Koa - next generation web framework for node.jsIntroductionKoa 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. Through leveraging generators Koa allows you
to ditch callbacks and greatly increase error-handling. Koa does not bundle any
middleware within core, and provides an elegant suite of methods that make
writing servers fast and enjoyable.
Installation
Koa requires node v7.6.0 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
Async Functions with Babel
To use async functions in Koa in versions of node & 7.6, we recommend using .
require('babel-core/register');
// require the rest of the app that needs to be transpiled after the hook
const app = require('./app');
To parse and transpile async functions,
you should at a minimum have the
For example, in your .babelrc file, you should have:
&plugins&: [&transform-async-to-generator&]
You can also use the
with a target option &node&: &current& instead.
Application
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';
app.listen(3000);
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 continue
to yield 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();
// x-response-time
app.use(async (ctx, next) =& {
const start = Date.now();
await next();
const ms = Date.now() -
ctx.set('X-Response-Time', `${ms}ms`);
app.use(async (ctx, next) =& {
const start = Date.now();
await next();
const ms = Date.now() -
console.log(`${ctx.method} ${ctx.url} - ${ms}`);
// response
app.use(async ctx =& {
ctx.body = 'Hello World';
app.listen(3000);
Application settings are properties on the app instance, currently
the following are supported:
app.env defaulting to the NODE_ENV or &development&
app.proxy when true proxy header fields will be trusted
app.subdomainOffset offset of .subdomains to ignore [2]
app.listen(...)
A Koa application is not a 1-to-1 representation of a 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();
app.listen(3000);
The app.listen(...) method is simply sugar for the following:
const http = require('http');
const Koa = require('koa');
const app = new Koa();
http.createServer(app.callback()).listen(3000);
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();
http.createServer(app.callback()).listen(3000);
https.createServer(app.callback()).listen(3001);
app.callback()
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.
app.use(function)
Add the given middleware function to this application. See
more information.
Set signed cookie keys.
These are passed to ,
however you may also pass your own KeyGrip instance. For
example the following are acceptable:
app.keys = ['im a newer secret', 'i like turtle'];
app.keys = new KeyGrip(['im a newer secret', 'i like turtle'], 'sha256');
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
app.context is the prototype from which ctx is created from.
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 =& {
console.log(ctx.db);
Many properties on ctx are defined using getters, setters, and Object.defineProperty(). You can only edit these properties (not recommended) by using Object.defineProperty() on app.context. See .
Mounted apps currently use its parent's ctx and settings. Thus, mounted apps are really just groups of middleware.
Error Handling
By default outputs all errors to stderr unless app.silent is true.
The default error handler also won't outputs 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
app.use(async ctx =& {
// is the Context
ctx. // is a koa Request
ctx. // 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:
res.statusCode
res.writeHead()
res.write()
ctx.request
A koa Request object.
ctx.response
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.
ctx.cookies.get(name, [options])
Get cookie name with options:
signed the cookie requested should be signed
koa uses the
module where options are simply passed.
ctx.cookies.set(name, value, [options])
Set cookie name to value with options:
maxAge a number representing the milliseconds from Date.now() for expiry
signed sign the cookie value
expires a Date for cookie expiration
path cookie path, /' by default
domain cookie domain
secure secure cookie
httpOnly server-accessible cookie, true by default
overwrite a boolean indicating whether to overwrite previously set cookies of the same name (false by default). If this is true, all cookies set during the same request with the same name (regardless of path or domain) are filtered out of the Set-Cookie header when setting this cookie.
koa uses the
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);
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 =
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
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 });
to create errors.
ctx.assert(value, [status], [msg], [properties])
Helper method to throw an error similar to .throw()
when !value. Similar to node's
ctx.assert(ctx.state.user, 401, 'User not found. Please login!');
for assertions.
ctx.respond
To bypass Koa's built-in response handling, you may explicitly set ctx.respond =. 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
equivalents:
ctx.header
ctx.headers
ctx.method
ctx.method=
ctx.originalUrl
ctx.origin
ctx.query=
ctx.querystring
ctx.querystring=
ctx.hostname
ctx.socket
ctx.protocol
ctx.secure
ctx.subdomains
ctx.accepts()
ctx.acceptsEncodings()
ctx.acceptsCharsets()
ctx.acceptsLanguages()
Response aliases
The following accessors and alias
equivalents:
ctx.status
ctx.status=
ctx.message
ctx.message=
ctx.length=
ctx.length
ctx.headerSent
ctx.redirect()
ctx.attachment()
ctx.append()
ctx.remove()
ctx.lastModified=
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.
