next generation web framework for node.js


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. By leveraging async functions, Koa allows you to ditch callbacks and greatly increase error-handling. Koa does not bundle any middleware within its core, and it provides an elegant suite of methods that make writing servers fast and enjoyable.


Koa requires node v12 or higher for ES2015 and async function support.

You can quickly install a supported version of node with your favorite version manager:

$ nvm install 7
$ npm i koa
$ node my-koa-app.js


A Koa application is an object containing an array of middleware functions which are composed and executed in a stack-like manner upon request. Koa is similar to many other middleware systems that you may have encountered such as Ruby's Rack, Connect, and so on - however a key design decision was made to provide high level "sugar" at the otherwise low-level middleware layer. This improves interoperability, robustness, and makes writing middleware much more enjoyable.

This includes methods for common tasks like content-negotiation, cache freshness, proxy support, and redirection among others. Despite supplying a reasonably large number of helpful methods Koa maintains a small footprint, as no middleware are bundled.

The obligatory hello world application:

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

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



Koa middleware cascade in a more traditional way as you may be used to with similar tools - this was previously difficult to make user friendly with node's use of callbacks. However with async functions we can achieve "true" middleware. Contrasting Connect's implementation which simply passes control through series of functions until one returns, Koa invoke "downstream", then control flows back "upstream".

The following example responds with "Hello World", however first the request flows through the x-response-time and logging middleware to mark when the request started, then yields control through the response middleware. When a middleware invokes next() the function suspends and passes control to the next middleware defined. After there are no more middleware to execute downstream, the stack will unwind and each middleware is resumed to perform its upstream behaviour.

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

// logger

app.use(async (ctx, next) => {
  await next();
  const rt = ctx.response.get('X-Response-Time');
  console.log(`${ctx.method} ${ctx.url} - ${rt}`);

// x-response-time

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

// response

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



Application settings are properties on the app instance, currently the following are supported:


A Koa application is not a 1-to-1 representation of an HTTP server. One or more Koa applications may be mounted together to form larger applications with a single HTTP server.

Create and return an HTTP server, passing the given arguments to Server#listen(). These arguments are documented on The following is a useless Koa application bound to port 3000:

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

The app.listen(...) method is simply sugar for the following:

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

This means you can spin up the same application as both HTTP and HTTPS or on multiple addresses:

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


Return a callback function suitable for the http.createServer() method to handle a request. You may also use this callback function to mount your Koa app in a Connect/Express app.


Add the given middleware function to this application. app.use() returns this, so is chainable.


Is the same as


See Middleware for more information.


Set signed cookie keys.

These are passed to KeyGrip, however you may also pass your own KeyGrip instance. For example the following are acceptable:

app.keys = ['OEK5zjaAMPc3L6iK7PyUjCOziUH3rsrMKB9u8H07La1SkfwtuBoDnHaaPCkG5Brg', 'MNKeIebviQnCPo38ufHcSfw3FFv8EtnAe1xE02xkN1wkCV1B2z126U44yk2BQVK7'];
app.keys = new KeyGrip(['OEK5zjaAMPc3L6iK7PyUjCOziUH3rsrMKB9u8H07La1SkfwtuBoDnHaaPCkG5Brg', 'MNKeIebviQnCPo38ufHcSfw3FFv8EtnAe1xE02xkN1wkCV1B2z126U44yk2BQVK7'], 'sha256');

For security reasons, please ensure that the key is long enough and random.

These keys may be rotated and are used when signing cookies with the { signed: true } option:

ctx.cookies.set('name', 'tobi', { signed: true });


app.context is the prototype from which ctx is created. You may add additional properties to ctx by editing app.context. This is useful for adding properties or methods to ctx to be used across your entire app, which may be more performant (no middleware) and/or easier (fewer require()s) at the expense of relying more on ctx, which could be considered an anti-pattern.

