fetch-readablestream

Compatibility layer for efficient streaming of binary data using WHATWG Streams

Why

This library provides a consistent, cross browser API for streaming a response from an HTTP server based on the WHATWG Streams specification. At the time of writing, Chrome is the only browser to nativley support returning a ReadableStream from its fetch implementation - all other browsers need to fall back to XMLHttpRequest.

FireFox does provide the ability to efficiently retrieve a byte-stream from a server; however only via it's XMLHttpRequest implementation (when using responsetype=moz-chunked-arraybuffer). Other browsers do not provide access to the underlying byte-stream and must therefore fall-back to concatenating the response string and then encoding it into it's UTF-8 byte representation using the TextEncoder API.

Nb: If you are happy using a node-style API (using callbacks and events) I would suggest taking a look at stream-http.

Installation

This package can be installed with npm:

$ npm install fetch-readablestream --save

Once installed you can import it directly:

import fetchStream from 'fetch-readablestream';

Or you can add a script tag pointing to the dist/fetch-readablestream.js bundle and use the fetchStream global:

<script src="./node_modules/fetch-readablestream/dist/fetch-readablestream.js"></script>
<script>
  window.fetchStream('...')
</script>

Usage

The fetchStream api provides a subset of the fetch API; in particular, the ability to get a ReadableStream back from the Response object which can be used to efficiently stream a chunked-transfer encoded response from the server.

function readAllChunks(readableStream) {
  const reader = readableStream.getReader();
  const chunks = [];

  function pump() {
    return reader.read().then(({ value, done }) => {
      if (done) {
        return chunks;
      }
      chunks.push(value);
      return pump();
    });
  }

  return pump();
}

fetchStream('/endpoint')
  .then(response => readAllChunks(response.body))
  .then(chunks => console.dir(chunks))

AbortController is supported in many environments, and allows you to abort ongoing requests. This is fully supported in any environment that supports both ReadableStreams & AbortController directly (e.g. Chrome 66+), and has basic support in most other environments, though you may need a polyfill in your own code to use it. To abort a request:

const controller = new AbortController();

fetchStream('/endpoint', {
  signal: controller.signal
}).then(() => {
  // ...
});

// To abort the ongoing request:
controller.abort();

Browser Compatibility

fetch-readablestream makes the following assumptions on the environment; legacy browsers will need to provide Polyfills for this functionality:

Feature	Browsers	Polyfill
ReadableStream	Firefox, Safari, IE11, PhantomJS	web-streams-polyfill
TextEncoder	Safari, IE11, PhantomJS	text-encoding
Promise, Symbol, Object.assign	IE11, PhantomJS	babel-polyfill

Contributing

Use npm run watch to fire up karma with live-reloading. Visit http://localhost:9876/ in a bunch of browsers to capture them - the test suite will run automatically and report any failures.