The docs mention a RecordFactory<T> type for Flow and TypeScript, but at least in TypeScript it's actually Record.Factory<T>, which took some time to figure out. My Immutable.js version is 4.0.0-rc.12; I haven't tried it in Flow. It's also a little odd that given this, RecordOf<T> isn't also Record.Of<T>, but maybe there's a reason for that.