For example, to add a reference to your database from ctx:

app.context.db = db();

app.use(async ctx => {


Error Handling

By default outputs all errors to stderr unless app.silent is true. The default error handler also won't output errors when err.status is 404 or err.expose is true. To perform custom error-handling logic such as centralized logging you can add an "error" event listener:

app.on('error', err => {
  log.error('server error', err)

If an error is in the req/res cycle and it is not possible to respond to the client, the Context instance is also passed:

app.on('error', (err, ctx) => {
  log.error('server error', err, ctx)

When an error occurs and it is still possible to respond to the client, aka no data has been written to the socket, Koa will respond appropriately with a 500 "Internal Server Error". In either case an app-level "error" is emitted for logging purposes.


A Koa Context encapsulates node's request and response objects into a single object which provides many helpful methods for writing web applications and APIs. These operations are used so frequently in HTTP server development that they are added at this level instead of a higher level framework, which would force middleware to re-implement this common functionality.

A Context is created per request, and is referenced in middleware as the receiver, or the ctx identifier, as shown in the following snippet:

app.use(async ctx => {
  ctx; // is the Context
  ctx.request; // is a Koa Request
  ctx.response; // is a Koa Response

Many of the context's accessors and methods simply delegate to their ctx.request or ctx.response equivalents for convenience, and are otherwise identical. For example ctx.type and ctx.length delegate to the response object, and ctx.path and ctx.method delegate to the request.


Context specific methods and accessors.


Node's request object.


Node's response object.

Bypassing Koa's response handling is not supported. Avoid using the following node properties:


A Koa Request object.


A Koa Response object.


The recommended namespace for passing information through middleware and to your frontend views.

ctx.state.user = await User.find(id);

Application instance reference.

Koa applications extend an internal EventEmitter. emits an event with a type, defined by the first argument. For each event you can hook up "listeners", which is a function that is called when the event is emitted. Consult the error handling docs for more information.

ctx.cookies.get(name, [options])

Get cookie name with options:

Koa uses the cookies module where options are simply passed.

ctx.cookies.set(name, value, [options])

Set cookie name to value with options:

Koa uses the cookies module where options are simply passed.

ctx.throw([status], [msg], [properties])

Helper method to throw an error with a .status property defaulting to 500 that will allow Koa to respond appropriately. The following combinations are allowed:

ctx.throw(400, 'name required');
ctx.throw(400, 'name required', { user: user });

For example ctx.throw(400, 'name required') is equivalent to:

const err = new Error('name required');
err.status = 400;
err.expose = true;
throw err;

Note that these are user-level errors and are flagged with err.expose meaning the messages are appropriate for client responses, which is typically not the case for error messages since you do not want to leak failure details.

You may optionally pass a properties object which is merged into the error as-is, useful for decorating machine-friendly errors which are reported to the requester upstream.

ctx.throw(401, 'access_denied', { user: user });

Koa uses http-errors to create errors. status should only be passed as the first parameter.

ctx.assert(value, [status], [msg], [properties])

Helper method to throw an error similar to .throw() when !value. Similar to node's assert() method.

ctx.assert(ctx.state.user, 401, 'User not found. Please login!');

Koa uses http-assert for assertions.


To bypass Koa's built-in response handling, you may explicitly set ctx.respond = false;. Use this if you want to write to the raw res object instead of letting Koa handle the response for you.

Note that using this is not supported by Koa. This may break intended functionality of Koa middleware and Koa itself. Using this property is considered a hack and is only a convenience to those wishing to use traditional fn(req, res) functions and middleware within Koa.

Request aliases

The following accessors and alias Request equivalents:

Response aliases

The following accessors and alias Response equivalents:


A Koa Request object is an abstraction on top of node's vanilla request object, providing additional functionality that is useful for every day HTTP server development.



Request header object. This is the same as the headers field on node's http.IncomingMessage.


Set request header object.


Request header object. Alias as request.header.


Set request header object. Alias as request.header=.


Request method.


Set request method, useful for implementing middleware such as methodOverride().


Return request Content-Length as a number when present, or undefined.


Get request URL.


Set request URL, useful for url rewrites.


Get request original URL.


Get origin of URL, include protocol and host.

// =>


Get full request URL, include protocol, host and url.

// =>


Get request pathname.


Set request pathname and retain query-string when present.


Get raw query string void of ?.


Set raw query string.

