TransformStream
===============

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` Read only 

The `readable` end of a `TransformStream`.

`TransformStream.writable` Read only 

The `writable` end of a `TransformStream`.

Instance methods
----------------

None

Examples
--------

### Anything-to-uint8array stream

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


```js
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() });
  }
}
```

### Polyfilling TextEncoderStream and TextDecoderStream

Note that this is deprecated by the native constructors. This is intended as a polyfill for unsupported platforms.


```js
const tes = {
  start() {
    this.encoder = new TextEncoder();
  },
  transform(chunk, controller) {
    controller.enqueue(this.encoder.encode(chunk));
  },
};

let _jstes_wm = new WeakMap(); /\* info holder \*/
class JSTextEncoderStream extends TransformStream {
  constructor() {
    let t = { ...tes };

    super(t);
    _jstes_wm.set(this, t);
  }
  get encoding() {
    return _jstes_wm.get(this).encoder.encoding;
  }
}
```


Similarly, `TextDecoderStream` can be written as such:


```js
const tds = {
  start() {
    this.decoder = new TextDecoder(this.encoding, this.options);
  },
  transform(chunk, controller) {
    controller.enqueue(this.decoder.decode(chunk, { stream: true }));
  },
};

let _jstds_wm = new WeakMap(); /\* info holder \*/
class JSTextDecoderStream extends TransformStream {
  constructor(encoding = "utf-8", { ...options } = {}) {
    let t = { ...tds, encoding, options };

    super(t);
    _jstds_wm.set(this, t);
  }
  get encoding() {
    return _jstds_wm.get(this).decoder.encoding;
  }
  get fatal() {
    return _jstds_wm.get(this).decoder.fatal;
  }
  get ignoreBOM() {
    return _jstds_wm.get(this).decoder.ignoreBOM;
  }
}
```

### 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.


```js
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 Standard # ts-class |

Browser compatibility
---------------------

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See also
--------

* WHATWG Stream Visualizer, for a basic visualization of readable, writable, and transform streams.
* Streams—The Definitive Guide