Streams—The definitive guide

Learn how to use readable, writable, and transform streams with the Streams API.

Thomas Steiner

The Streams API allows you to programmatically access streams of data received over the network or created by whatever means locally and process them with JavaScript. Streaming involves breaking down a resource that you want to receive, send, or transform into small chunks, and then processing these chunks bit by bit. While streaming is something browsers do anyway when receiving assets like HTML or videos to be shown on webpages, this capability has never been available to JavaScript before fetch with streams was introduced in 2015.

Previously, if you wanted to process a resource of some kind (be it a video, or a text file, etc.), you would have to download the entire file, wait for it to be deserialized into a suitable format, and then process it. With streams being available to JavaScript, this all changes. You can now process raw data with JavaScript progressively as soon as it is available on the client, without needing to generate a buffer, string, or blob. This unlocks a number of use cases, some of which I list below:

• Video effects: piping a readable video stream through a transform stream that applies effects in real time.
• Data (de)compression: piping a file stream through a transform stream that selectively (de)compresses it.
• Image decoding: piping an HTTP response stream through a transform stream that decodes bytes into bitmap data, and then through another transform stream that translates bitmaps into PNGs. If installed inside the fetch handler of a service worker, this allows you to transparently polyfill new image formats like AVIF.

Browser support

ReadableStream and WritableStream

TransformStream

Core concepts

Before I go into details on the various types of streams, let me introduce some core concepts.

Chunks

A chunk is a single piece of data that is written to or read from a stream. It can be of any type; streams can even contain chunks of different types. Most of the time, a chunk will not be the most atomic unit of data for a given stream. For example, a byte stream might contain chunks consisting of 16 KiB Uint8Array units, instead of single bytes.

Readable streams

A readable stream represents a source of data from which you can read. In other words, data comes out of a readable stream. Concretely, a readable stream is an instance of the ReadableStream class.

Writable streams

A writable stream represents a destination for data into which you can write. In other words, data goes in to a writable stream. Concretely, a writable stream is an instance of the WritableStream class.

Transform streams

A transform stream consists of a pair of streams: a writable stream, known as its writable side, and a readable stream, known as its readable side. A real-world metaphor for this would be a simultaneous interpreter who translates from one language to another on-the-fly. In a manner specific to the transform stream, writing to the writable side results in new data being made available for reading from the readable side. Concretely, any object with a writable property and a readable property can serve as a transform stream. However, the standard TransformStream class makes it easier to create such a pair that is properly entangled.

Pipe chains

Streams are primarily used by piping them to each other. A readable stream can be piped directly to a writable stream, using the readable stream's pipeTo() method, or it can be piped through one or more transform streams first, using the readable stream's pipeThrough() method. A set of streams piped together in this way is referred to as a pipe chain.

Backpressure

Once a pipe chain is constructed, it will propagate signals regarding how fast chunks should flow through it. If any step in the chain cannot yet accept chunks, it propagates a signal backwards through the pipe chain, until eventually the original source is told to stop producing chunks so fast. This process of normalizing flow is called backpressure.

Teeing

A readable stream can be teed (named after the shape of an uppercase 'T') using its tee() method. This will lock the stream, that is, make it no longer directly usable; however, it will create two new streams, called branches, which can be consumed independently. Teeing also is important because streams cannot be rewound or restarted, more about this later.

The mechanics of a readable stream

A readable stream is a data source represented in JavaScript by a ReadableStream object that flows from an underlying source. The ReadableStream() constructor creates and returns a readable stream object from the given handlers. There are two types of underlying source:

• Push sources constantly push data at you when you have accessed them, and it is up to you to start, pause, or cancel access to the stream. Examples include live video streams, server-sent events, or WebSockets.
• Pull sources require you to explicitly request data from them once connected to. Examples include HTTP operations via fetch() or XMLHttpRequest calls.

Stream data is read sequentially in small pieces called chunks. The chunks placed in a stream are said to be enqueued. This means they are waiting in a queue ready to be read. An internal queue keeps track of the chunks that have not yet been read.

A queuing strategy is an object that determines how a stream should signal backpressure based on the state of its internal queue. The queuing strategy assigns a size to each chunk, and compares the total size of all chunks in the queue to a specified number, known as the high water mark.

The chunks inside the stream are read by a reader. This reader retrieves the data one chunk at a time, allowing you to do whatever kind of operation you want to do on it. The reader plus the other processing code that goes along with it is called a consumer.

