Type guarding against BigInt using typeof not working - but using instanceof works, even though that javascript code will not work

[arnorhs]

🔎 Search Terms

bigInt typeof, bigint instanceof

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about bigint

⏯ Playground Link

https://www.typescriptlang.org/play?target=99&ts=5.5.0-dev.20240417#code/FAMwrgdgxgLglgewgAgggQnA5gSQjAVQGc4IsAVATwAcBTAeRAAoA3AQwBsAuVMAWwBGtAE7IAPsky58ASh5EYw0lnG9BI1RDAcOyAN7BkR5AHoTyKnVRthwhAHdlyOEVQIYyKAlu1YAGmQBMA8YAAsXTwQAE1pkKIRaV3jke1C2DzgPDgQEAGtXDjhc2kNjOBBkJhgaWgQK9l0AXmbkAHIBbFIYVpl9UuNjYVoYMGEULR1+5ABfZFoOIliDAYGhkbHkBqnp4B3QSFhEcYxsPEISMjwFNmgGZgaeLXVRCSkzuWQFJTJNfiEX3g6PoDMwWGrWWwOJwRLw+fyBYKfUhQWJhGHRWLxRIQVoeezeXIBTLOVxsWBgTgcSgpOxkKblSoNZwQa63OqSU6yYErIxrUbjbQcbZzBZLKarYb8zacba7YDAIA

💻 Code

function noBigIntUsingTypeOf(val: number | BigInt): string | number | null {
    // Type narrowing is not correct, but this code does do what it looks like
    if (typeof val === 'bigint') {
        return null
    } else {
        return val
    }
}

function noBigIntUsingInstanceOf(val: number | BigInt): string | number | null {
    // Type narrowing is correct, but since this code doesn't work, it is actually wrong
    if (val instanceof BigInt) {
        return null
    } else {
        return val
    }
}

🙁 Actual behavior

typescript believes the typeof val === 'bigint' check does not narrow the type down, when in reality (JS) it does - and similarly for the instanceof case, typescript believes the type to be narrowed down, when reality (JS) it does not work at all

🙂 Expected behavior

typeof val === 'bigint' would narrow the type down to not include BigInt

Additional information about the issue

No response

[arnorhs]

im so stupid.. bigint is not BigInt