Get raw query string with the ?.

Set raw query string.

Get host (hostname:port) when present. Supports X-Forwarded-Host when app.proxy is true, otherwise Host is used.


Get hostname when present. Supports X-Forwarded-Host when app.proxy is true, otherwise Host is used.

If host is IPv6, Koa delegates parsing to WHATWG URL API, Note This may impact performance.


Get WHATWG parsed URL object.


Get request Content-Type void of parameters such as "charset".

const ct = ctx.request.type;
// => "image/png"


Get request charset when present, or undefined:

// => "utf-8"


Get parsed query-string, returning an empty object when no query-string is present. Note that this getter does not support nested parsing.

For example "color=blue&size=small":

  color: 'blue',
  size: 'small'


Set query-string to the given object. Note that this setter does not support nested objects.

ctx.query = { next: '/login' };


Check if a request cache is "fresh", aka the contents have not changed. This method is for cache negotiation between If-None-Match / ETag, and If-Modified-Since and Last-Modified. It should be referenced after setting one or more of these response headers.

// freshness check requires status 20x or 304
ctx.status = 200;
ctx.set('ETag', '123');

// cache is ok
if (ctx.fresh) {
  ctx.status = 304;

// cache is stale
// fetch new data
ctx.body = await db.find('something');


Inverse of request.fresh.


Return request protocol, "https" or "http". Supports X-Forwarded-Proto when app.proxy is true.

Shorthand for ctx.protocol == "https" to check if a request was issued via TLS.


Request remote address. Supports X-Forwarded-For when app.proxy is true.


When X-Forwarded-For is present and app.proxy is enabled an array of these ips is returned, ordered from upstream -> downstream. When disabled an empty array is returned.

For example if the value were "client, proxy1, proxy2", you would receive the array ["client", "proxy1", "proxy2"].

Most of the reverse proxy(nginx) set x-forwarded-for via proxy_add_x_forwarded_for, which poses a certain security risk. A malicious attacker can forge a client's ip address by forging a X-Forwarded-Forrequest header. The request sent by the client has an X-Forwarded-For request header for 'forged'. After being forwarded by the reverse proxy, request.ips will be ['forged', 'client', 'proxy1', 'proxy2'].

Koa offers two options to avoid being bypassed.

If you can control the reverse proxy, you can avoid bypassing by adjusting the configuration, or use the app.proxyIpHeader provided by koa to avoid reading x-forwarded-for to get ips.

  const app = new Koa({
    proxy: true,
    proxyIpHeader: 'X-Real-IP',

If you know exactly how many reverse proxies are in front of the server, you can avoid reading the user's forged request header by configuring app.maxIpsCount:

  const app = new Koa({
    proxy: true,
    maxIpsCount: 1, // only one proxy in front of the server

  // request.header['X-Forwarded-For'] === [ '', '' ];
  // ctx.ips === [ '' ];


Return subdomains as an array.

Subdomains are the dot-separated parts of the host before the main domain of the app. By default, the domain of the app is assumed to be the last two parts of the host. This can be changed by setting app.subdomainOffset.

For example, if the domain is "": If app.subdomainOffset is not set, ctx.subdomains is ["ferrets", "tobi"]. If app.subdomainOffset is 3, ctx.subdomains is ["tobi"].

Check if the incoming request contains the "Content-Type" header field, and it contains any of the give mime types. If there is no request body, null is returned. If there is no content type, or the match fails false is returned. Otherwise, it returns the matching content-type.

// With Content-Type: text/html; charset=utf-8'html'); // => 'html''text/html'); // => 'text/html''text/*', 'text/html'); // => 'text/html'

// When Content-Type is application/json'json', 'urlencoded'); // => 'json''application/json'); // => 'application/json''html', 'application/*'); // => 'application/json''html'); // => false

For example if you want to ensure that only images are sent to a given route:

if ('image/*')) {
  // process
} else {
  ctx.throw(415, 'images only!');

Content Negotiation

Koa's request object includes helpful content negotiation utilities powered by accepts and negotiator. These utilities are:

If no types are supplied, all acceptable types are returned.