Set request header object.
Request header object. Alias as request.header.
Set request header object. Alias as request.header=.
request.method
Request method.
request.method=
Set request method, useful for implementing middleware
such as methodOverride().
request.length
Return request Content-Length as a number when present, or undefined.
request.url
Get request URL.
request.url=
Set request URL, useful for url rewrites.
request.originalUrl
Get request original URL.
request.origin
Get origin of URL, include protocol and host.
ctx.request.origin
request.href
Get full request URL, include protocol, host and url.
ctx.request.
// =& /foo/bar?q=1
request.path
Get request pathname.
request.path=
Set request pathname and retain query-string when present.
request.querystring
Get raw query string void of ?.
request.querystring=
Set raw query string.
Get raw query string with the ?.
Set raw query string.
request.host
Get host (hostname:port) when present. Supports X-Forwarded-Host
when app.proxy is true, otherwise Host is used.
request.hostname
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
Note This may impact performance.
request.URL
Get WHATWG parsed URL object.
request.type
Get request Content-Type void of parameters such as &charset&.
const ct = ctx.request.
// =& &image/png&
request.charset
Get request charset when present, or undefined:
ctx.request.
// =& &utf-8&
request.query
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'
request.query=
Set query-string to the given object. Note that this
setter does not support nested objects.
ctx.query = { next: '/login' };
request.fresh
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');
request.stale
Inverse of request.fresh.
request.protocol
Return request protocol, &https& or &http&. Supports X-Forwarded-Proto
when app.proxy is true.
request.secure
Shorthand for ctx.protocol == &https& to check if a request was
issued via TLS.
request.ip
Request remote address. Supports X-Forwarded-For when app.proxy
request.ips
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.
request.subdomains
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 &tobi.&:
If app.subdomainOffset is not set, ctx.subdomains is [&ferrets&, &tobi&].
If app.subdomainOffset is 3, ctx.subdomains is [&tobi&].
request.is(types...)
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/ charset=utf-8
ctx.is('html'); // =& 'html'
ctx.is('text/html'); // =& 'text/html'
ctx.is('text/*', 'text/html'); // =& 'text/html'
// When Content-Type is application/json
ctx.is('json', 'urlencoded'); // =& 'json'
ctx.is('application/json'); // =& 'application/json'
ctx.is('html', 'application/*'); // =& 'application/json'
ctx.is('html'); // =& false
For example if you want to ensure that
only images are sent to a given route:
if (ctx.is('image/*')) {
// process
ctx.throw(415, 'images only!');
Content Negotiation
Koa's request object includes helpful content negotiation utilities powered by
and . These utilities are:
request.accepts(types)
request.acceptsEncodings(types)
request.acceptsCharsets(charsets)
request.acceptsLanguages(langs)
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.
request.accepts(types)
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
ctx.accepts('html');
// =& &html&
// Accept: text/*, application/json
ctx.accepts('html');
// =& &html&
ctx.accepts('text/html');
// =& &text/html&
ctx.accepts('json', 'text');
// =& &json&
ctx.accepts('application/json');
// =& &application/json&
// Accept: text/*, application/json
ctx.accepts('image/png');
ctx.accepts('png');
// =& 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':
case 'html':
case 'text':
default: ctx.throw(406, 'json, html, or text only');
request.acceptsEncodings(encodings)
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
ctx.acceptsEncodings();
// =& [&gzip&, &deflate&, &identity&]
Note that the identity encoding (which means no encoding) could be unacceptable if the client explicitly sends q=0. Although this is an edge case, you should still handle the case where this method returns false.
request.acceptsCharsets(charsets)
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
ctx.acceptsCharsets();
// =& [&utf-8&, &utf-7&, &iso-8859-1&]
request.acceptsLanguages(langs)
Check if langs are acceptable, returning
the best match when true, otherwise false.
