--- title: "Web Platform APIs" description: "A guide to the Web Platform APIs available in Deno. Learn about fetch, events, workers, storage, and other web standard APIs, including implementation details and deviations from browser specifications." oldUrl: - /runtime/manual/runtime/navigator_api/ - /runtime/manual/runtime/web_platform_apis/ - /runtime/manual/runtime/location_api/ - /runtime/manual/runtime/web_storage_api/ - /runtime/manual/runtime/workers/ --- One way Deno simplifies web and cloud development is by using standard Web Platform APIs (like `fetch`, WebSockets and more) over proprietary APIs. This means if you've ever built for the browser, you're likely already familiar with Deno, and if you're learning Deno, you're also investing in your knowledge of the web. Explore supported Web APIs Below we'll highlight some of the standard Web APIs that Deno supports. To check if a Web Platform API is available in Deno, you can click on [the interface on MDN](https://developer.mozilla.org/en-US/docs/Web/API#interfaces) and refer to [its Browser Compatibility table](https://developer.mozilla.org/en-US/docs/Web/API/AbortController#browser_compatibility). ## fetch The [`fetch`](/api/web/~/fetch) API can be used to make HTTP requests. It is implemented as specified in the [WHATWG `fetch` spec](https://fetch.spec.whatwg.org/). ### 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](https://fetch.spec.whatwg.org/#scheme-fetch). 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](https://github.com/whatwg/fetch/pull/1346). ### Fetching local files 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 works both with 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`](#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 a resource, relative to the current module, which would work if the module is local or remote, you should to use `import.meta.url` as the base. For example: ```js 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 exist 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 and EventTarget The [DOM Event API](/api/web/~/Event) can be used to dispatch and listen to events happening in an application. It is implemented as specified in the [WHATWG DOM spec](https://dom.spec.whatwg.org/#events). ### 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. - `timeStamp` property is always set to `0`. ## Typings The TypeScript definitions for the implemented web APIs can be found in the [`lib.deno.shared_globals.d.ts`](https://github.com/denoland/deno/blob/main/cli/tsc/dts/lib.deno.shared_globals.d.ts) and [`lib.deno.window.d.ts`](https://github.com/denoland/deno/blob/main/cli/tsc/dts/lib.deno.window.d.ts) files. Definitions that are specific to workers can be found in the [`lib.deno.worker.d.ts`](https://github.com/denoland/deno/blob/main/cli/tsc/dts/lib.deno.worker.d.ts) file. ## Location Deno supports the [`location`](/api/web/~/Location) global from the web. ### Location flag There is no "web page" whose URL we can use for a location in a Deno process. We instead allow users to emulate a document location by specifying one on the CLI using the `--location` flag. It can be a `http` or `https` URL. ```ts // deno run --location https://example.com/path main.ts console.log(location.href); // "https://example.com/path" ``` You must pass `--location ` for this to work. If you don't, any access to the `location` global will throw an error. ```ts // deno run main.ts console.log(location.href); // error: Uncaught ReferenceError: Access to "location", run again with --location . ``` Setting `location` or any of its fields will normally cause navigation in browsers. This is not applicable in Deno, so it will throw in this situation. ```ts // deno run --location https://example.com/path main.ts location.pathname = "./foo"; // error: Uncaught NotSupportedError: Cannot set "location.pathname". ``` ### Extended usage On the web, resource resolution (excluding modules) typically uses the value of `location.href` as the root on which to base any relative URLs. This affects some web APIs adopted by Deno. #### Fetch API ```ts // deno run --location https://api.github.com/ --allow-net main.ts const response = await fetch("./orgs/denoland"); // Fetches "https://api.github.com/orgs/denoland". ``` The `fetch()` call above would throw if the `--location` flag was not passed, since there is no web-analogous location to base it onto. #### Worker modules ```ts // deno run --location https://example.com/index.html --allow-net main.ts const worker = new Worker("./workers/hello.ts", { type: "module" }); // Fetches worker module at "https://example.com/workers/hello.ts". ``` :::note