Function start_object_reader 

pub fn start_object_reader(
    real_git: &Path,
    global_prefix: &[OsString],
    overlay: &[(OsString, OsString)],
) -> Result<ObjectReader, CandidateError>

What: Start the reader for the repository the caller’s global options select. &[OsString] borrows the arguments before the subcommand, unchanged; &[(OsString, OsString)] borrows added environment pairs. Result<T, E> is “a value or a failure”. Why: Replaying the global prefix makes the reader open the same repository as the forwarded command. The overlay may point it at a private object directory, so one reader is bound to one overlay for its whole life.

In TS you’d write (pseudocode):

function startObjectReader(realGit, globalPrefix, overlay): ObjectReader;