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 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.desiredSizeRead 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 # ts-default-controller-class |
Browser compatibility
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