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()
.
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() {},
async transform(chunk, controller) {
chunk = await chunk;
switch (typeof chunk) {
case "object":
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);
} 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() {
},
};
class AnyToU8Stream extends TransformStream {
constructor() {
super({ ...transformContent, textencoder: new TextEncoder() });
}
}