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Application

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

The following is a useless Koa application bound to port 3000:

var koa = require('koa');
var app = koa();
app.listen(3000);

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

var http = require('http');
var koa = require('koa');
var app = 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:

var http = require('http');
var koa = require('koa');
var app = koa();
http.createServer(app.callback()).listen(3000);
http.createServer(app.callback()).listen(3001);

Settings

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.jsonSpaces default JSON response spaces [2]
app.outputErrors output err.stack to stderr [true in "development"]

app.listen(...)

Create and return an HTTP server, passing the given arguments to Server#listen(). These arguments are documented on nodejs.org.

app.callback()

Return a callback function suitable for the http.createServer() method to handle a request.

app.use(function)

Add the given middleware function to this application. See Middleware for more information.

app.context(obj)

Each Application has its own Context instance, meaning you may extend the prototype of one, and the other will remain untouched with the default prototype. To extend an app's context you may invoke app.context() any number of times with an object of extensions:

app.context({
  get something(){
    return 'hi';
  },

  set something(val){
    this._something = val;
  },

  render: function(){
    this.body = '<html></html>';
  }
});

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 middlware to re-implement this common functionality.

A Context is created per request, and is referenced in middleware as the receiver, or the this variable.

ctx.req

Node's request object.

ctx.res

Node's response object.

ctx.app

Application instance reference.

ctx.header

Request header object.

ctx.responseHeader

Response header object.

ctx.method

Request method.

ctx.method=

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

ctx.status

Get response status.

ctx.status=

Set response status via numeric code or string:

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"
300 "multiple choices"
301 "moved permanently"
302 "moved temporarily"
303 "see other"
304 "not modified"
305 "use proxy"
307 "temporary 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 time-out"
409 "conflict"
410 "gone"
411 "length required"
412 "precondition failed"
413 "request entity too large"
414 "request-uri too large"
415 "unsupported media type"
416 "requested range not satisfiable"
417 "expectation failed"
418 "i'm a teapot"
422 "unprocessable entity"
423 "locked"
424 "failed dependency"
425 "unordered collection"
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 time-out"
505 "http version not supported"
506 "variant also negotiates"
507 "insufficient storage"
509 "bandwidth limit exceeded"
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.

ctx.length

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

ctx.responseLength

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

ctx.body

Get response body. When ctx.body is null and ctx.status is still 200 it is considered a 404. This is to prevent the developer from manually specifying this.status = 200 on every response.

ctx.body=

Set response body to one of the following:

string written
Buffer written
Stream piped
Object json stringified

When a Koa application is created it injects a middleware named respond, which handles each of these ctx.body values. The Content-Length header field is set when possible, and objects are passed through JSON.stringify().

To alter the JSON response formatting use the app.jsonSpaces setting, for example to compress JSON responses set:

app.jsonSpaces = 0;

ctx.get(field)

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

var etag = this.get('If-None-Match');

ctx.set(field, value)

Set response header field to value:

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

ctx.set(fields)

Set several response header fields with an object:

this.set({
  'Etag': '1234',
  'Last-Modified': date
});

ctx.type

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

var ct = this.type;
// => "image/png"

ctx.type=

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

this.type = 'image/png';
this.type = '.png';
this.type = 'png';

NOTE: when ctx.body is an object the content-type is set for you.

ctx.url

Get request URL.

ctx.url=

Set request URL, useful for url rewrites.

ctx.path

Get request pathname.

ctx.path=

Set request pathname and retain query-string when present.

ctx.query

Get parsed query-string using qs. For example with the url "/shoes?page=2&sort=asc&filters[color]=blue" this.query would be the following object:

{
  page: '2',
  sort: 'asc',
  filters: {
    color: 'blue'
  }
}

NOTE: this property returns {} when no query-string is present.

ctx.query=

Set query-string to the given object.

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

ctx.querystring

Get raw query string void of ?.

ctx.querystring=

Set raw query string.

ctx.host

Get host void of port number when present. Supports X-Forwarded-Host when app.proxy is true, otherwise Host is used.

ctx.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.

this.set('ETag', '123');

// cache is ok
if (this.fresh) {
  this.status = 304;
  return;
} 

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

ctx.stale

Inverse of ctx.fresh.

ctx.protocol

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

ctx.secure

Shorthand for this.protocol == "https" to check if a requset was issued via TLS.

ctx.ip

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

ctx.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.

ctx.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, this.subdomains is ["ferrets", "tobi"]. If app.subdomainOffset is 3, this.subdomains is ["tobi"].

ctx.is(type)

Check if the incoming request contains the Content-Type header field, and it contains the give mime type.

// With Content-Type: text/html; charset=utf-8
this.is('html');
this.is('.html');
this.is('text/html');
this.is('text/*');
// => true

// When Content-Type is application/json
this.is('json');
this.is('.json');
this.is('application/json');
this.is('application/*');
// => true

this.is('html');
// => false

ctx.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.

this.redirect('back');
this.redirect('back', '/index.html');
this.redirect('/login');
this.redirect('http://google.com');

To alter the default status of 302 or the response body simply re-assign after this call:

this.redirect('/cart');
this.status = 301;
this.body = 'Redirecting to shopping cart';

ctx.attachment([filename])

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

ctx.accept(types)

Check if the given type(s) is acceptable, returning the best match when true, otherwise undefined, in which case you should respond with 406 "Not Acceptable".

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"]. When a list or array is given the best match, if any is returned.

// Accept: text/html
this.accepts('html');
// => "html"

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

// Accept: text/*, application/json
this.accepts('image/png');
this.accepts('png');
// => undefined

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

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

switch (this.accepts('json', 'html', 'text')) {
  case 'json': break;
  case 'html': break;
  case 'text': break;
}

ctx.accepted

Return accepted mime types ordered by quality.

ctx.acceptedEncodings

Return accepted content encodings ordered by quality.

ctx.acceptedCharsets

Return accepted charsets ordered by quality.

ctx.acceptedLanguages

Return accepted languages ordered by quality.

ctx.headerSent

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

ctx.socket

Request socket object.

Notes

HEAD support

Koa's upstream response middleware supports HEAD for you, however expensive requests would benefit from custom handling. For example instead of reading a file into memory and piping it to the client, you may wish to stat() and set the Content-* header fields appropriately to bypass the read.

License

MIT

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