const max = 2n ** (64n - 1n) - 1n;

function check64bit(number) {
  number > max
    ? console.log("Number doesn't fit in signed 64-bit integer!")
    : console.log(BigInt.asIntN(64, number));
}

check64bit(2n ** 64n);
// Expected output: "Number doesn't fit in signed 64-bit integer!"
check64bit(2n ** 32n);
// Expected output: 4294967296n