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115 lines
7.2 KiB
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115 lines
7.2 KiB
Markdown
# web-streams-polyfill
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Web Streams, based on the WHATWG spec reference implementation.
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[](https://travis-ci.com/MattiasBuelens/web-streams-polyfill)
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[](https://www.npmjs.com/package/web-streams-polyfill)
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[](https://github.com/MattiasBuelens/web-streams-polyfill/blob/master/LICENSE)
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[](https://gitter.im/web-streams-polyfill/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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## Links
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- [Official spec][spec]
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- [Reference implementation][ref-impl]
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## Usage
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This library comes in multiple variants:
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* `web-streams-polyfill`: a polyfill that replaces the native stream implementations.
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Recommended for use in web apps supporting older browsers through a `<script>` tag.
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* `web-streams-polyfill/es6`: a polyfill targeting ES2015+ environments.
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Recommended for use in web apps supporting modern browsers through a `<script>` tag.
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* `web-streams-polyfill/es2018`: a polyfill targeting ES2018+ environments.
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* `web-streams-polyfill/ponyfill`: a [ponyfill] that provides
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the stream implementations without replacing any globals.
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Recommended for use in legacy Node applications, or in web libraries supporting older browsers.
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* `web-streams-polyfill/ponyfill/es6`: a ponyfill targeting ES2015+ environments.
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Recommended for use in Node 6+ applications, or in web libraries supporting modern browsers.
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* `web-streams-polyfill/ponyfill/es2018`: a ponyfill targeting ES2018+ environments.
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Recommended for use in Node 10+ applications.
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Each variant also includes TypeScript type definitions, compatible with the DOM type definitions for streams included in TypeScript.
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Usage as a polyfill:
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```html
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<!-- option 1: hosted by unpkg CDN -->
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<script src="https://unpkg.com/web-streams-polyfill/dist/polyfill.min.js"></script>
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<!-- option 2: self hosted -->
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<script src="/path/to/web-streams-polyfill/dist/polyfill.min.js"></script>
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<script>
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var readable = new ReadableStream();
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</script>
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```
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Usage as a Node module:
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```js
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var streams = require("web-streams-polyfill/ponyfill");
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var readable = new streams.ReadableStream();
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```
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Usage as a ES2015 module:
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```js
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import { ReadableStream } from "web-streams-polyfill/ponyfill";
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const readable = new ReadableStream();
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```
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### Compatibility
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The `polyfill` and `ponyfill` variants work in any ES5-compatible environment that has a global `Promise`.
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If you need to support older browsers or Node versions that do not have a native `Promise` implementation
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(check the [support table][promise-support]), you must first include a `Promise` polyfill
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(e.g. [promise-polyfill][promise-polyfill]).
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The `polyfill/es6` and `ponyfill/es6` variants work in any ES2015-compatible environment.
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The `polyfill/es2018` and `ponyfill/es2018` variants work in any ES2018-compatible environment.
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[Async iterable support for `ReadableStream`][rs-asynciterator] is available in all variants, but requires an ES2018-compatible environment or a polyfill for `Symbol.asyncIterator`.
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[`WritableStreamDefaultController.signal`][ws-controller-signal] is available in all variants, but requires a global `AbortController` constructor. If necessary, consider using a polyfill such as [abortcontroller-polyfill].
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### Compliance
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The polyfill implements [version `4b6b93c` (25 Oct 2021)][spec-snapshot] of the streams specification.
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The polyfill is tested against the same [web platform tests][wpt] that are used by browsers to test their native implementations.
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The polyfill aims to pass all tests, although it allows some exceptions for practical reasons:
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* The `es2018` variant passes all of the tests, except for the ["bad buffers and views" tests for readable byte streams][wpt-bad-buffers].
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These tests require the implementation to synchronously transfer the contents of an `ArrayBuffer`, which is not yet possible from JavaScript (although there is a [proposal][proposal-arraybuffer-transfer] to make it possible).
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The reference implementation "cheats" on these tests [by making a copy instead][ref-impl-transferarraybuffer], but that is unacceptable for the polyfill's performance ([#3][issue-3]).
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* The `es6` variant passes the same tests as the `es2018` variant, except for the [test for the prototype of `ReadableStream`'s async iterator][wpt-async-iterator-prototype].
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Retrieving the correct `%AsyncIteratorPrototype%` requires using an async generator (`async function* () {}`), which is invalid syntax before ES2018.
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Instead, the polyfill [creates its own version][stub-async-iterator-prototype] which is functionally equivalent to the real prototype.
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* The `es5` variant passes the same tests as the `es6` variant, except for various tests about specific characteristics of the constructors, properties and methods.
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These test failures do not affect the run-time behavior of the polyfill.
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For example:
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* The `name` property of down-leveled constructors is incorrect.
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* The `length` property of down-leveled constructors and methods with optional arguments is incorrect.
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* Not all properties and methods are correctly marked as non-enumerable.
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* Down-leveled class methods are not correctly marked as non-constructable.
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The type definitions are compatible with the built-in stream types of TypeScript 3.3.
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### Contributors
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Thanks to these people for their work on [the original polyfill][creatorrr-polyfill]:
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- Diwank Singh Tomer ([creatorrr](https://github.com/creatorrr))
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- Anders Riutta ([ariutta](https://github.com/ariutta))
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[spec]: https://streams.spec.whatwg.org
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[ref-impl]: https://github.com/whatwg/streams
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[ponyfill]: https://github.com/sindresorhus/ponyfill
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[promise-support]: https://kangax.github.io/compat-table/es6/#test-Promise
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[promise-polyfill]: https://www.npmjs.com/package/promise-polyfill
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[rs-asynciterator]: https://streams.spec.whatwg.org/#rs-asynciterator
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[ws-controller-signal]: https://streams.spec.whatwg.org/#ws-default-controller-signal
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[abortcontroller-polyfill]: https://www.npmjs.com/package/abortcontroller-polyfill
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[spec-snapshot]: https://streams.spec.whatwg.org/commit-snapshots/4b6b93c69e531e2fe45a6ed4cb1484a7ba4eb8bb/
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[wpt]: https://github.com/web-platform-tests/wpt/tree/96ca25f0f7526282c0d47e6bf6a7edd439da1968/streams
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[wpt-bad-buffers]: https://github.com/web-platform-tests/wpt/blob/96ca25f0f7526282c0d47e6bf6a7edd439da1968/streams/readable-byte-streams/bad-buffers-and-views.any.js
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[proposal-arraybuffer-transfer]: https://github.com/domenic/proposal-arraybuffer-transfer
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[ref-impl-transferarraybuffer]: https://github.com/whatwg/streams/blob/4b6b93c69e531e2fe45a6ed4cb1484a7ba4eb8bb/reference-implementation/lib/abstract-ops/ecmascript.js#L16
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[issue-3]: https://github.com/MattiasBuelens/web-streams-polyfill/issues/3
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[wpt-async-iterator-prototype]: https://github.com/web-platform-tests/wpt/blob/96ca25f0f7526282c0d47e6bf6a7edd439da1968/streams/readable-streams/async-iterator.any.js#L24
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[stub-async-iterator-prototype]: https://github.com/MattiasBuelens/web-streams-polyfill/blob/v2.0.0/src/target/es5/stub/async-iterator-prototype.ts
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[creatorrr-polyfill]: https://github.com/creatorrr/web-streams-polyfill
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