Streams

Streams are an important abstraction for working with binary data without loading it all into memory at once. They are commonly used for reading and writing files, sending and receiving network requests, and processing large amounts of data.

Bun implements the Web APIs ReadableStream and WritableStream.

To create a ReadableStream:

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("hello");
    controller.enqueue("world");
    controller.close();
  },
});

You can read the contents of a ReadableStream chunk-by-chunk with for await syntax.

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

// hello
// world

Direct ReadableStream#

Bun implements an optimized version of ReadableStream that avoids unnecessary queue management.

With a traditional ReadableStream, you enqueue chunks of data. The stream adds each chunk to a queue, where it sits until the stream is ready to send more data.

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue("hello");
    controller.enqueue("world");
    controller.close();
  },
});

With a direct ReadableStream, you write chunks of data directly to the stream. No queueing happens. The controller API reflects this: you call .write() instead of .enqueue().

const stream = new ReadableStream({
  type: "direct", 
  pull(controller) {
    controller.write("hello");
    controller.write("world");
  },
});

When using a direct ReadableStream, the destination handles all chunk queueing. The destination receives the bytes you pass to controller.write(). When the stream is read from JavaScript, Bun buffers the writes and delivers them as Uint8Array chunks (strings are UTF-8 encoded).

Handling backpressure#

controller.write() returns the number of bytes written, or a pending Promise<number> when the destination's internal buffer is full (for example, a slow HTTP client). The chunk is accepted either way. The promise resolves once the destination has drained, so awaiting the result is enough:

const stream = new ReadableStream({
  type: "direct",
  async pull(controller) {
    for (const chunk of chunks) {
      await controller.write(chunk);
    }
    controller.close();
  },
});

await controller.flush(true) is equivalent, and you can use it after a write returns a Promise.

For default (non-direct) ReadableStreams and async-generator response bodies, Bun applies this backpressure automatically: it pauses the producer while the destination is backed up.

Async generator streams#

Bun also supports async generator functions as a source for Response and Request. Use async generators to create a ReadableStream that fetches data from an asynchronous source.

const response = new Response(
  (async function* () {
    yield "hello";
    yield "world";
  })(),
);

await response.text(); // "helloworld"

You can also use [Symbol.asyncIterator] directly.

const response = new Response({
  [Symbol.asyncIterator]: async function* () {
    yield "hello";
    yield "world";
  },
});

await response.text(); // "helloworld"

For more control over the stream, yield returns the direct ReadableStream controller.

const response = new Response({
  [Symbol.asyncIterator]: async function* () {
    const controller = yield "hello";
    await controller.end();
  },
});

await response.text(); // "hello"

Bun.ArrayBufferSink#

The Bun.ArrayBufferSink class is a fast incremental writer for constructing an ArrayBuffer of unknown size.

const sink = new Bun.ArrayBufferSink();

sink.write("h");
sink.write("e");
sink.write("l");
sink.write("l");
sink.write("o");

sink.end();
// ArrayBuffer(5) [ 104, 101, 108, 108, 111 ]

To instead retrieve the data as a Uint8Array, pass the asUint8Array option to the start method.

const sink = new Bun.ArrayBufferSink();
sink.start({
  asUint8Array: true, 
});

sink.write("h");
sink.write("e");
sink.write("l");
sink.write("l");
sink.write("o");

sink.end();
// Uint8Array(5) [ 104, 101, 108, 108, 111 ]

The .write() method supports strings, typed arrays, ArrayBuffer, and SharedArrayBuffer.

sink.write("h");
sink.write(new Uint8Array([101, 108]));
sink.write(Buffer.from("lo").buffer);

sink.end();

Once you call .end(), you can't write any more data to the ArrayBufferSink. However, when buffering a stream you may want to keep writing data and periodically .flush() the contents (say, into a WritableStream). To support this, pass stream: true to the start method.

const sink = new Bun.ArrayBufferSink();
sink.start({
  stream: true, 
});

sink.write("h");
sink.write("e");
sink.write("l");
sink.flush();
// ArrayBuffer(3) [ 104, 101, 108 ]

sink.write("l");
sink.write("o");
sink.flush();
// ArrayBuffer(2) [ 108, 111 ]

The .flush() method returns the buffered data as an ArrayBuffer (or Uint8Array if asUint8Array: true) and clears the internal buffer.

To manually set the size of the internal buffer in bytes, pass a value for highWaterMark:

const sink = new Bun.ArrayBufferSink();
sink.start({
  highWaterMark: 1024 * 1024, // 1 MB
});

Reference#

/**
 * Fast incremental writer that becomes an `ArrayBuffer` on end().
 */
export class ArrayBufferSink {
  constructor();

  start(options?: {
    asUint8Array?: boolean;
    /**
     * Preallocate an internal buffer of this size
     * This can significantly improve performance when the chunk size is small
     */
    highWaterMark?: number;
    /**
     * On {@link ArrayBufferSink.flush}, return the written data as a `Uint8Array`.
     * Writes will restart from the beginning of the buffer.
     */
    stream?: boolean;
  }): void;

  write(chunk: string | ArrayBufferView | ArrayBuffer | SharedArrayBuffer): number;
  /**
   * Flush the internal buffer
   *
   * If {@link ArrayBufferSink.start} was passed a `stream` option, this will return a `ArrayBuffer`
   * If {@link ArrayBufferSink.start} was passed a `stream` option and `asUint8Array`, this will return a `Uint8Array`
   * Otherwise, this will return the number of bytes written since the last flush
   *
   * This API might change later to separate Uint8ArraySink and ArrayBufferSink
   */
  flush(): number | Uint8Array<ArrayBuffer> | ArrayBuffer;
  end(): ArrayBuffer | Uint8Array<ArrayBuffer>;
}