WritableStreamDefaultWriter: close() method

Baseline 2022

Newly available

Since June 2022, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

Note: This feature is available in Web Workers.

The close() method of the WritableStreamDefaultWriter interface closes the associated writable stream.

The underlying sink will finish processing any previously-written chunks, before invoking the close behavior. During this time any further attempts to write will fail (without erroring the stream).

Syntax

close()

Return value

A Promise, which fulfills with the undefined if all remaining chunks were successfully written before the close, or rejects with an error if a problem was encountered during the process.

Exceptions

TypeError: The stream you are trying to close is not a WritableStream.

The following example shows the creation of a 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. Each chunk of the encoded string is written to the stream using the write() method, and the forEach() method of the encoded Uint8Array to process it byte-by-byte. Finally, close() is called and the Promise it returns is handled to deal with success (or any failures) of the chunked write operations.

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 view a live demonstration of this at our simple writer example.

Specifications

Specification
Streams Standard # ref-for-default-writer-close⑦

Browser compatibility

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