TransformStreamDefaultController

Note: This feature is available in Web Workers.

The TransformStreamDefaultController interface of the Streams API provides methods to manipulate the associated ReadableStream and WritableStream.

When constructing a TransformStream, the TransformStreamDefaultController is created. It therefore has no constructor. The way to get an instance of TransformStreamDefaultController is via the callback methods of TransformStream().

Instance properties

TransformStreamDefaultController.desiredSize Read only: Returns the desired size to fill the readable side of the stream's internal queue.

Instance methods

TransformStreamDefaultController.enqueue(): Enqueues a chunk (single piece of data) in the readable side of the stream.
TransformStreamDefaultController.error(): Errors both the readable and writable side of the transform stream.
TransformStreamDefaultController.terminate(): Closes the readable side and errors the writable side of the stream.

Examples

In the following example, a transform stream passes through all chunks it receives as Uint8Array values, using the error() and enqueue() methods.

const transformContent = {
  start() {}, // required.
  async transform(chunk, controller) {
    chunk = await chunk;
    switch (typeof chunk) {
      case "object":
        // just say the stream is done I guess
        if (chunk === null) {
          controller.terminate();
        } else if (ArrayBuffer.isView(chunk)) {
          controller.enqueue(
            new Uint8Array(chunk.buffer, chunk.byteOffset, chunk.byteLength),
          );
        } else if (
          Array.isArray(chunk) &&
          chunk.every((value) => typeof value === "number")
        ) {
          controller.enqueue(new Uint8Array(chunk));
        } else if (
          typeof chunk.valueOf === "function" &&
          chunk.valueOf() !== chunk
        ) {
          this.transform(chunk.valueOf(), controller); // hack
        } else if ("toJSON" in chunk) {
          this.transform(JSON.stringify(chunk), controller);
        }
        break;
      case "symbol":
        controller.error("Cannot send a symbol as a chunk part");
        break;
      case "undefined":
        controller.error("Cannot send undefined as a chunk part");
        break;
      default:
        controller.enqueue(this.textencoder.encode(String(chunk)));
        break;
    }
  },
  flush() {
    /* do any destructor work here */
  },
};

class AnyToU8Stream extends TransformStream {
  constructor() {
    super({ ...transformContent, textencoder: new TextEncoder() });
  }
}

Specifications

Specification
Streams Standard # ts-default-controller-class

Browser compatibility

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