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 is a transferable object.

ReadableStream()

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

ReadableStream.locked Read only

Returns a boolean indicating whether or not the readable stream is locked to a reader.

ReadableStream.from() Experimental

Returns ReadableStream from a provided iterable or async iterable object, such as an array, a set, an async generator, and so on.

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.

ReadableStream implements the async iterable protocol. This enables asynchronous iteration over the chunks in a stream using the for await...of syntax:

const stream = new ReadableStream(getSomeSource());

for await (const chunk of stream) {
  // Do something with each 'chunk'
}

The async iterator consumes the stream until it runs out of data or otherwise terminates. The loop can also exit early due to a break, throw, or return statement.

While iterating, the stream is locked to prevent other consumers from acquiring a reader (attempting to iterate over a stream that is already locked will throw a TypeError). This lock is released when the loop exits.

By default, exiting the loop will also cancel the stream, so that it can no longer be used. To continue to use a stream after exiting the loop, pass { preventCancel: true } to the stream's values() method:

for await (const chunk of stream.values({ preventCancel: true })) {
  // Do something with 'chunk'
  break;
}
// Acquire a reader for the stream and continue reading ...

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

The from() static method can convert an iterator, such as an Array or Map, or an (async) iterator to a readable stream:

const myReadableStream = ReadableStream.from(iteratorOrAsyncIterator);

On browsers that don't support the from() method you can instead create your own custom readable stream to achieve the same result:

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

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

This example shows how you can process the fetch() response using a for await...of loop to iterate through the arriving chunks.

const response = await fetch("https://www.example.org");
let total = 0;

// Iterate response.body (a ReadableStream) asynchronously
for await (const chunk of response.body) {
  // Do something with each chunk
  // Here we just accumulate the size of the response.
  total += chunk.length;
}

// Do something with the total
console.log(total);

• Streams API concepts
• Using readable streams
• Using readable byte stream
• WHATWG Stream Visualizer, for a basic visualization of readable, writable, and transform streams.
• Web-streams-polyfill or sd-streams - polyfills