Ecma262: Why would BigInt not be kept as it in ToInteger?

[axross]

As I read the specs, if you convert a BigInt value to some "integer" value with ToInteger, it would not be a BigInt, cause a TypeError instead.

Only definitions about ToNumber and ToNumeric are added in https://tc39.github.io/proposal-bigint/ .

For example, start in Array.prototype.slice(start, end) will be converted an integer value with ToInteger. ToInteger changes the value to be an integer value with ToNumber (ref). But ToNumber causes a TypeError for BigInt (ref).

I think ToInteger shouldn't convert a value and keep as it is if the value is BigInt same as ToNumeric (ref).

If you have any information of the reason, please tell me. Thank you.

[devsnek]

It is disallowed because it would be an implicit coercion that loses precision.

[axross]

Thank you for your response.

I still have the question. I meant ToInteger(BigInt) should returns the BigInt as it is. Of course I think ToInteger(Number) should returns a rounded Number. I didn't mean about coercion. Why would ToInteger(BigInt) lose precision?

[littledan]

BigInt is still at Stage 3, so the appropriate place to file the issue is at https://github.com/tc39/proposal-bigint/ . In this case, you can find an explanation of the answer at https://github.com/tc39/proposal-bigint/#interoperation-with-number-and-string .

[axross]

@littledan Thank you for the navigation!