// Accept-Language: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:q=0.8, es, pt
ctx.acceptsLanguages();
// =& [&es&, &pt&, &en&]
request.idempotent
Check if the request is idempotent.
request.socket
Return the request socket.
request.get(field)
Return request header.
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
Request socket.
response.status
Get response status. By default, response.status is set to 404 unlike node's res.statusCode which defaults to 200.
response.status=
Set response status via numeric code:
100 &continue&
101 &switching protocols&
102 &processing&
201 &created&
202 &accepted&
203 &non-authoritative information&
204 &no content&
205 &reset content&
206 &partial content&
207 &multi-status&
208 &already reported&
226 &im used&
300 &multiple choices&
301 &moved permanently&
302 &found&
303 &see other&
304 &not modified&
305 &use proxy&
307 &temporary redirect&
308 &permanent redirect&
400 &bad request&
401 &unauthorized&
402 &payment required&
403 &forbidden&
404 &not found&
405 &method not allowed&
406 &not acceptable&
407 &proxy authentication required&
408 &request timeout&
409 &conflict&
410 &gone&
411 &length required&
412 &precondition failed&
413 &payload too large&
414 &uri too long&
415 &unsupported media type&
416 &range not satisfiable&
417 &expectation failed&
418 &I'm a teapot&
422 &unprocessable entity&
423 &locked&
424 &failed dependency&
426 &upgrade required&
428 &precondition required&
429 &too many requests&
431 &request header fields too large&
500 &internal server error&
501 &not implemented&
502 &bad gateway&
503 &service unavailable&
504 &gateway timeout&
505 &http version not supported&
506 &variant also negotiates&
507 &insufficient storage&
508 &loop detected&
510 &not extended&
511 &network authentication required&
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.
response.message
Get response status message. By default, response.message is
associated with response.status.
response.message=
Set response status message to the given value.
response.length=
Set response Content-Length to the given value.
response.length
Return response Content-Length as a number when present, or deduce
from ctx.body when possible, or undefined.
response.body
Get response body.
response.body=
Set response body to one of the following:
string written
Buffer written
Stream piped
Object || Array json-stringified
null no content response
If response.status has not been set, Koa will automatically set the status to 200 or 204.
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.
for more information.
Here's an example of stream error handling without automatically destroying the stream:
const PassThrough = require('stream').PassT
app.use(async ctx =& {
ctx.body = someHTTPStream.on('error', ctx.onerror).pipe(PassThrough());
The Content-Type is defaulted to application/json. This includes plain objects { foo: 'bar' } and arrays ['foo', 'bar'].
response.get(field)
Get a response header field value with case-insensitive field.
const etag = ctx.response.get('ETag');
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', '&http://127.0.0.1/&');
response.set(fields)
Set several response header fields with an object:
'Etag': '1234',
'Last-Modified': date
response.remove(field)
Remove header field.
response.type
Get response Content-Type void of parameters such as &charset&.
const ct = ctx.
// =& &image/png&
response.type=
Set response Content-Type via mime string or file extension.
ctx.type = 'text/ 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.
response.is(types...)
Very similar to ctx.request.is().
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 (!ctx.response.is('html'))
let body = ctx.
if (!body || body.pipe)
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');
ctx.redirect('back', '/index.html');
ctx.redirect('/login');
ctx.redirect('');
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.redirect('/cart');
ctx.body = 'Redirecting to shopping cart';
response.attachment([filename])
Set Content-Disposition to &attachment& to signal the client
to prompt for download. Optionally specify the filename of the
Check if a response header has already been sent. Useful for seeing
if the client may be notified on error.
response.lastModified
Return the Last-Modified header as a Date, if it exists.
response.lastModified=
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');
response.vary(field)
Vary on field.
Flush any set headers, and begin the body.
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LinksCommunity links to discover third-party middleware for Koa, full runnable examples,
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