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 v12 or higher for ES2015 and async function support.
$ npm install koaHello Koa
const Koa = require('koa');const app = new Koa();// responseapp.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.
- [Guide](docs/guide.md) - Go straight to the docs.
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](docs/migration.md) for more information on upgrading from v1.x andusing v1.x middleware with v2.x.
Context, Request and Response
Each middleware receives a Koa Context object that encapsulates an incominghttp message and the corresponding response to that message. ctx is often usedas 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 withhttp requests which delegate to an IncomingMessagefrom 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 withhttp responses which delegate to a ServerResponse.
Koa's pattern of delegating to Node's request and response objects rather than extending themprovides a cleaner interface and reduces conflicts between different middleware and with Nodeitself as well as providing better support for stream handling. The IncomingMessage can still bedirectly accessed as the req property on the Context and ServerResponse can be directlyaccessed 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 priorexamples, ctx.type can be used instead of ctx.response.type and ctx.accepts can be usedinstead of ctx.request.accepts.
For more information on Request, Response and Context, see the [Request API Reference](docs/api/request.md),[Response API Reference](docs/api/response.md) and [Context API Reference](docs/api/context.md).
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 registrationof middleware, dispatching to the middleware from http, default error handling, as well asconfiguration of the context, request and response objects.
Learn more about the application object in the [Application API Reference](docs/api/index.md).
Documentation
- [Usage Guide](docs/guide.md)
- [Error Handling](docs/error-handling.md)
- [Koa for Express Users](docs/koa-vs-express.md)
- [FAQ](docs/faq.md)
- [API documentation](docs/api/index.md)
Troubleshooting
Check the [Troubleshooting Guide](docs/troubleshooting.md) or [Debugging Koa](docs/guide.md#debugging-koa) inthe general Koa guide.
Running tests
$ npm testReporting 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](AUTHORS).
Community
- KoaJS Slack Group
- 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
*Note that all licence references and agreements mentioned in the Koa README section above are relevant to that project's source code only.
