This is going to be a work in progress over a few weeks, in part because the JS implementation is rather bare-bones with few support functions, and in part because with the BigInt type, the JS designers did away with some automated and implicit casting (uh?).
With the BigInt type, some strictness is reintroduced, and adding a BigInt to a Number will result in a type error:
a = 1; b = 1n + a; // Type error ! b = 1n + 1; // Type error here as well b = 1n + BigInt(a); // Explicit cast required