Conditionally-typed zero

const zeros = ([undefined, null, true, 5, 5n, 'txt', {}, []]).map(x => x ^ x)
console.log(zeros.every(z => z === 0)) // "false"

Explanation: The caret ^ is the XOR operator, XOR-ing a value with itself always returns zero, even in JS. But applying XOR to a BigInt (5n in this case) returns a zero of type BigInt.

typeof 0 == 'number' // true
typeof 0n == 'bigint' // true

So even though BigInt is a numeric-like value, it's not a Number.
The self-XOR is a shorthand for this:

typeof x?.valueOf() == 'bigint' ? 0n : 0

Except for the fact that Symbols will cause ^ to throw.

map replaces each element of an iterable (not in-place), every tests a predicate.

BigInt method that only works with Numbers

BigInt.asIntN(8, 257n) // 1n
BigInt.asIntN(8n, 257n) // throws an error

Explanation: The 1st parameter of that static method only accepts Number-typed arguments, more specifically, integer Numbers. That's because it specifies the exponent of the modulo divisor, which is expected to be small.

Links

More pitfalls/footguns/gotchas here: https://github.com/tc39/proposal-bigint#gotchas--exceptions
Spec here: https://tc39.es/ecma262/multipage/numbers-and-dates.html#sec-bigint-objects
Docs here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt