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oak ci deno doc

A middleware framework for Deno’s http server, including a router middleware.

This middleware framework is inspired by Koa and middleware router inspired by @koa/router.

This README focuses on the mechanics of the oak APIs and is intended for those who are familiar with JavaScript middleware frameworks like Express and Koa as well as a decent understanding of Deno. If you aren’t familiar with these, please check out documentation on

Also, check out our FAQs and the awesome-oak site of community resources.

Warning The examples in this README pull from main, which may not make sense to do when you are looking to actually deploy a workload. You would want to “pin” to a particular version which is compatible with the version of Deno you are using and has a fixed set of APIs you would expect. supports using git tags in the URL to direct you at a particular version. So to use version 3.0.0 of oak, you would want to import

Application, middleware, and context

The Application class wraps the serve() function from the http package. It has two methods: .use() and .listen(). Middleware is added via the .use() method and the .listen() method will start the server and start processing requests with the registered middleware.

A basic usage, responding to every request with Hello World!:

import { Application } from "";

const app = new Application();

app.use((ctx) => {
  ctx.response.body = "Hello World!";

await app.listen({ port: 8000 });

The middleware is processed as a stack, where each middleware function can control the flow of the response. When the middleware is called, it is passed a context and reference to the “next” method in the stack.

A more complex example:

import { Application } from "";

const app = new Application();

// Logger
app.use(async (ctx, next) => {
  await next();
  const rt = ctx.response.headers.get("X-Response-Time");
  console.log(`${ctx.request.method} ${ctx.request.url} - ${rt}`);

// Timing
app.use(async (ctx, next) => {
  const start =;
  await next();
  const ms = - start;
  ctx.response.headers.set("X-Response-Time", `${ms}ms`);

// Hello World!
app.use((ctx) => {
  ctx.response.body = "Hello World!";

await app.listen({ port: 8000 });

To provide an HTTPS server, then the app.listen() options need to include the options .secure option set to true and supply a .certFile and a .keyFile options as well.

An instance of application has some properties as well:

  • .keys

    Keys to be used when signing and verifying cookies. The value can be set to an array of keys, and instance of KeyStack, or an object which provides the same interface as KeyStack (e.g. an instance of keygrip). If just the keys are passed, oak will manage the keys via KeyStack which allows easy key rotation without requiring re-signing of data values.

  • .proxy

    This defaults to false, but can be set via the Application constructor options. This is intended to indicate the application is behind a proxy and will use X-Forwarded-Proto, X-Forwarded-Host, and X-Forwarded-For when processing the request, which should provide more accurate information about the request.

  • .state

    A record of application state, which can be strongly typed by specifying a generic argument when constructing an Application(), or inferred by passing a state object (e.g. Application({ state })).


The context passed to middleware has several properties:

  • .app

    A reference to the Application that is invoking this middleware.

  • .cookies

    The Cookies instance for this context which allows you to read and set cookies.

  • .request

    The Request object which contains details about the request.

  • .respond

    Determines if when middleware finishes processing, the application should send the .response to the client. If true the response will be sent, and if false the response will not be sent. The default is true but certain methods, like .upgrade() and .sendEvents() will set this to false.

  • .response

    The Response object which will be used to form the response sent back to the requestor.

  • .socket

    This will be undefined if the connection has not been upgraded to a web socket. If the connection has been upgraded, the .socket interface will be set.

  • .state

    A record of application state, which can be strongly typed by specifying a generic argument when constructing an Application(), or inferred by passing a state object (e.g. Application({ state })).

The context passed to middleware has some methods:

  • .assert()

    Makes an assertion, which if not true, throws an HTTPError, which subclass is identified by the second argument, with the message being the third argument.

  • .send()

    Stream a file to the requesting client. See Static content below for more information.

  • .sendEvents()

    Convert the current connection into a server-sent event response and return a ServerSentEventTarget where messages and events can be streamed to the client. This will set .respond to false.

  • .throw()

    Throws an HTTPError, which subclass is identified by the first argument, with the message being passed as the second.

  • .upgrade()

    Attempt to upgrade the connection to a web socket connection, and resolve with an web socket interface. This will set .respond to false.

Unlike other middleware frameworks, context does not have a significant amount of aliases. The information about the request is only located in .request and the information about the response is only located in .response.


