WritableStreamDefaultWriter.ready

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