ReadableStream: pipeThrough() method

transformStream

A TransformStream (or an object with the structure {writable, readable}) consisting of a readable stream and a writable stream working together to transform some data from one form to another. Data written to the writable stream can be read in some transformed state by the readable stream. For example, a TextDecoder, has bytes written to it and strings read from it, while a video decoder has encoded bytes written to it and uncompressed video frames read from it.

options Optional

The options that should be used when piping to the writable stream. Available options are:

preventClose

If this is set to true, the source ReadableStream closing will no longer cause the destination WritableStream to be closed. The method will return a fulfilled promise once this process completes, unless an error is encountered while closing the destination, in which case it will be rejected with that error.

preventAbort

If this is set to true, errors in the source ReadableStream will no longer abort the destination WritableStream. The method will return a promise rejected with the source's error, or with any error that occurs during aborting the destination.

preventCancel

If this is set to true, errors in the destination WritableStream will no longer cancel the source ReadableStream. In this case the method will return a promise rejected with the source's error, or with any error that occurs during canceling the source. In addition, if the destination writable stream starts out closed or closing, the source readable stream will no longer be canceled. In this case the method will return a promise rejected with an error indicating piping to a closed stream failed, or with any error that occurs during canceling the source.

signal

If set to an AbortSignal object, ongoing pipe operations can then be aborted via the corresponding AbortController.

The readable side of the transformStream.

TypeError

Thrown if the writable and/or readable property of transformStream are undefined.

In the following example (see Unpack chunks of a PNG for the full code running live, and png-transform-stream for the source code), an image is fetched and its body retrieved as a ReadableStream.

Next, we log the contents of the readable stream, use pipeThrough() to send it to a new function that creates a gray-scaled version of the stream, then log the new stream's contents too.

// Fetch the original image
fetch("png-logo.png")
  // Retrieve its body as ReadableStream
  .then((response) => response.body)
  .then((rs) => logReadableStream("Fetch Response Stream", rs))
  // Create a gray-scaled PNG stream out of the original
  .then((body) => body.pipeThrough(new PNGTransformStream()))
  .then((rs) => logReadableStream("PNG Chunk Stream", rs));

• ReadableStream() constructor
• Pipe chains