ReadableStream on Grafana Labs

cancel(reason)

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

cancel(reason)

The cancel() method of the ReadableStream 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.

Return value

A promise that resolves with the value undefined when the stream is canceled.

Exceptions

Exception	Description
TypeError	Thrown the stream is locked to a reader.

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();
  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, any enqueued chunks are lost
      await reader.cancel('cancelling the stream');
      break;
    }
  }
}

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);
    },
  });
}

getReader()

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

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.

Arguments

The getReader method takes a single optional options argument, which is an object with the following properties:

Name	Type	Default value	Description
mode	string	`undefined`	A property that specifies the type of reader to create. It currently only supports the `undefined` value.

Return value

A ReadableStreamDefaultReader object instance.

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);
    },
  });
}

CountQueuingStrategy

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

CountQueuingStrategy

The CountQueuingStrategy interface of the Streams API represents a built-in queuing strategy that counts the number of chunks in the queue.

Properties

Name	Type	Description
highWaterMark	number	The maximum number of chunks that the queue can contain.

Example

import { CountQueuingStrategy, 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;

  // Using the CountQueuingStrategy to limit the number of chunks in the queue to 5
  const queuingStrategy = new CountQueuingStrategy({ highWaterMark: 5 });

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

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