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WritableStreamDefaultWriter: write() method

The write() method of the WritableStreamDefaultWriter interface writes a passed chunk of data to a WritableStream and its underlying sink, then returns a Promise that resolves to indicate the success or failure of the write operation.

Note that what "success" means is up to the underlying sink; it might indicate that the chunk has been accepted, and not necessarily that it is safely saved to its ultimate destination.

Syntax

write(chunk)

Parameters

chunk: A block of binary data to pass to the WritableStream.

Return value

A Promise, which fulfills with the undefined upon a successful write, or rejects if the write fails or stream becomes errored before the writing process is initiated.

Exceptions

TypeError: The target stream is not a writable stream, or it does not have an owner.

Examples

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. 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, { stream: true });
  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.

Specifications

Specification
Streams Standard # ref-for-default-writer-write①②

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`write`	59	16	100	No	46	14.1	59	59	100	43	14.5	7.0

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https://developer.mozilla.org/en-US/docs/Web/API/WritableStreamDefaultWriter/write