The man demanded money from the girl at a home on Hillside Avenue around 3:30 p.m. Monday, Montclair police said.
When she didn't agree, he assaulted her, then fled with a laptop and cell phone, according to authorities.
The girl was taken to the hospital by a family member.
She described the man as being around 6 feet tall with a very thin and bony build, police said. He had large hands, big eyes and a gap between his two front teeth. He was dressed in all black "raggedy and dirty" clothing with an Army camouflaged face mask.
He is also believed to have a backpack possibly containing a hammer, the girl said.
Anyone with further information is urged to call Joseph Anderson at (973) 509-4707 or Sgt. Terence Turner at (973) 509-4730.
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