The next construct in this context is called a controller. Each readable stream has an associated controller that, as the name suggests, allows you to control the stream.

Only one reader can read a stream at a time; when a reader is created and starts reading a stream (that is, becomes an active reader), it is locked to it. If you want another reader to take over reading your stream, you typically need to release the first reader before you do anything else (although you can tee streams).

Creating a readable stream

You create a readable stream by calling its constructor ReadableStream(). The constructor has an optional argument underlyingSource, which represents an object with methods and properties that define how the constructed stream instance will behave.

The underlyingSource

This can use the following optional, developer-defined methods:

• start(controller): Called immediately when the object is constructed. The method can access the stream source, and do anything else required to set up the stream functionality. If this process is to be done asynchronously, the method can return a promise to signal success or failure. The controller parameter passed to this method is a ReadableStreamDefaultController.
• pull(controller): Can be used to control the stream as more chunks are fetched. It is called repeatedly as long as the stream's internal queue of chunks is not full, up until the queue reaches its high water mark. If the result of calling pull() is a promise, pull() will not be called again until said promise fulfills. If the promise rejects, the stream will become errored.
• cancel(reason): Called when the stream consumer cancels the stream.

const readableStream = new ReadableStream({
  start(controller) {
    /* … */
  },

  pull(controller) {
    /* … */
  },

  cancel(reason) {
    /* … */
  },
});

The ReadableStreamDefaultController supports the following methods:

• ReadableStreamDefaultController.close() closes the associated stream.
• ReadableStreamDefaultController.enqueue() enqueues a given chunk in the associated stream.
• ReadableStreamDefaultController.error() causes any future interactions with the associated stream to error.

/* … */
start(controller) {
  controller.enqueue('The first chunk!');
},
/* … */

The queuingStrategy

The second, likewise optional, argument of the ReadableStream() constructor is queuingStrategy. It is an object that optionally defines a queuing strategy for the stream, which takes two parameters:

• highWaterMark: A non-negative number indicating the high water mark of the stream using this queuing strategy.
• size(chunk): A function that computes and returns the finite non-negative size of the given chunk value. The result is used to determine backpressure, manifesting via the appropriate ReadableStreamDefaultController.desiredSize property. It also governs when the underlying source's pull() method is called.

const readableStream = new ReadableStream({
    /* … */
  },
  {
    highWaterMark: 10,
    size(chunk) {
      return chunk.length;
    },
  },
);

The getReader() and read() methods

To read from a readable stream, you need a reader, which will be a ReadableStreamDefaultReader. The getReader() method of the ReadableStream interface 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.

The read() method of the ReadableStreamDefaultReader interface returns a promise providing access to the next chunk in the stream's internal queue. It fulfills or rejects with a result depending on the state of the stream. The different possibilities are as follows:

• If a chunk is available, the promise will be fulfilled with an object of the form
  { value: chunk, done: false }.
• If the stream becomes closed, the promise will be fulfilled with an object of the form
  { value: undefined, done: true }.
• If the stream becomes errored, the promise will be rejected with the relevant error.

const reader = readableStream.getReader();
while (true) {
  const { done, value } = await reader.read();
  if (done) {
    console.log('The stream is done.');
    break;
  }
  console.log('Just read a chunk:', value);
}

The locked property

You can check if a readable stream is locked by accessing its ReadableStream.locked property.

const locked = readableStream.locked;
console.log(`The stream is ${locked ? 'indeed' : 'not'} locked.`);

Readable stream code samples

The code sample below shows all the steps in action. You first create a ReadableStream that in its underlyingSource argument (that is, the TimestampSource class) defines a start() method. This method tells the stream's controller to enqueue() a timestamp every second during ten seconds. Finally, it tells the controller to close() the stream. You consume this stream by creating a reader via the getReader() method and calling read() until the stream is done.

