa';
});
app.listen(3000);- 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 two different kinds of functions as middleware:
- async function
- common function
Here is an example of logger middleware with each of the different functions:
app.use(async (ctx, next) => {
const start = Date.now();
await next();
const ms = Date.now() - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});// Middleware normally takes two parameters (ctx, next), ctx is the context for one request,
// next is a function that is invoked to execute the downstream middleware. It returns a Promise with a then function for running code after completion.
app.use((ctx, next) => {
const start = Date.now();
return next().then(() => {
const ms = Date.now() - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});
});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.
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.
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.
Check the Troubleshooting Guide or Debugging Koa in the general Koa guide.