TransformStreamDefaultController
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since June 2022.
Note: This feature is available inWeb Workers.
TheTransformStreamDefaultController interface of theStreams API provides methods to manipulate the associatedReadableStream andWritableStream.
When constructing aTransformStream, theTransformStreamDefaultController is created. It therefore has no constructor. The way to get an instance ofTransformStreamDefaultController is via the callback methods ofTransformStream().
Instance properties
TransformStreamDefaultController.desiredSizeRead onlyReturns 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 asUint8Array values, using theerror() andenqueue() 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 |
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| Streams # ts-default-controller-class |