ReadableStreamDefaultReader

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨January 2020⁩.

* Some parts of this feature may have varying levels of support.

Note: This feature is available inWeb Workers.

TheReadableStreamDefaultReader interface of theStreams API represents a default reader that can be used to read stream data supplied from a network (such as a fetch request).

AReadableStreamDefaultReader can be used to read from aReadableStream that has an underlying source of any type (unlike aReadableStreamBYOBReader, which can only be used with readable streams that have anunderlying byte source).

Note however that zero-copy transfer from an underlying source is only supported for underlying byte sources that autoallocate buffers.In other words, the stream must have beenconstructed specifying bothtype="bytes" andautoAllocateChunkSize.For any other underlying source, the stream will always satisfy read requests with data from internal queues.

Constructor

ReadableStreamDefaultReader(): Creates and returns aReadableStreamDefaultReader object instance.

Instance properties

ReadableStreamDefaultReader.closedRead only: Returns aPromise that fulfills when the stream closes, or rejects if the stream throws an error or the reader's lock is released. This property enables you to write code that responds to an end to the streaming process.

Instance methods

ReadableStreamDefaultReader.cancel(): Returns aPromise that resolves when the stream is canceled. Calling this method signals a loss of interest in the stream by a consumer. The suppliedreason argument will be given to the underlying source, which may or may not use it.
ReadableStreamDefaultReader.read(): Returns a promise providing access to the next chunk in the stream's internal queue.
ReadableStreamDefaultReader.releaseLock(): Releases the reader's lock on the stream.

Examples

In the following example, an artificialResponse is created to stream HTML fragments fetched from another resource to the browser.

It demonstrates the usage of aReadableStream in combination with aUint8Array.

fetch("https://www.example.org/").then((response) => {  const reader = response.body.getReader();  const stream = new ReadableStream({    start(controller) {      // The following function handles each data chunk      function push() {        // "done" is a Boolean and value a "Uint8Array"        return reader.read().then(({ done, value }) => {          // Is there no more data to read?          if (done) {            // Tell the browser that we have finished sending data            controller.close();            return;          }          // Get the data and send it to the browser via the controller          controller.enqueue(value);          push();        });      }      push();    },  });  return new Response(stream, { headers: { "Content-Type": "text/html" } });});