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

The WritableStreamDefaultWriter interface of the the Streams API is the object returned by WritableStream.getWriter() and once created locks the < writer to the WritableStream ensuring that no other streams can write to the underlying sink.


Creates a new WritableStreamDefaultWriter object instance.


WritableStreamDefaultWriter.closedRead only
Allows you to write code that responds to an end to the streaming process. Returns a promise that fulfills if the stream becomes closed or the writer's lock is released, or rejects if the stream errors.
WritableStreamDefaultWriter.desiredSizeRead only
Returns the desired size required to fill the stream's internal queue.
WritableStreamDefaultWriter.readyRead only
Returns a Promise that resolves when the desired size of the stream's internal queue transitions from non-positive to positive, signaling that it is no longer applying backpressure.


Aborts the stream, signaling that the producer can no longer successfully write to the stream and it is to be immediately moved to an error state, with any queued writes discarded.
Closes the associated writable stream.
Releases the writer's lock on the corresponding stream. After the lock is released, the writer is no longer active. If the associated stream is errored when the lock is released, the writer will appear errored in the same way from now on; otherwise, the writer will appear closed.
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.


The following example shows the creation of a WritableStream with a custom sink and an API-supplied queueing 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 'WritableStreamDefaultWriter' in that specification.
Living Standard Initial definition.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support5916 No No46 ?
WritableStreamDefaultWriter() constructor.5916 No No46 ?
closed5916 No No46 ?
desiredSize5916 No No46 ?
ready5916 No No46 ?
abort5916 No No46 ?
close5916 No No46 ?
releaseLock5916 No No46 ?
write5916 No No46 ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support595916 No46 ? ?
WritableStreamDefaultWriter() constructor.595916 No46 ? ?
closed595916 No46 ? ?
desiredSize595916 No46 ? ?
ready595916 No46 ? ?
abort595916 No46 ? ?
close595916 No46 ? ?
releaseLock595916 No46 ? ?
write595916 No46 ? ?

Document Tags and Contributors

Contributors to this page: connorshea, chrisdavidmills, ksm2, jpmedley
Last updated by: connorshea,