WritableStreamDefaultWriter: ready property

Baseline 2022

Newly available

Since June 2022, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

Note: This feature is available in Web Workers.

The ready read-only property of the WritableStreamDefaultWriter interface 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.



The following example shows two uses of the ready property. The first uses ready to ensure that the WritableStream is done writing and thus able to receive data before sending a binary chunk. The second also checks whether the WritableStream is done writing, but this time because the writing must be finished before the writer can be closed.

function sendMessage(message, writableStream) {
  // defaultWriter is of type WritableStreamDefaultWriter
  const defaultWriter = writableStream.getWriter();
  const encoder = new TextEncoder();
  const encoded = encoder.encode(message);
  encoded.forEach((chunk) => {
    // Make sure the stream and its writer are able to
    //   receive data.
      .then(() => defaultWriter.write(chunk))
      .then(() => {
        console.log("Chunk written to sink.");
      .catch((err) => {
        console.error(`Chunk error: ${err}`);
    // Call ready again to ensure that all chunks are written
    //   before closing the writer.
      .then(() => defaultWriter.close())
      .then(() => {
        console.log("All chunks written");
      .catch((err) => {
        console.error(`Stream error: ${err}`);


Streams Standard
# ref-for-default-writer-ready⑨

Browser compatibility

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