class TimestampSource {
  #interval

  start(controller) {
    this.#interval = setInterval(() => {
      const string = new Date().toLocaleTimeString();
      // Add the string to the stream.
      controller.enqueue(string);
      console.log(`Enqueued ${string}`);
    }, 1_000);

    setTimeout(() => {
      clearInterval(this.#interval);
      // Close the stream after 10s.
      controller.close();
    }, 10_000);
  }

  cancel() {
    // This is called if the reader cancels.
    clearInterval(this.#interval);
  }
}

const stream = new ReadableStream(new TimestampSource());

async function concatStringStream(stream) {
  let result = '';
  const reader = stream.getReader();
  while (true) {
    // The `read()` method returns a promise that
    // resolves when a value has been received.
    const { done, value } = await reader.read();
    // Result objects contain two properties:
    // `done`  - `true` if the stream has already given you all its data.
    // `value` - Some data. Always `undefined` when `done` is `true`.
    if (done) return result;
    result += value;
    console.log(`Read ${result.length} characters so far`);
    console.log(`Most recently read chunk: ${value}`);
  }
}
concatStringStream(stream).then((result) => console.log('Stream complete', result));

Asynchronous iteration

Checking upon each read() loop iteration if the stream is done may not be the most convenient API. Luckily there will soon be a better way to do this: asynchronous iteration.

for await (const chunk of stream) {
  console.log(chunk);
}

A workaround to use asynchronous iteration today is to implement the behavior with a polyfill.

if (!ReadableStream.prototype[Symbol.asyncIterator]) {
  ReadableStream.prototype[Symbol.asyncIterator] = async function* () {
    const reader = this.getReader();
    try {
      while (true) {
        const {done, value} = await reader.read();
        if (done) {
          return;
          }
        yield value;
      }
    }
    finally {
      reader.releaseLock();
    }
  }
}

Teeing a readable stream

The tee() method of the ReadableStream interface tees the current readable stream, returning a two-element array containing the two resulting branches as new ReadableStream instances. This allows two readers to read a stream simultaneously. You might do this, for example, in a service worker if you want to fetch a response from the server and stream it to the browser, but also stream it to the service worker cache. Since a response body cannot be consumed more than once, you need two copies to do this. To cancel the stream, you then need to cancel both resulting branches. Teeing a stream will generally lock it for the duration, preventing other readers from locking it.

const readableStream = new ReadableStream({
  start(controller) {
    // Called by constructor.
    console.log('[start]');
    controller.enqueue('a');
    controller.enqueue('b');
    controller.enqueue('c');
  },
  pull(controller) {
    // Called `read()` when the controller's queue is empty.
    console.log('[pull]');
    controller.enqueue('d');
    controller.close();
  },
  cancel(reason) {
    // Called when the stream is canceled.
    console.log('[cancel]', reason);
  },
});

// Create two `ReadableStream`s.
const [streamA, streamB] = readableStream.tee();

// Read streamA iteratively one by one. Typically, you
// would not do it this way, but you certainly can.
const readerA = streamA.getReader();
console.log('[A]', await readerA.read()); //=> {value: "a", done: false}
console.log('[A]', await readerA.read()); //=> {value: "b", done: false}
console.log('[A]', await readerA.read()); //=> {value: "c", done: false}
console.log('[A]', await readerA.read()); //=> {value: "d", done: false}
console.log('[A]', await readerA.read()); //=> {value: undefined, done: true}

// Read streamB in a loop. This is the more common way
// to read data from the stream.
const readerB = streamB.getReader();
while (true) {
  const result = await readerB.read();
  if (result.done) break;
  console.log('[B]', result);
}

Readable byte streams

For streams representing bytes, an extended version of the readable stream is provided to handle bytes efficiently, in particular by minimizing copies. Byte streams allow for bring-your-own-buffer (BYOB) readers to be acquired. The default implementation can give a range of different outputs such as strings or array buffers in the case of WebSockets, whereas byte streams guarantee byte output. In addition, BYOB readers have stability benefits. This is because if a buffer detaches, it can guarantee that one does not write into the same buffer twice, hence avoiding race conditions. BYOB readers can reduce the number of times the browser needs to run garbage collection, because it can reuse buffers.

Creating a readable byte stream

You can create a readable byte stream by passing an additional type parameter to the ReadableStream() constructor.

new ReadableStream({ type: 'bytes' });

The underlyingSource

The underlying source of a readable byte stream is given a ReadableByteStreamController to manipulate. Its ReadableByteStreamController.enqueue() method takes a chunk argument whose value is an ArrayBufferView. The property ReadableByteStreamController.byobRequest returns the current BYOB pull request, or null if there is none. Finally, the ReadableByteStreamController.desiredSize property returns the desired size to fill the controlled stream's internal queue.