The context.cookies allows access to the values of cookies in the request, and allows cookies to be set in the response. It automatically secures cookies if the .keys property is set on the application. It has several methods:

  • .get(key: string, options?: CookieGetOptions)

    Attempts to retrieve the cookie out of the request and returns the value of the cookie based on the key. If the applications .keys is set, then the cookie will be verified against a signed version of the cookie. If the cookie is valid, the value will be returned. If it is invalid, the cookie signature will be set to deleted on the response. If the cookie was not signed by the current key, it will be resigned and added to the response.

  • .set(key: string, value: string, options?: CookieSetDeleteOptions)

    Will set a cookie in the response based on the provided key, value and any options. If the applications .keys is set, then the cookie will be signed and the signature added to the response.


The context.request contains information about the request. It contains several properties:

  • .hasBody

    Set to true if the request has a body, or false if it does not. It does not validate if the body is supported by the built in body parser though.

  • .headers

    The headers for the request, an instance of Headers.

  • .method

    A string that represents the HTTP method for the request.

  • .secure

    A shortcut for .protocol, returning true if HTTPS otherwise false.

  • .serverRequest

    The original net server request.

  • .url

    An instance of URL which is based on the full URL for the request. This is in place of having parts of the URL exposed on the rest of the request object.

And several methods:

  • .accepts(...types: string[])

    Negotiates the content type supported by the request for the response. If no content types are passed, the method returns a prioritized array of accepted content types. If content types are passed, the best negotiated content type is returned. If there is no content type matched, then undefined is returned.

  • .acceptsCharsets(...charsets: string[])

    Negotiates the character encoding supported by the request for the response. If no character encodings are passed, the method returns a prioritized array of accepted character encodings. If character encodings are passed, the best negotiated charset is returned. If there are no encodings matched, then undefined is returned.

    Most browsers simply to not send a character encoding header anymore, and it is just expected UTF-8 will be used.

  • .acceptsEncodings(...encodings: string[])

    Negotiates the content encoding supported by the request for the response. If no encodings are passed, the method returns a prioritized array of accepted encodings. If encodings are passed, the best negotiated encoding is returned. If there are no encodings matched, then undefined is returned.

  • .acceptsLanguages(...languages: string[])

    Negotiates the language the client is able to understand. Where a locale variant takes preference. If no encodings are passed, the method returns a prioritized array of understood languages. If languages are passed, the best negotiated language is returned. If there are no languages matched, then undefined is returned.

  • .body(options?: BodyOptions)

    The method returns a representation of the request body. When no options are passed, the request headers are used to determine the type of the body, which will be parsed and returned. The returned object contains two properties. type contains the type of "json", "text", "form", "form-data", "raw" or "undefined".

    The type of the value can be determined by the value of the type property:

    type value
    "form" Promise<URLSearchParams>
    "form-data" FormDataReader
    "json" Promise<unknown>
    "raw" Promise<Uint8Array>
    "text" Promise<string>
    "undefined" undefined

    If there is no body, the type of "undefined" is returned. If the content type of the request is not recognised, then the type of "raw" is returned.

    You can use the option type to specifically request the body to be returned in a particular format. If you need access to the Deno HTTP server’s body, then you can use the type of "reader" which will return the body object of type "reader" with a value as a Deno.Reader:

    app.use(async (ctx) => {
      const result = ctx.request.body({ type: "reader" });
      result.type; // "reader"
      await Deno.readAll(result.value); // a "raw" Uint8Array of the body

    Another use case for the type option is if certain middleware always needs the body in a particular format, but wants other middleware to consume it in a content type resolved way:

    app.use(async (ctx) => {
      if (ctx.request.hasBody) {
        const result = ctx.request.body({ type: "text" });
        const text = await result.value;
        // do some validation of the body as a string
    app.use(async (ctx) => {
      const result = ctx.request.body(); // content type automatically detected
      if (result.type === "json") {
        const value = await result.value; // an object of parsed JSON

    When specifying a type, it is always good to check that .request.hasBody is true, as the .request.body() will throw if the body is undefined.

