Socket.sendBinary(data)

import ws from 'k6/ws';

const binFile = open('./file.pdf', 'b');

export default function () {
  ws.connect('http://wshost/', function (socket) {
    socket.on('open', function () {
      socket.sendBinary(binFile);
    });

    socket.on('binaryMessage', function (msg) {
      // msg is an ArrayBuffer, so we can wrap it in a typed array directly.
      new Uint8Array(msg);
    });
  });
}