TheReadableStream interface of theStreams API represents a readable stream of byte data. TheFetch API offers a concrete instance of aReadableStream through thebody property of aResponse object.

ReadableStream is atransferable object.

Constructor

ReadableStream()

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

Instance properties

ReadableStream.lockedRead only

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

Static methods

ReadableStream.from()Experimental

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

Instance methods

ReadableStream.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.

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 givenWritableStream and returns aPromise that fulfills when the piping process completes successfully, or rejects if any errors were encountered.

ReadableStream.tee()

Thetee methodtees this readable stream, returning a two-element array containing the two resulting branches as newReadableStream instances. Each of those streams receives the same incoming data.

Async iteration

ReadableStream implements theasync iterable protocol.This enables asynchronous iteration over the chunks in a stream using thefor 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 abreak,throw, orreturn 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 aTypeError).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'svalues() method:

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

Examples

Fetch stream

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) => 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);  });

Convert an iterator or async iterator to a stream

Thefrom() static method can convert an iterator, such as anArray orMap, or an(async) iterator to a readable stream:

const myReadableStream = ReadableStream.from(iteratorOrAsyncIterator);

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

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

Async iteration of a stream using for await...of

This example shows how you can process thefetch() response using afor 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) asynchronouslyfor 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 totalconsole.log(total);

Specifications

Browser compatibility

See also

• Streams API concepts
• Using readable streams
• Using readable byte stream
• Web-streams-polyfill