ReadableStream

The ReadableStream interface of the Streams API represents a readable stream of byte data. The Fetch API offers a concrete instance of a ReadableStream through the body property of a Response object.

ReadableStream()

Creates and returns a readable stream object from the given handlers.

ReadableStream.locked Read only

The locked getter returns whether or not the readable stream is locked to a reader.

ReadableStream.cancel()

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.

ReadableStream.getReader()

Creates a reader and locks the stream to it. While the stream is locked, no other reader can be acquired until this one is released.

ReadableStream.pipeThrough()

Provides a chainable way of piping the current stream through a transform stream or any other writable/readable pair.

ReadableStream.pipeTo()

Pipes the current ReadableStream to a given WritableStream and returns a promise that fulfills when the piping process completes successfully, or rejects if any errors were encountered.

ReadableStream.tee()

The tee method tees this readable stream, returning a two-element array containing the two resulting branches as new ReadableStream instances. Each of those streams receives the same incoming data.

In the following example, an artificial 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"
        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" } });
});

Converting an (async) iterator to a readable stream:

function iteratorToStream(iterator) {
  return new ReadableStream({
    async pull(controller) {
      const { value, done } = await iterator.next();

      if (done) {
        controller.close();
      } else {
        controller.enqueue(value);
      }
    },
  });
}

This works with both async and non-async iterators.

• WHATWG Stream Visualiser, for a basic visualisation of readable, writable, and transform streams.