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