XMLHttpRequest: getResponseHeader() メソッド

getResponseHeader(headerName)

const client = new XMLHttpRequest();
client.open("GET", "unicorns-are-awesome.txt", true);
client.send();
client.onreadystatechange = () => {
  if (client.readyState === client.HEADERS_RECEIVED) {
    const contentType = client.getResponseHeader("Content-Type");
    if (contentType !== my_expected_type) {
      client.abort();
    }
  }
};