Web Platform APIs

Deno aims to use web platform APIs (like fetch) instead of inventing a new proprietary API where it makes sense. These APIs generally follow the specifications and should match the implementation in Chrome and Firefox. In some cases it makes sense to deviate from the spec slightly, because of the different security model Deno has.

Here is a list of web platform APIs Deno implements:

fetch API


The fetch API can be used to make HTTP requests. It is implemented as specified in the WHATWG fetch spec.

You can find documentation about this API on MDN.

Spec deviations

  • The Deno user agent does not have a cookie jar. As such, the set-cookie header on a response is not processed, or filtered from the visible response headers.
  • Deno does not follow the same-origin policy, because the Deno user agent currently does not have the concept of origins, and it does not have a cookie jar. This means Deno does not need to protect against leaking authenticated data cross origin. Because of this Deno does not implement the following sections of the WHATWG fetch specification:
    • Section 3.1. 'Origin' header.
    • Section 3.2. CORS protocol.
    • Section 3.5. CORB.
    • Section 3.6. 'Cross-Origin-Resource-Policy' header.
    • Atomic HTTP redirect handling.
    • The opaqueredirect response type.
  • A fetch with a redirect mode of manual will return a basic response rather than an opaqueredirect response.
  • The specification is vague on how file: URLs are to be handled. Firefox is the only mainstream browser that implements fetching file: URLs, and even then it doesn't work by default. As of Deno 1.16, Deno supports fetching local files. See the next section for details.
  • The request and response header guards are implemented, but unlike browsers do not have any constraints on which header names are allowed.
  • The referrer, referrerPolicy, mode, credentials, cache, integrity, keepalive, and window properties and their relevant behaviours in RequestInit are not implemented. The relevant fields are not present on the Request object.
  • Request body upload streaming is supported (on HTTP/1.1 and HTTP/2). Unlike the current fetch proposal, the implementation supports duplex streaming.
  • The set-cookie header is not concatenated when iterated over in the headers iterator. This behaviour is in the process of being specified.

Fetching local files

As of Deno 1.16, Deno supports fetching file: URLs. This makes it easier to write code that uses the same code path on a server as local, as well as easier to author code that work both under the Deno CLI and Deno Deploy.

Deno only supports absolute file URLs, this means that fetch("./some.json") will not work. It should be noted though that if --location is specified, relative URLs use the --location as the base, but a file: URL cannot be passed as the --location.

To be able to fetch some resource, relative to the current module, which would work if the module is local or remote, you would want to use import.meta.url as the base. For example, something like:

const response = await fetch(new URL("./config.json", import.meta.url));
const config = await response.json();

Notes on fetching local files:

  • Permissions are applied to reading resources, so an appropriate --allow-read permission is needed to be able to read a local file.
  • Fetching locally only supports the GET method, and will reject the promise with any other method.
  • A file that does not exists simply rejects the promise with a vague TypeError. This is to avoid the potential of fingerprinting attacks.
  • No headers are set on the response. Therefore it is up to the consumer to determine things like the content type or content length.
  • Response bodies are streamed from the Rust side, so large files are available in chunks, and can be cancelled.

CustomEvent, EventTarget and EventListener


The DOM Event API can be used to dispatch and listen to events happening in an application. It is implemented as specified in the WHATWG DOM spec.

You can find documentation about this API on MDN.

Spec deviations

  • Events do not bubble, because Deno does not have a DOM hierarchy, so there is no tree for Events to bubble/capture through.


The TypeScript definitions for the implemented web APIs can be found in the lib.deno.shared_globals.d.ts and lib.deno.window.d.ts files.

Definitions that are specific to workers can be found in the lib.deno.worker.d.ts file.