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This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The ReadableStreamDefaultReader interface of the Streams API represents a default reader that can be used to read stream data supplied from a network (e.g. a fetch request). 


Creates and returns a ReadableStreamDefaultReader() object instance.


Allows you to write code that responds to an end to the streaming process. Returns a promise that fulfills if the stream becomes closed or the reader's lock is released, or rejects if the stream errors.


Cancels the stream, signaling a loss of interest in the stream by a consumer. The supplied reason argument will be given to the underlying source, which may or may not use it.
Returns a promise providing access to the next chunk in the stream's internal queue.
Releases the reader's lock on the stream.


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

It demonstrates the usage of a ReadableStream in combination with a Uint8Array.

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

          // Get the data and send it to the browser via the controller

  return new Response(stream, { headers: { "Content-Type": "text/html" } });


Specification Status Comment
The definition of 'ReadableStreamDefaultReader' in that specification.
Living Standard Initial definition

Browser Compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support


57.0 (57.0) ?


Feature Android Webview Chrome for Android Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile
Basic support 52 52 ? ? ? 39 ?

Document Tags and Contributors

Contributors to this page: chrisdavidmills
Last updated by: chrisdavidmills,