Function base_revision 

pub fn base_revision(capture: &InvocationCapture) -> String

What: The revision the transaction reads as its baseline tree: the base commit, or the empty tree when unborn. Why: Every baseline read of the transaction uses this one revision.

In TS you’d write (pseudocode):

baseRevision(capture) // capture.base.kind === 'commit' ? capture.base.oid : capture.emptyTreeOid