There is now a setBigInt64() and a getBigInt64() method on DataView!
It does, as suggested in the other answers, return a bigint.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView/getBigInt64 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/DataView/setBigInt64
(There are also BigUint64 versions)