NetworkInformation: свойство downlinkMax

function logConnectionType() {
  let connectionType = "not supported";
  let downlinkMax = "not supported";

  if ("connection" in navigator) {
    connectionType = navigator.connection.effectiveType;

    if ("downlinkMax" in navigator.connection) {
      downlinkMax = navigator.connection.downlinkMax;
    }
  }

  console.log(
    `Current connection type: ${connectionType} (downlink max: ${downlinkMax})`,
  );
}

logConnectionType();
navigator.connection.addEventListener("change", logConnectionType);