バイナリーデータの送受信

const req = new XMLHttpRequest();
req.open("GET", "/myfile.png", true);
req.responseType = "arraybuffer";
req.onload = (event) => {
  const arrayBuffer = req.response; // Note: not req.responseText
  if (arrayBuffer) {
    const byteArray = new Uint8Array(arrayBuffer);
    byteArray.forEach((element, index) => {
      // do something with each byte in the array
    });
  }
};
req.send(null);

const req = new XMLHttpRequest();
req.open("GET", "/myfile.png", true);
req.responseType = "blob";
req.onload = (event) => {
  const blob = req.response;
  // …
};
req.send();