If multiple types are supplied, the best match will be returned. If no matches are found, a false is returned, and you should send a 406 "Not Acceptable" response to the client.

In the case of missing accept headers where any type is acceptable, the first type will be returned. Thus, the order of types you supply is important.


Check if the given type(s) is acceptable, returning the best match when true, otherwise false. The type value may be one or more mime type string such as "application/json", the extension name such as "json", or an array ["json", "html", "text/plain"].

// Accept: text/html
// => "html"

// Accept: text/*, application/json
// => "html"
// => "text/html"
ctx.accepts('json', 'text');
// => "json"
// => "application/json"

// Accept: text/*, application/json
// => false

// Accept: text/*;q=.5, application/json
ctx.accepts(['html', 'json']);
ctx.accepts('html', 'json');
// => "json"

// No Accept header
ctx.accepts('html', 'json');
// => "html"
ctx.accepts('json', 'html');
// => "json"

You may call ctx.accepts() as many times as you like, or use a switch:

switch (ctx.accepts('json', 'html', 'text')) {
  case 'json': break;
  case 'html': break;
  case 'text': break;
  default: ctx.throw(406, 'json, html, or text only');


Check if encodings are acceptable, returning the best match when true, otherwise false. Note that you should include identity as one of the encodings!

// Accept-Encoding: gzip
ctx.acceptsEncodings('gzip', 'deflate', 'identity');
// => "gzip"

ctx.acceptsEncodings(['gzip', 'deflate', 'identity']);
// => "gzip"

When no arguments are given all accepted encodings are returned as an array:

// Accept-Encoding: gzip, deflate
// => ["gzip", "deflate", "identity"]

Note that the identity encoding (which means no encoding) could be unacceptable if the client explicitly sends identity;q=0. Although this is an edge case, you should still handle the case where this method returns false.


Check if charsets are acceptable, returning the best match when true, otherwise false.

// Accept-Charset: utf-8, iso-8859-1;q=0.2, utf-7;q=0.5
ctx.acceptsCharsets('utf-8', 'utf-7');
// => "utf-8"

ctx.acceptsCharsets(['utf-7', 'utf-8']);
// => "utf-8"

When no arguments are given all accepted charsets are returned as an array:

// Accept-Charset: utf-8, iso-8859-1;q=0.2, utf-7;q=0.5
// => ["utf-8", "utf-7", "iso-8859-1"]


Check if langs are acceptable, returning the best match when true, otherwise false.

// Accept-Language: en;q=0.8, es, pt
ctx.acceptsLanguages('es', 'en');
// => "es"

ctx.acceptsLanguages(['en', 'es']);
// => "es"

When no arguments are given all accepted languages are returned as an array:

// Accept-Language: en;q=0.8, es, pt
// => ["es", "pt", "en"]


Check if the request is idempotent.


Return the request socket.


Return request header with case-insensitive field.


A Koa Response object is an abstraction on top of node's vanilla response object, providing additional functionality that is useful for every day HTTP server development.



Response header object.


Response header object. Alias as response.header.


Response socket. Points to net.Socket instance as request.socket.


Get response status. By default, response.status is set to 404 unlike node's res.statusCode which defaults to 200.


Set response status via numeric code:

NOTE: don't worry too much about memorizing these strings, if you have a typo an error will be thrown, displaying this list so you can make a correction.

Since response.status default is set to 404, to send a response without a body and with a different status is to be done like this:

ctx.response.status = 200;

// Or whatever other status
ctx.response.status = 204;


Get response status message. By default, response.message is associated with response.status.


Set response status message to the given value.


Set response Content-Length to the given value.


Return response Content-Length as a number when present, or deduce from ctx.body when possible, or undefined.


Get response body.


