Node version v12.16.1
64-bit Windows 10 Pro v2004

The BigInt value BigInt(0xFFFFFFFFFFFFFFFF) should be 2^64 - 1 but it's evaluating as 2^64

//Example

const v1 = BigInt(0xFFFFFFFFFFFFFFFF); // 2^64 - 1
const v2 = BigInt(0x10000000000000000); // 2^64

console.log(v1 === v2);
console.log(v1,v2);

// Output

true
18446744073709551616n 18446744073709551616n