Function parse_add_region 

pub fn parse_add_region(
    region: &[OsString],
    wrapper_flags: &[&[u8]],
) -> Result<AddRegion, OptionError>

What: Parse the region after add. Result<A, B> is “either success A or failure B”. Why: A region Git itself would refuse yields the refusal, never a guessed fact set.

In TS you’d write (pseudocode):

function parseAddRegion(region: string[], wrapperFlags: string[]): AddRegion; // throws