Harpur, 17, made his first-team debut as a 15-year-old in the FAI Cup against Killester-Donnycarney in July of last year.
His League of Ireland debut came earlier this season when he came off the bench for the final four minutes of the 1-1 draw Derry City.
Bohemian manager Alan Reynolds was full of praise for Harpur as he confirmed he had left to link up with Fabregas at Como.
Reynolds said: “Josh is a really exciting young striker. He has a brilliant attitude, and I’m confident he will go on to have a really good career.
“You don’t want to put too much pressure on a 17-year-old but he has serious potential to push on. He’s a brilliant striker for his age, scores goals and holds up the ball well.
“He’s really hungry to do well and if he maintains that, and continues to progress and learn in the way he has been, he will do very well. We wish him the very best of luck.”