TransformStream

Learn about TransformStream.

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The TransformStream interface of the Streams API represents a concrete implementation of the pipe chain transform stream concept.

It may be passed to the ReadableStream.pipeThrough() method in order to transform a stream of data from one format into another. For example, it might be used to decode (or encode) video frames, decompress data, or convert the stream from XML to JSON.

A transformation algorithm may be provided as an optional argument to the object constructor. If not supplied, data is not modified when piped through the stream.

TransformStream is a transferable object.

Constructor

TransformStream() Creates and returns a transform stream object, optionally specifying a transformation object and queuing strategies for the streams.

Instance properties

TransformStream.readable The readable end of a TransformStream.

TransformStream.writable The writable end of a TransformStream.

Examples

Anything-to-uint8array stream

In the following example, a transform stream passes through all chunks it receives as Uint8Array values.

const transformContent = {
start() {}, // required.
async transform(chunk, controller) {
chunk = await chunk;
switch (typeof chunk) {
case 'object':
// just say the stream is done
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()})
}
}

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