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web-streams-adapter

Adapters for converting between different implementations of WHATWG Streams.

Why?

When you've got a ReadableStream from a native web API, you might be disappointed to find out that not all browser support the latest and greatest features from the streams spec yet:

const response = await fetch('http://example.com/data.txt');
const readable = response.body;
const writable = new WritableStream({ write(chunk) { console.log(chunk) } });
await readable.pipeTo(writable); // TypeError: Object doesn't support property or method 'pipeTo'

This is because although many browsers have already started implementing streams, most of them are not yet fully up-to-date with the latest specification:

  • Chrome 59 supports ReadableStream and WritableStream, and TransformStream will arrive in Chrome 67. Readable byte streams are not yet supported.
  • Firefox 57 supports ReadableStream, but only behind an experimental flag.
  • Edge 16 supports ReadableStream, but does not yet support developer-created streams using new ReadableStream. WritableStream however is fully supported.
  • Safari supports ReadableStream, but no readable byte streams or writable streams.

For up-to-date information, check caniuse.com and the browser compatibility tables on MDN for ReadableStream and WritableStream.

What?

web-streams-adapter provides adapter functions that take any readable/writable/transform stream and wraps it into a different readable/writable/stream with a different (more complete) implementation of your choice, for example web-streams-polyfill.

// setup
import { ReadableStream as PolyfillReadableStream } from 'web-streams-polyfill';
import { createReadableStreamWrapper } from '@mattiasbuelens/web-streams-adapter';
const toPolyfillReadable = createReadableStreamWrapper(PolyfillReadableStream);

// when handling a fetch response
const response = await fetch('http://example.com/data.txt');
const readable = toPolyfillReadable(response.body);
console.log(readable instanceof PolyfillReadableStream); // -> true
await readable.pipeTo(writable); // works!

You can also use an adapter to convert from your polyfilled stream back to a native stream:

// setup
const toNativeReadable = createReadableStreamWrapper(self.ReadableStream);

// when starting a fetch with a streaming POST body
const readable = new PolyfillReadableStream({ /* amazingness */ });
const response = await fetch(url, {
  method: 'POST',
  body: toNativeReadable(readable) // works!
});

How?

For readable streams, web-streams-adapter creates an underlying source that pulls from the given readable stream using the primitive reader API. This source can then be used by any other readable stream implementation, both native and polyfilled ones.

For writable and transform streams, it uses a very similar approach to create an underlying sink or transformer using primitive reader and writer APIs on the given stream.