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.