    You can use the option contentTypes to set additional media types that when present as the content type for the request, the body will be parsed accordingly. The options takes possibly four keys: json, form, text, and raw. For example if you wanted JavaScript sent to the server to be parsed as text, you would do something like this:

    app.use(async (ctx) => {
      const result = ctx.request.body({
        contentTypes: {
          text: ["application/javascript"],
      result.type; // "text"
      await result.value; // a string containing the text

    Because of the nature of how the body is parsed, once the body is requested and returned in a particular format, it can’t be requested in certain other ones, and .request.body() will throw if an incompatible type is requested. The type "reader" is incompatible with all other types, bodies which are resolved as type "form-data" or "undefined" are incompatible with all other types. While "json", "form", "raw", "text" are all compatible with each other, although if there are invalid data for that type, they may throw if coerced into that type.

    In particular the contentTypes.raw can be used to override default types that are supported that you would want the middleware to handle itself. For example if you wanted the middleware to parse all text media types itself, you would do something like this:

    app.use(async (ctx) => {
      const result = ctx.request.body({
        contentTypes: {
          raw: ["text"],
      result.type; // "raw"
      await result.value; // a Uint8Array of all of the bytes read from the request


The context.response contains information about the response which will be sent back to the requestor. It contains several properties:

  • .body

    The body of the response, which can often be handled by the automatic response body handling documented below.

  • .headers

    A Headers instance which contains the headers for the response.

  • .status

    An HTTP Status code that will be sent back with the response. If this is not set before responding, oak will default to 200 OK if there is a .body, otherwise 404 Not Found.

  • .type

    A media type or extension to set the Content-Type header for the response. For example, you can provide txt or text/plain to describe the body.

And a method:

  • .redirect(url?: string | URL | REDIRECT_BACK, alt?: string | URL)

    A method to simplify redirecting the response to another URL. It will set the Location header to the supplied url and the status to 302 Found (unless the status is already a 3XX status). The use of symbol REDIRECT_BACK as the url indicates that the Referrer header in the request should be used as the direction, with the alt being the alternative location if the Referrer is not set. If neither the alt nor the Referrer are set, the redirect will occur to /. A basic HTML (if the requestor supports it) or a text body will be set explaining they are being redirected.

Automatic response body handling

When the response Content-Type is not set in the headers of the .response, oak will automatically try to determine the appropriate Content-Type. First it will look at .response.type. If assigned, it will try to resolve the appropriate media type based on treating the value of .type as either the media type, or resolving the media type based on an extension. For example if .type was set to "html", then the Content-Type will be set to "text/html".

If .type is not set with a value, then oak will inspect the value of .response.body. If the value is a string, then oak will check to see if the string looks like HTML, if so, Content-Type will be set to text/html otherwise it will be set to text/plain. If the value is an object, other than a Uint8Array, a Deno.Reader, or null, the object will be passed to JSON.stringify() and the Content-Type will be set to application/json.

If the type of body is a number, bigint or symbol, it will be coerced to a string and treated as text.

If the value of body is a function, the function will be called with no arguments. If the return value of the function is promise like, that will be await, and the resolved value will be processed as above. If the value is not promise like, it will be processed as above.

Opening the server

The application method .listen() is used to open the server, start listening for requests, and processing the registered middleware for each request. This method returns a promise when the server closes.

Once the server is open, before it starts processing requests, the application will fire a "listen" event, which can be listened for via the .addEventListener() method. For example:

import { Application } from "";

const app = new Application();

app.addEventListener("listen", ({ hostname, port, secure }) => {
    `Listening on: ${secure ? "https://" : "http://"}${
      hostname ?? "localhost"

// register some middleware

await app.listen({ port: 80 });

Closing the server

If you want to close the application, the application supports the option of an abort signal. Here is an example of using the signal:

import { Application } from "";

const app = new Application();

const controller = new AbortController();
const { signal } = controller;

// Add some middleware using `app.use()`

const listenPromise = app.listen({ port: 8000, signal });

// In order to close the sever...

// Listen will stop listening for requests and the promise will resolve...
await listenPromise;
// and you can do something after the close to shutdown

Just handling requests

In situations where you don’t want the application to listen for requests on the std/http/server, but you still want the application to process requests, you can use the .handle() method. For example if you are in a serverless function where the requests are arriving in a different way.

