WritableStreamDefaultWriter.write()

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

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

Syntax

Promise<> = defaultWriter.write(chunk)

Parameters

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

Return value

A Promise that resolves to indicate the success or failure of the write operation.

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 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) => {
    defaultWriter.ready
      .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.
  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) => {
      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;
      list.appendChild(listItem);
      result += decoded;
      resolve();
    });
  },
  close() {
    var 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 Status Comment
Streams
The definition of 'write()' in that specification.
Living Standard Initial definition.

Browser Compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 58 (Yes) ? ? 45 ?
Feature Android Webview Chrome for Android Edge Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile
Basic support 58 58 (Yes) ? ? ? 45 ?

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, rexxars, abbycar, jpmedley
 Last updated by: chrisdavidmills,