J.R. Smith was asked about LeBron James ditching the Cavs ... and went COMPLETELY NUCLEAR -- telling the guy who brought it up to get the "f*ck outta here!"
It all went down outside Poppy in L.A. -- one of the hottest new restaurants in West Hollywood -- when a photog hit a nerve by asking if LeBron was gonna leave Cleveland (again) after this season.
Smith, one of Bron's Cavs BFFs, wasn't havin' it -- ripping the dude (not our guy) to shreds ... before blowing up over a question on The King joining the Lakers, specifically.
Photog: "LeBron's gone man. Deal with it."
J.R.: "LeBron's still here. He's still here."
Photog: "You think LeBron's gonna come to L.A.?
J.R.: "I think LeBron's gonna be wherever the f**k he wanna be at!"
The best part of the video is when J.R. fashion shames the photog for wearing a striped hoodie. It's legitimately funny stuff.