Web/API/WritableStreamDefaultWriter/WritableStreamDefaultWriter

Draft This page is not complete.

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

The WritableStreamDefaultWriter() constructor creates a new WritableStreamDefaultWriter object instance.

Note: You generally wouldn't use this constructor manually; instead, you'd use the WritableStream.getWriter() method.

Syntax

var writableStreamDefaultWriter = new WritableStreamDefaultWriter(stream);

Parameters

stream: The WritableStream to be written to.

Return value

An instance of the WritableStreamDefaultWriter object.

Exceptions

TypeError: The provided stream value is not a WritableStream, or it is locked to another writer already.

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(() => {
        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 [[../../../../../../../mdn.github.io/dom-examples/streams/simple-writer/index|Simple writer example]].

Specifications

Specification	Status	Comment
StreamsThe definition of 'WritableStreamDefaultWriter()' in that specification.	Living Standard	Initial definition.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`WritableStreamDefaultWriter()` constructor Experimental'	Chrome Full support 59	Edge Full support 16	Firefox No support No	IE No support No	Opera Full support 46	Safari ?	WebView Android Full support 59	Chrome Android Full support 59	Firefox Android No support No	Opera Android Full support 43	Safari iOS ?	Samsung Internet Android Full support 7.0

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
Experimental. Expect behavior to change in the future.': Experimental. Expect behavior to change in the future.

WritableStreamDefaultWriter.WritableStreamDefaultWriter() by Mozilla Contributors is licensed under CC-BY-SA 2.5.