ReadableStreamDefaultReader on Grafana Labs

cancel(reason)

Mon, 21 Sep 2026 15:16:28 +0000

cancel(reason)

The cancel() method of the ReadableStreamDefaultReader interface returns a Promise that resolves when the stream is canceled.

Cancel is used when you’ve completely finished with the stream and don’t need any more data from it, even if chunks are enqueued waiting to be read. That data is lost after cancel is called, and the stream is not readable any more. To close the stream without getting rid of enqueued chunks, use ReadableStreamDefaultController.close().

Arguments

Name	Type	Description
reason	any	An optional human-readable value that represents the reason for canceling the stream.

Returns

A promise that resolves with the provided reason when the stream is canceled.

Exceptions

Exception	Description
TypeError	Thrown when the source object is not a ReadableStreamDefaultReader or the stream has no owner.

Example

import { ReadableStream } from 'k6/experimental/streams';
import { setTimeout } from 'k6/timers';

export default async function () {
  // Define a number stream that emits numbers from 1 to 10 every second
  const stream = numbersStream();

  // We use the getReader method to create a reader and lock the stream to it
  const reader = stream.getReader();

  while (true) {
    const { done, value } = await reader.read();
    if (done) break;
    if (value === 8) {
      // Cancel the stream when the number is 8
      await reader.cancel('cancelling the stream');
      break;
    }

    console.log(`received number ${value} from stream`);
  }
}

function numbersStream() {
  let currentNumber = 0;

  return new ReadableStream({
    start(controller) {
      const fn = () => {
        if (currentNumber < 10) {
          controller.enqueue(++currentNumber);
          setTimeout(fn, 1000);
          return;
        }

        controller.close();
      };
      setTimeout(fn, 1000);
    },
  });
}

read()

Mon, 21 Sep 2026 15:16:28 +0000

read()

The read() method of the ReadableStreamDefaultReader interface returns a promise providing access to the next chunk in the stream’s internal queue.

Returns

A promise which fullfils or rejects with a value depending on the state of the stream:

If a chunk is available, the promise resolves with an object of the form: { done: false, value: chunkValue }.
If the stream is closed and no more data is available, the promise resolves with an object of the form: { done: true, value: undefined }.
If the stream is errored, the promise rejects with the error that caused the stream to error.

Exceptions

Exception	Description
TypeError	Thrown when the source object is not a ReadableStreamDefaultReader, the stream has no owner, or ReadableStreamDefaultReader.releaseLock() is called.

Example

import { ReadableStream } from 'k6/experimental/streams';
import { setTimeout } from 'k6/timers';

export default async function () {
  // Define a number stream that emits numbers from 1 to 10 every second
  const stream = numbersStream();

  // We use the getReader method to create a reader and lock the stream to it
  const reader = stream.getReader();

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

    console.log(`received number ${value} from stream`);
  }
}

function numbersStream() {
  let currentNumber = 0;

  return new ReadableStream({
    start(controller) {
      const fn = () => {
        if (currentNumber < 10) {
          controller.enqueue(++currentNumber);
          setTimeout(fn, 1000);
          return;
        }

        controller.close();
      };
      setTimeout(fn, 1000);
    },
  });
}

releaseLock()

Mon, 21 Sep 2026 15:16:28 +0000

releaseLock()

The releaseLock() method of the ReadableStreamDefaultReader interface releases the reader’s lock on the stream.

If the associated stream is errored as the lock is released, the reader will be errored as well. This method is useful when you’re done with the stream and want to release the lock on it.

If the reader’s lock is released as pending read operations are still in progress, the reader’s ReadableStreamDefaultReader.read() calls are immediately rejected with a TypeError.

Exceptions

Exception	Description
TypeError	Thrown when the source object is not a ReadableStreamDefaultReader

Example

import { ReadableStream } from 'k6/experimental/streams';
import { setTimeout } from 'k6/timers';

export default async function () {
  // Define a number stream that emits numbers from 1 to 10 every second
  const stream = numbersStream();

  // We use the getReader method to create a reader and lock the stream to it
  const reader = stream.getReader();

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

  // Release the lock on the stream, so we're free to obtain another reader
  // and read from the stream again
  reader.releaseLock();
}

function numbersStream() {
  let currentNumber = 0;

  return new ReadableStream({
    start(controller) {
      const fn = () => {
        if (currentNumber < 10) {
          controller.enqueue(++currentNumber);
          setTimeout(fn, 1000);
          return;
        }

        controller.close();
      };
      setTimeout(fn, 1000);
    },
  });
}