The victim, who police were initially unable to locate, has finally come forward.
She left the scene of Wednesday's attack before authorities arrived, but after seeing the surveillance video of the crime on television, she walked into LAPD's Hollywood station Saturday around 7:30 p.m.
Her testimony is considered critical to the case.
Investigators say 22-year-old David Victor Thomas approached the victim from behind and used the stun gun on her neck.
He then allegedly tried to steal purse, but witnesses held him until police arrived.
CNS contributed to this report.