WritableStream: WritableStream() constructor

underlyingSink Optional

An object containing methods and properties that define how the constructed stream instance will behave. The controller parameter passed to this object's methods is a WritableStreamDefaultController that provides abort and error signaling. underlyingSink can contain the following:

start(controller) Optional

This is a method, called immediately when the object is constructed. The contents of this method are defined by the developer, and should aim to get access to the underlying sink. If this process is to be done asynchronously, it can return a promise to signal success or failure.

write(chunk, controller) Optional

This method, also defined by the developer, will be called when a new chunk of data (specified in the chunk parameter) is ready to be written to the underlying sink. It can return a promise to signal success or failure of the write operation. This method will be called only after previous writes have succeeded, and never after the stream is closed or aborted (see below).

close(controller) Optional

This method, also defined by the developer, will be called if the app signals that it has finished writing chunks to the stream. The contents should do whatever is necessary to finalize writes to the underlying sink, and release access to it. If this process is asynchronous, it can return a promise to signal success or failure. This method will be called only after all queued-up writes have succeeded.

abort(reason) Optional

This method, also defined by the developer, will be called if the app signals that it wishes to abruptly close the stream and put it in an errored state. It can clean up any held resources, much like close(), but abort() will be called even if writes are queued up — those chunks will be thrown away. If this process is asynchronous, it can return a promise to signal success or failure. The reason parameter contains a string describing why the stream was aborted.

queuingStrategy Optional

An object that optionally defines a queuing strategy for the stream. This takes two parameters:

highWaterMark

A non-negative integer — this defines the total number of chunks that can be contained in the internal queue before backpressure is applied.

size(chunk)

A method containing a parameter chunk — this indicates the size to use for each chunk, in bytes.

Note: You could define your own custom queuingStrategy, or use an instance of ByteLengthQueuingStrategy or CountQueuingStrategy for this object value. If no queuingStrategy is supplied, the default used is the same as a CountQueuingStrategy with a high water mark of 1.

An instance of the WritableStream object.

The following example illustrates several features of this interface. It shows the creation of the WritableStream with a custom sink and an API-supplied queuing strategy. It then calls a function called sendMessage(), passing the newly created stream and a string. Inside this function it calls the stream's getWriter() method, which returns an instance of WritableStreamDefaultWriter. A forEach() call is used to write each chunk of the string to the stream. Finally, write() and close() return promises that are processed to deal with success or failure of chunks and streams.

const list = document.querySelector("ul");

function sendMessage(message, writableStream) {
  // defaultWriter is of type WritableStreamDefaultWriter
  const defaultWriter = writableStream.getWriter();
  const encoder = new TextEncoder();
  const encoded = encoder.encode(message);
  encoded.forEach((chunk) => {
    defaultWriter.ready
      .then(() => defaultWriter.write(chunk))
      .then(() => {
        console.log("Chunk written to sink.");
      })
      .catch((err) => {
        console.log("Chunk error:", err);
      });
  });
  // Call ready again to ensure that all chunks are written
  //   before closing the writer.
  defaultWriter.ready
    .then(() => {
      defaultWriter.close();
    })
    .then(() => {
      console.log("All chunks written");
    })
    .catch((err) => {
      console.log("Stream error:", err);
    });
}

const decoder = new TextDecoder("utf-8");
const queuingStrategy = new CountQueuingStrategy({ highWaterMark: 1 });
let result = "";
const writableStream = new WritableStream(
  {
    // Implement the sink
    write(chunk) {
      return new Promise((resolve, reject) => {
        const buffer = new ArrayBuffer(1);
        const view = new Uint8Array(buffer);
        view[0] = chunk;
        const decoded = decoder.decode(view, { stream: true });
        const listItem = document.createElement("li");
        listItem.textContent = `Chunk decoded: ${decoded}`;
        list.appendChild(listItem);
        result += decoded;
        resolve();
      });
    },
    close() {
      const listItem = document.createElement("li");
      listItem.textContent = `[MESSAGE RECEIVED] ${result}`;
      list.appendChild(listItem);
    },
    abort(err) {
      console.log("Sink error:", err);
    },
  },
  queuingStrategy,
);

sendMessage("Hello, world.", writableStream);

You can find the full code in our Simple writer example.

Because of how backpressure is supported in the API, its implementation in code may be less than obvious. To see how backpressure is implemented look for three things.

• The highWaterMark property, which is set when creating the counting strategy using new CountQueuingStrategy, sets the maximum amount of data that the WritableStream instance will handle in a single write() operation. In this example, it's the maximum amount of data that can be sent to defaultWriter.write(), in the sendMessage function.
• The defaultWriter.ready property returns a promise that resolves when the sink (the first property of the WritableStream constructor) is done writing data. The data source can either write more data using defaultWriter.write() or call defaultWriter.close(), as demonstrated in the example above. Calling close() too early can prevent data from being written. This is why the example calls defaultWriter.ready twice.
• The Promise returned by the sink's write() method tells the WritableStream and its writer when to resolve defaultWriter.ready.