TransformStreamDefaultController

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().

Properties

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

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 ('function' === typeof chunk.valueOf && 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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