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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 => {
  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.

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.

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.app.emit

Koa applications extend an internal EventEmitter. ctx.app.emit 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:

  • 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.
  • expires: a Date object indicating the cookie's expiration date (expires at the end of session by default).
  • path: a string indicating the path of the cookie (/ by default).
  • domain: a string indicating the domain of the cookie (no default).
  • secure: a boolean indicating whether the cookie is only to be sent over HTTPS (false by default for HTTP, true by default for HTTPS). Read more about this option.
  • httpOnly: a boolean indicating whether the cookie is only to be sent over HTTP(S), and not made available to client JavaScript (true by default).
  • sameSite: a boolean or string indicating whether the cookie is a "same site" cookie (false by default). This can be set to 'strict', 'lax', 'none', or true (which maps to 'strict').
  • signed: a boolean indicating whether the cookie is to be signed (false by default). If this is true, another cookie of the same name with the .sig suffix appended will also be sent, with a 27-byte url-safe base64 SHA1 value representing the hash of cookie-name=cookie-value against the first Keygrip key. This signature key is used to detect tampering the next time a cookie is received.
  • 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])

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. 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.

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
  • 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=