JavaScript snippet does not handle non-integer value correctly

https://reproducible-builds.org/docs/source-date-epoch/#javascript--nodejs

$ SOURCE_DATE_EPOCH=a node -e 'console.log(new Date(process.env.SOURCE_DATE_EPOCH ? (process.env.SOURCE_DATE_EPOCH * 1000) : new Date().getTime()))'
Invalid Date

This works:

const epoch = Number(process.env["SOURCE_DATE_EPOCH"]);

const timestamp = Number.isInteger(epoch) ? new Date(epoch * 1000) : new Date();