TransformStream
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.
* Some parts of this feature may have varying levels of support.
Note: This feature is available in Web Workers.
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.readableRead only-
The
readableend of aTransformStream. TransformStream.writableRead only-
The
writableend of aTransformStream.
Instance methods
None
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 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() });
}
}
Chaining multiple ReadableStreams together
This is a useful one, where multiple streams can be conjoined. Examples include building a PWA with progressive loading and progressive streaming.
let responses = [
/* conjoined response tree */
];
let { readable, writable } = new TransformStream();
responses.reduce(
(a, res, i, arr) =>
a.then(() => res.pipeTo(writable, { preventClose: i + 1 !== arr.length })),
Promise.resolve(),
);
Note that this is not resilient to other influences.
Specifications
| Specification |
|---|
| Streams # ts-class |
Browser compatibility
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