For the above use cases, it is preferable to pass URLs in full rather than relying on `--location`. You can manually base a relative URL using the `URL` constructor if needed. ::: The `--location` flag is intended for those who have a specific purpose in mind for emulating a document location and are aware that this will only work at application-level. However, you may also use it to silence errors from a dependency which is frivolously accessing the `location` global. ## Web Storage The [Web Storage API](/api/web/storage) provides an API for storing string keys and values. Persisting data works similar to a browser, and has a 10MB storage limit. The global `sessionStorage` object only persists data for the current execution context, while `localStorage` persists data from execution to execution. In a browser, `localStorage` persists data uniquely per origin (effectively the protocol plus hostname plus port). As of Deno 1.16, Deno has a set of rules to determine what is a unique storage location: - When using the `--location` flag, the origin for the location is used to uniquely store the data. That means a location of `http://example.com/a.ts` and `http://example.com/b.ts` and `http://example.com:80/` would all share the same storage, but `https://example.com/` would be different. - If there is no location specifier, but there is a `--config` configuration file specified, the absolute path to that configuration file is used. That means `deno run --config deno.jsonc a.ts` and `deno run --config deno.jsonc b.ts` would share the same storage, but `deno run --config tsconfig.json a.ts` would be different. - If there is no configuration or location specifier, Deno uses the absolute path to the main module to determine what storage is shared. The Deno REPL generates a "synthetic" main module that is based off the current working directory where `deno` is started from. This means that multiple invocations of the REPL from the same path will share the persisted `localStorage` data. To set, get and remove items from `localStorage`, you can use the following: ```ts // Set an item in localStorage localStorage.setItem("myDemo", "Deno App"); // Read an item from localStorage const cat = localStorage.getItem("myDemo"); // Remove an item from localStorage localStorage.removeItem("myDemo"); // Remove all items from localStorage localStorage.clear(); ``` ## Web Workers Deno supports the [`Web Worker API`](/api/web/workers). Workers can be used to run code on multiple threads. Each instance of `Worker` is run on a separate thread, dedicated only to that worker. Currently Deno supports only `module` type workers; thus it's essential to pass the `type: "module"` option when creating a new worker. Use of relative module specifiers in the main worker are only supported with `--location ` passed on the CLI. This is not recommended for portability. You can instead use the `URL` constructor and `import.meta.url` to easily create a specifier for some nearby script. Dedicated workers, however, have a location and this capability by default. ```ts // Good new Worker(import.meta.resolve("./worker.js"), { type: "module" }); // Bad new Worker(import.meta.resolve("./worker.js")); new Worker(import.meta.resolve("./worker.js"), { type: "classic" }); new Worker("./worker.js", { type: "module" }); ``` As with regular modules, you can use top-level `await` in worker modules. However, you should be careful to always register the message handler before the first `await`, since messages can be lost otherwise. This is not a bug in Deno, it's just an unfortunate interaction of features, and it also happens in all browsers that support module workers. ```ts import { delay } from "jsr:@std/async@1/delay"; // First await: waits for a second, then continues running the module. await delay(1000); // The message handler is only set after that 1s delay, so some of the messages // that reached the worker during that second might have been fired when no // handler was registered. self.onmessage = (evt) => { console.log(evt.data); }; ``` ### Instantiation permissions Creating a new `Worker` instance is similar to a dynamic import; therefore Deno requires appropriate permission for this action. For workers using local modules; `--allow-read` permission is required: ```ts title="main.ts" new Worker(import.meta.resolve("./worker.ts"), { type: "module" }); ``` ```ts title="worker.ts" console.log("hello world"); self.close(); ``` ```shell $ deno run main.ts error: Uncaught PermissionDenied: read access to "./worker.ts", run again with the --allow-read flag $ deno run --allow-read