Actually, Joey says: Howyoudoin'? Not only is it grammatically incorrect, but the phrase is spoken as if it is one word with a tiny inflection on the "you." Stereotypical, old-school Brooklynese. Funny, though.

Notice the tone in Joey's voice when he says, 'Howyoudoin?' to a girl. It is flirtatious and he leans in towards her. Another way of expressing this would be , 'Hello how are you?'

Correct English: it is how are you doing. But, the native English speakers always raise or lower their voice, it may change the meaning from positive to negative and vise versa. For example: it is very very funny, it means very funny ( possitive). But when it say in the tone as: it is, very very funny ah! it means, it is not funny at all (negative). We also have the sound elimination, " How are you doing" it is pronounce as " How're you doing", it sounds like how, but it is how're. Thus, how you doing is wrong. The native Americans always speak with eliminated sounds. I wanna banana ( I want a banana) Peanut butter, pronounce: peanut budda (tt=d sound)