Set response body to one of the following:

If response.status has not been set, Koa will automatically set the status to 200 or 204 depending on response.body. Specifically, if response.body has not been set or has been set as null or undefined, Koa will automatically set response.status to 204. If you really want to send no content response with other status, you should override the 204 status as the following way:

// This must be always set first before status, since null | undefined
// body automatically sets the status to 204
ctx.body = null;
// Now we override the 204 status with the desired one
ctx.status = 200;

Koa doesn't guard against everything that could be put as a response body -- a function doesn't serialise meaningfully, returning a boolean may make sense based on your application, and while an error works, it may not work as intended as some properties of an error are not enumerable. We recommend adding middleware in your app that asserts body types per app. A sample middleware might be:

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

  ctx.assert.equal('object', typeof ctx.body, 500, 'some dev did something wrong')


The Content-Type is defaulted to text/html or text/plain, both with a default charset of utf-8. The Content-Length field is also set.


The Content-Type is defaulted to application/octet-stream, and Content-Length is also set.


The Content-Type is defaulted to application/octet-stream.

Whenever a stream is set as the response body, .onerror is automatically added as a listener to the error event to catch any errors. In addition, whenever the request is closed (even prematurely), the stream is destroyed. If you do not want these two features, do not set the stream as the body directly. For example, you may not want this when setting the body as an HTTP stream in a proxy as it would destroy the underlying connection.

See: for more information.

Here's an example of stream error handling without automatically destroying the stream:

const PassThrough = require('stream').PassThrough;

app.use(async ctx => {
  ctx.body = someHTTPStream.on('error', (err) => ctx.onerror(err)).pipe(PassThrough());


The Content-Type is defaulted to application/json. This includes plain objects { foo: 'bar' } and arrays ['foo', 'bar'].


Get a response header field value with case-insensitive field.

const etag = ctx.response.get('ETag');


Returns true if the header identified by name is currently set in the outgoing headers. The header name matching is case-insensitive.

const rateLimited = ctx.response.has('X-RateLimit-Limit');

response.set(field, value)

Set response header field to value:

ctx.set('Cache-Control', 'no-cache');

response.append(field, value)

Append additional header field with value val.

ctx.append('Link', '<>');


Set several response header fields with an object:

  'Etag': '1234',
  'Last-Modified': date

This delegates to setHeader which sets or updates headers by specified keys and doesn't reset the entire header.


Remove header field.


Get response Content-Type void of parameters such as "charset".

const ct = ctx.type;
// => "image/png"


Set response Content-Type via mime string or file extension.

ctx.type = 'text/plain; charset=utf-8';
ctx.type = 'image/png';
ctx.type = '.png';
ctx.type = 'png';

Note: when appropriate a charset is selected for you, for example response.type = 'html' will default to "utf-8". If you need to overwrite charset, use ctx.set('Content-Type', 'text/html') to set response header field to value directly.

Very similar to Check whether the response type is one of the supplied types. This is particularly useful for creating middleware that manipulate responses.

For example, this is a middleware that minifies all HTML responses except for streams.

const minify = require('html-minifier');

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

  if (!'html')) return;

  let body = ctx.body;
  if (!body || body.pipe) return;

  if (Buffer.isBuffer(body)) body = body.toString();
  ctx.body = minify(body);

response.redirect(url, [alt])

Perform a [302] redirect to url.

The string "back" is special-cased to provide Referrer support, when Referrer is not present alt or "/" is used.

ctx.redirect('back', '/index.html');

To alter the default status of 302, simply assign the status before or after this call. To alter the body, assign it after this call:

ctx.status = 301;
ctx.body = 'Redirecting to shopping cart';

response.attachment([filename], [options])

Set Content-Disposition to "attachment" to signal the client to prompt for download. Optionally specify the filename of the download and some options.


Check if a response header has already been sent. Useful for seeing if the client may be notified on error.


Return the Last-Modified header as a Date, if it exists.


Set the Last-Modified header as an appropriate UTC string. You can either set it as a Date or date string.

ctx.response.lastModified = new Date();


Set the ETag of a response including the wrapped "s. Note that there is no corresponding response.etag getter.

ctx.response.etag = crypto.createHash('md5').update(ctx.body).digest('hex');


Vary on field.


Flush any set headers, and begin the body.


Apex Ping is a beautiful uptime monitoring solution for websites and APIs, by one of the original authors of Koa.


Community links to discover third-party middleware for Koa, full runnable examples, thorough guides and more! If you have questions join us in IRC!