The queuingStrategy

The second, likewise optional, argument of the ReadableStream() constructor is queuingStrategy. It is an object that optionally defines a queuing strategy for the stream, which takes one parameter:

• highWaterMark: A non-negative number of bytes indicating the high water mark of the stream using this queuing strategy. This is used to determine backpressure, manifesting via the appropriate ReadableByteStreamController.desiredSize property. It also governs when the underlying source's pull() method is called.

The getReader() and read() methods

You can then get access to a ReadableStreamBYOBReader by setting the mode parameter accordingly: ReadableStream.getReader({ mode: "byob" }). This allows for more precise control over buffer allocation in order to avoid copies. To read from the byte stream, you need to call ReadableStreamBYOBReader.read(view), where view is an ArrayBufferView.

Readable byte stream code sample

const reader = readableStream.getReader({ mode: "byob" });

let startingAB = new ArrayBuffer(1_024);
const buffer = await readInto(startingAB);
console.log("The first 1024 bytes, or less:", buffer);

async function readInto(buffer) {
  let offset = 0;

  while (offset < buffer.byteLength) {
    const { value: view, done } =
        await reader.read(new Uint8Array(buffer, offset, buffer.byteLength - offset));
    buffer = view.buffer;
    if (done) {
      break;
    }
    offset += view.byteLength;
  }

  return buffer;
}

The following function returns readable byte streams that allow for efficient zero-copy reading of a randomly generated array. Instead of using a predetermined chunk size of 1,024, it attempts to fill the developer-supplied buffer, allowing for full control.

const DEFAULT_CHUNK_SIZE = 1_024;

function makeReadableByteStream() {
  return new ReadableStream({
    type: 'bytes',

    pull(controller) {
      // Even when the consumer is using the default reader,
      // the auto-allocation feature allocates a buffer and
      // passes it to us via `byobRequest`.
      const view = controller.byobRequest.view;
      view = crypto.getRandomValues(view);
      controller.byobRequest.respond(view.byteLength);
    },

    autoAllocateChunkSize: DEFAULT_CHUNK_SIZE,
  });
}

The mechanics of a writable stream

A writable stream is a destination into which you can write data, represented in JavaScript by a WritableStream object. This serves as an abstraction over the top of an underlying sink—a lower-level I/O sink into which raw data is written.

The data is written to the stream via a writer, one chunk at a time. A chunk can take a multitude of forms, just like the chunks in a reader. You can use whatever code you like to produce the chunks ready for writing; the writer plus the associated code is called a producer.

When a writer is created and starts writing to a stream (an active writer), it is said to be locked to it. Only one writer can write to a writable stream at one time. If you want another writer to start writing to your stream, you typically need to release it, before you then attach another writer to it.

An internal queue keeps track of the chunks that have been written to the stream but not yet been processed by the underlying sink.

A queuing strategy is an object that determines how a stream should signal backpressure based on the state of its internal queue. The queuing strategy assigns a size to each chunk, and compares the total size of all chunks in the queue to a specified number, known as the high water mark.

The final construct is called a controller. Each writable stream has an associated controller that allows you to control the stream (for example, to abort it).

Creating a writable stream

The WritableStream interface of the Streams API provides a standard abstraction for writing streaming data to a destination, known as a sink. This object comes with built-in backpressure and queuing. You create a writable stream by calling its constructor WritableStream(). It has an optional underlyingSink parameter, which represents an object with methods and properties that define how the constructed stream instance will behave.

The underlyingSink

The underlyingSink can include the following optional, developer-defined methods. The controller parameter passed to some of the methods is a WritableStreamDefaultController.

• start(controller): This method is called immediately when the object is constructed. The contents of this method should aim to get access to the underlying sink. If this process is to be done asynchronously, it can return a promise to signal success or failure.
• write(chunk, controller): This method will be called when a new chunk of data (specified in the chunk parameter) is ready to be written to the underlying sink. It can return a promise to signal success or failure of the write operation. This method will be called only after previous writes have succeeded, and never after the stream is closed or aborted.
• close(controller): This method will be called if the app signals that it has finished writing chunks to the stream. The contents should do whatever is necessary to finalize writes to the underlying sink, and release access to it. If this process is asynchronous, it can return a promise to signal success or failure. This method will be called only after all queued-up writes have succeeded.
• abort(reason): This method will be called if the app signals that it wishes to abruptly close the stream and put it in an errored state. It can clean up any held resources, much like close(), but abort() will be called even if writes are queued up. Those chunks will be thrown away. If this process is asynchronous, it can return a promise to signal success or failure. The reason parameter contains a DOMString describing why the stream was aborted.

