6/27/2017 Koa - next generation web framework for node.js
http://koajs.com/ 1/34
koa
next generation web framework for node.js
6/27/2017 Koa - next generation web framework for node.js
http://koajs.com/ 2/34
Introduction
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. 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:
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$ 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 babel's require hook.
To parse and transpile async functions, you should at a minimum
have the transform‑async‑to‑generator or transform‑async‑to‑
module‑method plugins. For example, in your .babelrc file, you
should have:
{
"plugins": ["transform-async-to-generator"]
}
You can also use the env preset 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,
require('babel-core/register');
// require the rest of the app that needs to be transpiled after the hook
const app = require('./app');
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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(ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);
Cascading
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
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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 function (ctx, next) {
const start = new Date();
await next();
const ms = new Date() - start;
ctx.set('X-Response-Time', `${ms}ms`);
});
// logger
app.use(async function (ctx, next) {
const start = new Date();
await next();
const ms = new Date() - start;
console.log(`${ctx.method} ${ctx.url} - ${ms}`);
});
// response
app.use(ctx => {
ctx.body = 'Hello World';
});
app.listen(3000);
Settings
Application settings are properties on the app instance, currently
the following are supported:
app.env defaulting to the NODE_ENV or "development"
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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
nodejs.org. 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 Koa = require('koa');
const app = new Koa();
http.createServer(app.callback()).listen(3000);
http.createServer(app.callback()).listen(3001);
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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
Middleware for more information.
app.keys=
Set signed cookie keys.
These are passed to KeyGrip, however you may also pass your
own KeyGrip instance. For example the following are acceptable:
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.
app.keys = ['im a newer secret', 'i like turtle'];
app.keys = new KeyGrip(['im a newer secret', 'i like turtle'], 'sha256');
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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);
});
Note:
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
https://github.com/koajs/koa/issues/652.
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:
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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.
Context
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, next) => {
ctx; // is the Context
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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.
API
Context specific methods and accessors.
ctx.req
Node's request object.
ctx.res
Node's response object.
Bypassing Koa's response handling is not supported. Avoid
using the following node properties:
res.statusCode
res.writeHead()
res.write()
res.end()
ctx.request
A koa Request object.
ctx.response
A koa Response object.
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ctx.state
The recommended namespace for passing information through
middleware and to your frontend views.
ctx.state.user = await User.find(id);
ctx.app
Application instance reference.
ctx.cookies.get(name, [options])
Get cookie name with options:
signed the cookie requested should be signed
koa uses the cookies 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 cookies module where options are simply passed.
ctx.throw([status], [msg], [properties])
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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 = 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.
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!');
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koa uses http‑assert for assertions.
ctx.respond
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:
ctx.header
ctx.headers
ctx.method
ctx.method=
ctx.url
ctx.url=
ctx.originalUrl
ctx.origin
ctx.href
ctx.path
ctx.path=
ctx.query
ctx.query=
ctx.querystring
ctx.querystring=
ctx.host
ctx.hostname
ctx.fresh
ctx.stale
ctx.socket
ctx.protocol
ctx.secure
ctx.ip
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ctx.ips
ctx.subdomains
ctx.is()
ctx.accepts()
ctx.acceptsEncodings()
ctx.acceptsCharsets()
ctx.acceptsLanguages()
ctx.get()
Response aliases
The following accessors and alias Response equivalents:
ctx.body
ctx.body=
ctx.status
ctx.status=
ctx.message
ctx.message=
ctx.length=
ctx.length
ctx.type=
ctx.type
ctx.headerSent
ctx.redirect()
ctx.attachment()
ctx.set()
ctx.append()
ctx.remove()
ctx.lastModified=
ctx.etag=
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Request
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.
API
request.header
Request header object.
request.headers
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
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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
// => http://example.com
request.href
Get full request URL, include protocol, host and url.
ctx.request.href
// => http://example.com/foo/bar?q=1
request.path
Get request pathname.
request.path=
Set request pathname and retain query‑string when present.
request.querystring
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Get raw query string void of ?.
request.querystring=
Set raw query string.
request.search
Get raw query string with the ?.
request.search=
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.
request.type
Get request Content-Type void of parameters such as "charset".
const ct = ctx.request.type
// => "image/png"
request.charset
Get request charset when present, or undefined:
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ctx.request.charset
// => "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;
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ctx.set('ETag', '123');
// cache is ok
if (ctx.fresh) {
ctx.status = 304;
return;
}
// cache is stale
// fetch new data
ctx.body = yield 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 is true.
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.
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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.ferrets.example.com": 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/html; 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
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} 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:
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');
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// => "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': break;
case 'html': break;
case 'text': break;
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!
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// 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 identity;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:
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// 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: 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
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.
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Response
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.
API
response.header
Response header object.
response.headers
Response header object. Alias as response.header.
response.socket
Request socket.
response.status
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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"
200 "ok"
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"
422 "unprocessable entity"
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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
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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.
String
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.
Buffer
The Content‑Type is defaulted to application/octet‑stream, and
Content‑Length is also set.
Stream
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: https://github.com/koajs/koa/pull/612 for more
information.
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Here's an example of stream error handling without
automatically destroying the stream:
Object
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.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/>');
const PassThrough = require('stream').PassThrough;
app.use(function * (next) {
ctx.body = someHTTPStream.on('error', ctx.onerror).pipe(PassThrough());
});
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response.set(fields)
Set several response header fields with an object:
ctx.set({
'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.type;
// => "image/png"
response.type=
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", however
when explicitly defined in full as response.type = 'text/html'
no charset is assigned.
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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(function * minifyHTML(next) {
yield next;
if (!ctx.response.is('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');
ctx.redirect('back', '/index.html');
ctx.redirect('/login');
ctx.redirect('http://google.com');
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:
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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
download.
response.headerSent
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();
response.etag=
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');
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Links
Community links to discover third‑party middleware for Koa, full
runnable examples, thorough guides and more! If you have
questions join us in IRC!
GitHub repository
Examples
Middleware
Wiki
G+ Community
Mailing list
Guide
FAQ
#koajs on freenode
