They are not in the dictionary. They are not real words. They are not proper written English. They are "colloquialisms" heard in spoken speech.
Writers can and do write them IN DIALOG, inside quotation marks.
--The boy at bat called to Nick: "Heh, you! Wanna come on and pitch?"
(Edna Ferber, "Gigolo")
Edna Ferber is showing us that the boy speaks colloquial English.