const v = 433791696840n;

// uses system endianness; usually little endian
const a = new Uint8Array(new BigUint64Array([ v ]).buffer);
console.log(a);
const b = new Uint8Array(new BigInt64Array([ -v ]).buffer);
console.log(b);

// endian could be set manually
const c = new Uint8Array(8);
const dv = new DataView(c.buffer);
dv.setBigUint64(0, v, true); // little endian
// dv.setBigUint64(0, v, false); // big endian
// dv.setBigUint64(0, v); // big endian
console.log(c);