Function setting_of 

pub fn setting_of(value: Option<&[u8]>) -> Setting

What: The setting the value of core.abbrev stands for. Why: Git reads auto as the automatic length, a false word (or an empty value) as the whole name and a number from four up as that many characters; anything else makes Git report the key and keep the automatic length.

In TS you’d write (pseudocode):

settingOf(value) // Auto | { length }