const writableStream = new WritableStream({
  start(controller) {
    /* … */
  },

  write(chunk, controller) {
    /* … */
  },

  close(controller) {
    /* … */
  },

  abort(reason) {
    /* … */
  },
});

The WritableStreamDefaultController interface of the Streams API represents a controller allowing control of a WritableStream's state during set up, as more chunks are submitted for writing, or at the end of writing. When constructing a WritableStream, the underlying sink is given a corresponding WritableStreamDefaultController instance to manipulate. The WritableStreamDefaultController has only one method: WritableStreamDefaultController.error(), which causes any future interactions with the associated stream to error. WritableStreamDefaultController also supports a signal property which returns an instance of AbortSignal, allowing a WritableStream operation to be stopped if needed.

/* … */
write(chunk, controller) {
  try {
    // Try to do something dangerous with `chunk`.
  } catch (error) {
    controller.error(error.message);
  }
},
/* … */

The queuingStrategy

The second, likewise optional, argument of the WritableStream() constructor is queuingStrategy. It is an object that optionally defines a queuing strategy for the stream, which takes two parameters:

• highWaterMark: A non-negative number indicating the high water mark of the stream using this queuing strategy.
• size(chunk): A function that computes and returns the finite non-negative size of the given chunk value. The result is used to determine backpressure, manifesting via the appropriate WritableStreamDefaultWriter.desiredSize property.

The getWriter() and write() methods

To write to a writable stream, you need a writer, which will be a WritableStreamDefaultWriter. The getWriter() method of the WritableStream interface returns a new instance of WritableStreamDefaultWriter and locks the stream to that instance. While the stream is locked, no other writer can be acquired until the current one is released.

The write() method of the WritableStreamDefaultWriter interface writes a passed chunk of data to a WritableStream and its underlying sink, then returns a promise that resolves to indicate the success or failure of the write operation. Note that what "success" means is up to the underlying sink; it might indicate that the chunk has been accepted, and not necessarily that it is safely saved to its ultimate destination.

const writer = writableStream.getWriter();
const resultPromise = writer.write('The first chunk!');

The locked property

You can check if a writable stream is locked by accessing its WritableStream.locked property.

const locked = writableStream.locked;
console.log(`The stream is ${locked ? 'indeed' : 'not'} locked.`);

Writable stream code sample

The code sample below shows all steps in action.

const writableStream = new WritableStream({
  start(controller) {
    console.log('[start]');
  },
  async write(chunk, controller) {
    console.log('[write]', chunk);
    // Wait for next write.
    await new Promise((resolve) => setTimeout(() => {
      document.body.textContent += chunk;
      resolve();
    }, 1_000));
  },
  close(controller) {
    console.log('[close]');
  },
  abort(reason) {
    console.log('[abort]', reason);
  },
});

const writer = writableStream.getWriter();
const start = Date.now();
for (const char of 'abcdefghijklmnopqrstuvwxyz') {
  // Wait to add to the write queue.
  await writer.ready;
  console.log('[ready]', Date.now() - start, 'ms');
  // The Promise is resolved after the write finishes.
  writer.write(char);
}
await writer.close();

Piping a readable stream to a writable stream

A readable stream can be piped to a writable stream through the readable stream's pipeTo() method. ReadableStream.pipeTo() pipes the current ReadableStreamto a given WritableStream and returns a promise that fulfills when the piping process completes successfully, or rejects if any errors were encountered.

const readableStream = new ReadableStream({
  start(controller) {
    // Called by constructor.
    console.log('[start readable]');
    controller.enqueue('a');
    controller.enqueue('b');
    controller.enqueue('c');
  },
  pull(controller) {
    // Called when controller's queue is empty.
    console.log('[pull]');
    controller.enqueue('d');
    controller.close();
  },
  cancel(reason) {
    // Called when the stream is canceled.
    console.log('[cancel]', reason);
  },
});

const writableStream = new WritableStream({
  start(controller) {
    // Called by constructor
    console.log('[start writable]');
  },
  async write(chunk, controller) {
    // Called upon writer.write()
    console.log('[write]', chunk);
    // Wait for next write.
    await new Promise((resolve) => setTimeout(() => {
      document.body.textContent += chunk;
      resolve();
    }, 1_000));
  },
  close(controller) {
    console.log('[close]');
  },
  abort(reason) {
    console.log('[abort]', reason);
  },
});

await readableStream.pipeTo(writableStream);
console.log('[finished]');