main.ts hello world ``` For workers using remote modules; `--allow-net` permission is required: ```ts title="main.ts" new Worker("https://example.com/worker.ts", { type: "module" }); ``` ```ts title="worker.ts" // This file is hosted at https://example.com/worker.ts console.log("hello world"); self.close(); ``` ```shell $ deno run main.ts error: Uncaught PermissionDenied: net access to "https://example.com/worker.ts", run again with the --allow-net flag $ deno run --allow-net main.ts hello world ``` ### Using Deno in a worker ```js title="main.js" const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", }); worker.postMessage({ filename: "./log.txt" }); ``` ```js title="worker.js" self.onmessage = async (e) => { const { filename } = e.data; const text = await Deno.readTextFile(filename); console.log(text); self.close(); }; ``` ```text title="log.txt" hello world ``` ```shell $ deno run --allow-read main.js hello world ``` ### Specifying worker permissions :::caution This is an unstable Deno feature. Learn more about [unstable features](/runtime/fundamentals/stability_and_releases/#unstable-apis). ::: The permissions available for the worker are analogous to the CLI permission flags, meaning every permission enabled there can be disabled at the level of the Worker API. You can find a more detailed description of each of the permission options [here](/runtime/fundamentals/security/). By default a worker will inherit permissions from the thread it was created in, however in order to allow users to limit the access of this worker we provide the `deno.permissions` option in the worker API. For permissions that support granular access you can pass in a list of the desired resources the worker will have access to, and for those who only have the on/off option you can pass true/false respectively: ```ts const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", deno: { permissions: { net: [ "deno.land", ], read: [ new URL("./file_1.txt", import.meta.url), new URL("./file_2.txt", import.meta.url), ], write: false, }, }, }); ``` Granular access permissions receive both absolute and relative routes as arguments, however take into account that relative routes will be resolved relative to the file the worker is instantiated in, not the path the worker file is currently in: ```ts const worker = new Worker( new URL("./worker/worker.js", import.meta.url).href, { type: "module", deno: { permissions: { read: [ "/home/user/Documents/deno/worker/file_1.txt", "./worker/file_2.txt", ], }, }, }, ); ``` Both `deno.permissions` and its children support the option `"inherit"`, which implies it will borrow its parent permissions: ```ts // This worker will inherit its parent permissions const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", deno: { permissions: "inherit", }, }); ``` ```ts // This worker will inherit only the net permissions of its parent const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", deno: { permissions: { env: false, hrtime: false, net: "inherit", ffi: false, read: false, run: false, write: false, }, }, }); ``` Not specifying the `deno.permissions` option or one of its children will cause the worker to inherit by default: ```ts // This worker will inherit its parent permissions const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", }); ``` ```ts // This worker will inherit all the permissions of its parent BUT net const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", deno: { permissions: { net: false, }, }, }); ``` You can disable the permissions of the worker all together by passing `"none"` to the `deno.permissions` option: ```ts // This worker will not have any permissions enabled const worker = new Worker(import.meta.resolve("./worker.js"), { type: "module", deno: { permissions: "none", }, }); ``` ## Deviations of other APIs from spec ### Cache API Only the following APIs are implemented: - [CacheStorage::open()](https://developer.mozilla.org/en-US/docs/Web/API/CacheStorage/open) - [CacheStorage::has()](https://developer.mozilla.org/en-US/docs/Web/API/CacheStorage/has) - [CacheStorage::delete()](https://developer.mozilla.org/en-US/docs/Web/API/CacheStorage/delete) - [Cache::match()](https://developer.mozilla.org/en-US/docs/Web/API/Cache/match) - [Cache::put()](https://developer.mozilla.org/en-US/docs/Web/API/Cache/put) - [Cache::delete()](https://developer.mozilla.org/en-US/docs/Web/API/Cache/delete) A few things that are different compared to browsers: 1. You cannot pass relative paths to the APIs. The request can be an instance of Request or URL or a url string. 2. `match()` & `delete()` don't support query options yet.