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.

Constructor

ReadableStream()

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

Properties

ReadableStream.locked Read only

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

Methods

ReadableStream.cancel()

Returns a Promise that resolves when the stream is canceled. Calling this method signals 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.

Examples

Fetch stream

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 => response.body)
.then(rb => {
  const reader = rb.getReader();

  return 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}) => {
          // If there is no more data to read
          if (done) {
            console.log('done', done);
            controller.close();
            return;
          }
          // Get the data and send it to the browser via the controller
          controller.enqueue(value);
          // Check chunks by logging to the console
          console.log(done, value);
          push();
        })
      }

      push();
    }
  });
})
.then(stream => {
  // Respond with our stream
  return new Response(stream, { headers: { "Content-Type": "text/html" } }).text();
})
.then(result => {
  // Do things with result
  console.log(result);
});

Async iterator to stream

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.

Specifications

Browser compatibility

See also

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