Creating a transform stream

The TransformStream interface of the Streams API represents a set of transformable data. You create a transform stream by calling its constructor TransformStream(), which creates and returns a transform stream object from the given handlers. The TransformStream() constructor accepts as its first argument an optional JavaScript object representing the transformer. Such objects can contain any of the following methods:

The transformer

• start(controller): This method is called immediately when the object is constructed. Typically this is used to enqueue prefix chunks, using controller.enqueue(). Those chunks will be read from the readable side but do not depend on any writes to the writable side. If this initial process is asynchronous, for example because it takes some effort to acquire the prefix chunks, the function can return a promise to signal success or failure; a rejected promise will error the stream. Any thrown exceptions will be re-thrown by the TransformStream() constructor.
• transform(chunk, controller): This method is called when a new chunk originally written to the writable side is ready to be transformed. The stream implementation guarantees that this function will be called only after previous transforms have succeeded, and never before start() has completed or after flush() has been called. This function performs the actual transformation work of the transform stream. It can enqueue the results using controller.enqueue(). This permits a single chunk written to the writable side to result in zero or multiple chunks on the readable side, depending on how many times controller.enqueue() is called. If the process of transforming is asynchronous, this function can return a promise to signal success or failure of the transformation. A rejected promise will error both the readable and writable sides of the transform stream. If no transform() method is supplied, the identity transform is used, which enqueues chunks unchanged from the writable side to the readable side.
• flush(controller): This method is called after all chunks written to the writable side have been transformed by successfully passing through transform(), and the writable side is about to be closed. Typically this is used to enqueue suffix chunks to the readable side, before that too becomes closed. If the flushing process is asynchronous, the function can return a promise to signal success or failure; the result will be communicated to the caller of stream.writable.write(). Additionally, a rejected promise will error both the readable and writable sides of the stream. Throwing an exception is treated the same as returning a rejected promise.

const transformStream = new TransformStream({
  start(controller) {
    /* … */
  },

  transform(chunk, controller) {
    /* … */
  },

  flush(controller) {
    /* … */
  },
});

The writableStrategy and readableStrategy queueing strategies

The second and third optional parameters of the TransformStream() constructor are optional writableStrategy and readableStrategy queueing strategies. They are defined as outlined in the readable and the writable stream sections respectively.

Transform stream code sample

The following code sample shows a simple transform stream in action.

// Note that `TextEncoderStream` and `TextDecoderStream` exist now.
// This example shows how you would have done it before.
const textEncoderStream = new TransformStream({
  transform(chunk, controller) {
    console.log('[transform]', chunk);
    controller.enqueue(new TextEncoder().encode(chunk));
  },
  flush(controller) {
    console.log('[flush]');
    controller.terminate();
  },
});

(async () => {
  const readStream = textEncoderStream.readable;
  const writeStream = textEncoderStream.writable;

  const writer = writeStream.getWriter();
  for (const char of 'abc') {
    writer.write(char);
  }
  writer.close();

  const reader = readStream.getReader();
  for (let result = await reader.read(); !result.done; result = await reader.read()) {
    console.log('[value]', result.value);
  }
})();

Piping a readable stream through a transform stream

The pipeThrough() method of the ReadableStream interface provides a chainable way of piping the current stream through a transform stream or any other writable/readable pair. Piping a stream will generally lock it for the duration of the pipe, preventing other readers from locking it.

const transformStream = new TransformStream({
  transform(chunk, controller) {
    console.log('[transform]', chunk);
    controller.enqueue(new TextEncoder().encode(chunk));
  },
  flush(controller) {
    console.log('[flush]');
    controller.terminate();
  },
});

const readableStream = new ReadableStream({
  start(controller) {
    // called by constructor
    console.log('[start]');
    controller.enqueue('a');
    controller.enqueue('b');
    controller.enqueue('c');
  },
  pull(controller) {
    // called read when controller's queue is empty
    console.log('[pull]');
    controller.enqueue('d');
    controller.close(); // or controller.error();
  },
  cancel(reason) {
    // called when rs.cancel(reason)
    console.log('[cancel]', reason);
  },
});

