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.


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:

  • 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"
  • 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.


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:

  • 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.

See: https://github.com/koajs/koa/pull/612 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', ctx.onerror).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');

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


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 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')) 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.

© 2018 Koa contributors
Licensed under the MIT License.