We got "The View" host out Tuesday in NYC outside ABC Studios -- and our photog asked her about Trump challenging Biden to take a drug test ahead of Thursday's debate ... something DT floated in an attempt to suggest Joe is on PEDs, which is unfounded.

Trump's been attempting to fan the flames on this speculation about Joe before big public events -- but Joy's seriously downplaying those concerns ... and flipping the tables on DT.

Check it out ... Joy says Biden's all there mentally -- saying she doesn't buy Trump's reckless claims that he uses anything to enhance his "performance" -- and goes on to explain it's Trump who's the one who needs to be tested ... and her reasoning is kinda hilarious.

Joy says Don oughta be hooked up to a lie detector test -- noting that's the only test she thinks would be worth using ... and one she believes he'd fail. She also jokes about drugs in Trump's system ... and yeah, it's a boner and fart crack here at DT's expense.

Regardless, Thursday's debate should be big for both Biden and Trump ... with Chris Cuomo recently saying the pressure is on Biden, from his POV.

CC thinks Biden might drop out of the race entirely if he flubs the first debate with Trump.