web-streams-polyfill

Web Streams, based on the WHATWG spec reference implementation.

Links

• Official spec
• Reference implementation

Usage

This library comes in multiple variants:

• web-streams-polyfill: a ponyfill that provides the stream implementations without replacing any globals, targeting ES2015+ environments. Recommended for use in Node 6+ applications, or in web libraries supporting modern browsers.
• web-streams-polyfill/es5: a ponyfill targeting ES5+ environments. Recommended for use in legacy Node applications, or in web libraries supporting older browsers.
• web-streams-polyfill/polyfill: a polyfill that replaces the native stream implementations, targeting ES2015+ environments. Recommended for use in web apps supporting modern browsers through a <script> tag.
• web-streams-polyfill/polyfill/es5: a polyfill targeting ES5+ environments. Recommended for use in web apps supporting older browsers through a <script> tag.

Each variant also includes TypeScript type definitions, compatible with the DOM type definitions for streams included in TypeScript. These type definitions require TypeScript version 5.7 or higher.

In version 4, the list of variants was reworked to have more modern defaults and to reduce the download size of the package. See the migration guide for more information.

Usage as a polyfill:

<!-- option 1: hosted by unpkg CDN -->
<script src="https://unpkg.com/web-streams-polyfill/dist/polyfill.js"></script>
<!-- option 2: self hosted -->
<script src="/path/to/web-streams-polyfill/dist/polyfill.js"></script>
<script>
var readable = new ReadableStream();
</script>

Usage as a Node module:

var streams = require("web-streams-polyfill");
var readable = new streams.ReadableStream();

Usage as a ponyfill from within a ES2015 module:

import { ReadableStream } from "web-streams-polyfill";
const readable = new ReadableStream();

Usage as a polyfill from within an ES2015 module:

import "web-streams-polyfill/polyfill";
const readable = new ReadableStream();

[!WARNING] Compatibility with built-in streams

If your browser or runtime already supports Web Streams, loading the polyfill will unconditionally replace the global ReadableStream, WritableStream and TransformStream classes with the polyfill's versions. However, browser APIs like fetch() will still return stream objects using the built-in stream classes. This can lead to surprising results, for example Response.body will not be instanceof ReadableStream after the polyfill replaces the global ReadableStream class.

Consider using ReadableStream.from() to convert a built-in stream (e.g. from fetch()) to a polyfilled stream, or try loading the polyfill conditionally if you don't always need the polyfill.

See issue #20 for more details.

Conditional loading

Web Streams are widely supported across all modern browsers (including Chrome, Firefox and Safari) and server runtimes (including Node.js, Deno and Bun). Consider using feature detection to check if your platform's built-in streams implementation can fulfill your app's needs, and load the polyfill only when needed.

Here are a couple of examples to load the polyfill conditionally:

// Check for basic ReadableStream support
if (!globalThis.ReadableStream) {
  await import("web-streams-polyfill/polyfill");
}

// Check for basic TransformStream support
if (!globalThis.TransformStream) {
  await import("web-streams-polyfill/polyfill");
}

// Check for async iteration support
if (typeof globalThis.ReadableStream?.prototype[Symbol.asyncIterator] !== 'function') {
  await import("web-streams-polyfill/polyfill");
}

Compatibility

The polyfill and ponyfill variants work in any ES2015-compatible environment.

The polyfill/es5 and ponyfill/es5 variants work in any ES5-compatible environment that has a global Promise. If you need to support older browsers or Node versions that do not have a native Promise implementation (check the support table), you must first include a Promise polyfill (e.g. promise-polyfill).

Async iterable support for ReadableStream is available in all variants, but requires an ES2018-compatible environment or a polyfill for Symbol.asyncIterator.

WritableStreamDefaultController.signal is available in all variants, but requires a global AbortController constructor. If necessary, consider using a polyfill such as abortcontroller-polyfill.

Reading with a BYOB reader is available in all variants, but requires ArrayBuffer.prototype.transfer() or structuredClone() to exist in order to correctly transfer the given view's buffer. If not available, then the buffer won't be transferred during the read.

Tooling compatibility

This package uses subpath exports for its variants. As such, you need Node 12 or higher in order to import or require() such a variant.

When using TypeScript, make sure your moduleResolution is set to "node16", "nodenext" or "bundler".

Compliance

The polyfill implements version 080852c (3 Apr 2025) of the streams specification.

The polyfill is tested against the same web platform tests that are used by browsers to test their native implementations. The polyfill aims to pass all tests, although it allows some exceptions for practical reasons:

• The default (ES2015) variant passes all of the tests, except for the test for the prototype of ReadableStream's async iterator. Retrieving the correct %AsyncIteratorPrototype% requires using an async generator (async function* () {}), which is invalid syntax before ES2018. Instead, the polyfill creates its own version which is functionally equivalent to the real prototype.
• The ES5 variant passes the same tests as the ES2015 variant, except for various tests about specific characteristics of the constructors, properties and methods. These test failures do not affect the run-time behavior of the polyfill. For example:
  • The name property of down-leveled constructors is incorrect.
  • The length property of down-leveled constructors and methods with optional arguments is incorrect.
  • Not all properties and methods are correctly marked as non-enumerable.
  • Down-leveled class methods are not correctly marked as non-constructable.

Contributors

Thanks to these people for their work on the original polyfill:

• Diwank Singh Tomer (creatorrr)
• Anders Riutta (ariutta)