(async () => {
  const reader = readableStream.pipeThrough(transformStream).getReader();
  for (let result = await reader.read(); !result.done; result = await reader.read()) {
    console.log('[value]', result.value);
  }
})();

The next code sample (a bit contrived) shows how you could implement a "shouting" version of fetch() that uppercases all text by consuming the returned response promise as a stream and uppercasing chunk by chunk. The advantage of this approach is that you do not need to wait for the whole document to be downloaded, which can make a huge difference when dealing with large files.

function upperCaseStream() {
  return new TransformStream({
    transform(chunk, controller) {
      controller.enqueue(chunk.toUpperCase());
    },
  });
}

function appendToDOMStream(el) {
  return new WritableStream({
    write(chunk) {
      el.append(chunk);
    }
  });
}

fetch('./lorem-ipsum.txt').then((response) =>
  response.body
    .pipeThrough(new TextDecoderStream())
    .pipeThrough(upperCaseStream())
    .pipeTo(appendToDOMStream(document.body))
);

Demo

The demo below shows readable, writable, and transform streams in action. It also includes examples of pipeThrough() and pipeTo() pipe chains, and also demonstrates tee(). You can optionally run the demo in its own window or view the source code.

Useful streams available in the browser

There are a number of useful streams built right into the browser. You can easily create a ReadableStream from a blob. The Blob interface's stream() method returns a ReadableStream which upon reading returns the data contained within the blob. Also recall that a File object is a specific kind of a Blob, and can be used in any context that a blob can.

const readableStream = new Blob(['hello world'], { type: 'text/plain' }).stream();

The streaming variants of TextDecoder.decode() and TextEncoder.encode() are called TextDecoderStream and TextEncoderStream respectively.

const response = await fetch('https://streams.spec.whatwg.org/');
const decodedStream = response.body.pipeThrough(new TextDecoderStream());

Compressing or decompressing a file is easy with the CompressionStream and DecompressionStream transform streams respectively. The code sample below shows how you can download the Streams spec, compress (gzip) it right in the browser, and write the compressed file directly to disk.

const response = await fetch('https://streams.spec.whatwg.org/');
const readableStream = response.body;
const compressedStream = readableStream.pipeThrough(new CompressionStream('gzip'));

const fileHandle = await showSaveFilePicker();
const writableStream = await fileHandle.createWritable();
compressedStream.pipeTo(writableStream);

The File System Access API's FileSystemWritableFileStream and the experimental fetch() request streams are examples of writable streams in the wild.

The Serial API makes heavy use of both readable and writable streams.

// Prompt user to select any serial port.
const port = await navigator.serial.requestPort();
// Wait for the serial port to open.
await port.open({ baudRate: 9_600 });
const reader = port.readable.getReader();

// Listen to data coming from the serial device.
while (true) {
  const { value, done } = await reader.read();
  if (done) {
    // Allow the serial port to be closed later.
    reader.releaseLock();
    break;
  }
  // value is a Uint8Array.
  console.log(value);
}

// Write to the serial port.
const writer = port.writable.getWriter();
const data = new Uint8Array([104, 101, 108, 108, 111]); // hello
await writer.write(data);
// Allow the serial port to be closed later.
writer.releaseLock();

Finally, the WebSocketStream API integrates streams with the WebSocket API.

const wss = new WebSocketStream(WSS_URL);
const { readable, writable } = await wss.connection;
const reader = readable.getReader();
const writer = writable.getWriter();

while (true) {
  const { value, done } = await reader.read();
  if (done) {
    break;
  }
  const result = await process(value);
  await writer.write(result);
}

Useful resources

• Streams specification
• Accompanying demos
• Streams polyfill
• 2016—the year of web streams
• Async iterators and generators
• Stream Visualizer

Acknowledgements

This article was reviewed by Jake Archibald, François Beaufort, Sam Dutton, Mattias Buelens, Surma, Joe Medley, and Adam Rice. Jake Archibald's blog posts have helped me a lot in understanding streams. Some of the code samples are inspired by GitHub user @bellbind's explorations and parts of the prose build heavily on the MDN Web Docs on Streams. The Streams Standard's authors have done a tremendous job on writing this spec. Hero image by Ryan Lara on Unsplash.