The .handle() method will invoke the middleware, just like the middleware gets invoked for each request that is processed by .listen(). It take up to two arguments, one being the request which conforms to the std/http/server’s ServerRequest interface and an optional second argument which is if the request is “secure” in the sense it originated from a TLS connection to the remote client. The method resolves with a response that conforms to the ServerResponse interface (which can be used with the std/http/server’s request.respond() method) or undefined if the ctx.respond === true (e.g. the connection was upgraded to a web socket or is sending back server sent events).

An example:

import { listenAndServe } from "";
import { Application } from "";

const app = new Application();

app.use((ctx) => {
  ctx.response.body = "Hello World!";

await listenAndServe({ port: 8000 }, async (request) => {
  const response = await app.handle(request);
  if (response) {

Error handling

Middleware can be used to handle other errors with middleware. Awaiting other middleware to execute while trapping errors works. So if you had an error handling middleware that provides a well managed response to errors would work like this:

import {
} from "";

const app = new Application();

app.use(async (ctx, next) => {
  try {
    await next();
  } catch (err) {
    if (isHttpError(err)) {
      switch (err.status) {
        case Status.NotFound:
          // handle NotFound
        // handle other statuses
    } else {
      // rethrow if you can't handle the error
      throw err;

Uncaught middleware exceptions will be caught by the application. Application extends the global EventTarget in Deno, and when uncaught errors occur in the middleware or sending of responses, an EventError will be dispatched to the application. To listen for these errors, you would add an event handler to the application instance:

import { Application } from "";

const app = new Application();

app.addEventListener("error", (evt) => {
  // Will log the thrown error to the console.

app.use((ctx) => {
  // Will throw a 500 on every request.

await app.listen({ port: 80 });


The Router class produces middleware which can be used with an Application to enable routing based on the pathname of the request.

Basic usage

The following example serves up a RESTful service of a map of books, where http://localhost:8000/book/ will return an array of books and http://localhost:8000/book/1 would return the book with ID "1":

import { Application, Router } from "";

const books = new Map<string, any>();
books.set("1", {
  id: "1",
  title: "The Hound of the Baskervilles",
  author: "Conan Doyle, Arthur",

const router = new Router();
  .get("/", (context) => {
    context.response.body = "Hello world!";
  .get("/book", (context) => {
    context.response.body = Array.from(books.values());
  .get("/book/:id", (context) => {
    if (context.params && && books.has( {
      context.response.body = books.get(;

const app = new Application();

await app.listen({ port: 8000 });

A route passed is converted to a regular expression using path-to-regexp, which means parameters expressed in the pattern will be converted. path-to-regexp has advanced usage which can create complex patterns which can be used for matching. Check out the documentation for that library if you have advanced use cases.

Static content

The function send() is designed to serve static content as part of a middleware function. In the most straight forward usage, a root is provided and requests provided to the function are fulfilled with files from the local file system relative to the root from the requested path.

A basic usage would look something like this:

import { Application, send } from "";

const app = new Application();

app.use(async (context) => {
  await send(context, context.request.url.pathname, {
    root: `${Deno.cwd()}/examples/static`,
    index: "index.html",

await app.listen({ port: 8000 });


The mod.ts also exports a variable named helpers which contains functions that help with managing contexts.

getQuery(ctx, options?)

The helpers.getQuery() function is designed to make it easier to determine what a request might be querying in the middleware. It takes the supplied context’s .request.url.searchParams and converts it to a record object of the keys and values. For example, it would convert the following request:


Into an object like this:

  foo: "bar",
  baz: "qat"

The function can take a couple of options. The asMap will result in a Map being returned instead of an object. The mergeParams will merge in parameters that were parsed out of the route. This only works with router contexts, and any params will be overwritten by the request’s search params. If the following URL was requested:


And the following was the router middleware:

router.get("/book/:id/page/:page", (ctx) => {
  getQuery(ctx, { mergeParams: true });

Would result in the return value being:

  id: "1234",
  page: "32",
  size: "24"

There are several modules that are directly adapted from other modules. They have preserved their individual licenses and copyrights. All of the modules, including those directly adapted are licensed under the MIT License.

All additional work is copyright 2018 - 2020 the oak authors. All rights reserved.