This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

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.


var promise = writableStreamDefaultWriter.write(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.


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


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) => {
      .then(() => {
        return 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.
    .then(() => {
    .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) => {
      var buffer = new ArrayBuffer(2);
      var view = new Uint16Array(buffer);
      view[0] = chunk;
      var decoded = decoder.decode(view, { stream: true });
      var listItem = document.createElement('li');
      listItem.textContent = "Chunk decoded: " + decoded;
      result += decoded;
  close() {
    var listItem = document.createElement('li');
    listItem.textContent = "[MESSAGE RECEIVED] " + result;
  abort(err) {
    console.log("Sink error:", err);
}, queuingStrategy);

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

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


Specification Status Comment
The definition of 'write()' in that specification.
Living Standard Initial definition.

Browser compatibility

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