“It’s probably just a food baby,” says 16-year-old Juno’s best friend Leah when Juno says she’s pregnant. “Did you have a big lunch?”

No, there was no big lunch. Juno is pregnant for realsies, and “Juno” is one of the year’s snappiest, snarkiest, most quotable comedies. Diablo Cody, a former stripper who’s now a writer, penned the screenplay, and she populated it with hip characters who speak not in the manner of real people but in the manner of self-aware movie characters. And yet, just as Joss Whedon accomplished with his TV series (notably “Buffy the Vampire Slayer” and “Firefly”), “Juno’s” people can pepper their language with stylized slang and still come off as believable, human characters.

Juno MacGuff (Ellen Page) is the kind of Middle American high schooler who’s too interesting to be popular and too savvy to care. She listens to cool ’70s bands and watches obscure foreign horror films, palling around with her best friend Leah (Olivia Thirlby) in her off hours. She also has a platonic guy friend, Paulie Bleeker (Michael Cera). They became slightly more than platonic on a recent night — boredom was a factor — and now Juno’s oven has a bun in it. Bleeker’s bun.

Her first instinct is to “take care of it” (“I’m just calling to procure a hasty abortion,” she says when she phones the clinic), but she thinks better of it and chooses the adoption route instead. She tells Leah they should give the baby to “a woman with a bum ovary, or a couple of nice lesbos.”

Instead, they find the Lorings, a generically nice couple who live in a generically nice home in a generically nice subdivision. Vanessa (Jennifer Garner) badly wants a baby; her husband Mark (Jason Bateman) seems to be going along with it, though it’s clear to us he has reservations. They meet with Juno and her father (J.K. Simmons) and offer to pay all her medical expenses. The arrangements are made. Everyone is happy.

Vanessa, a little nervous and uptight, reacts to the outspoken, seemingly carefree Juno the way a normal person would: Is this girl for real? Mark, on the other hand, “gets” Juno. Though he’s in his mid to late 30s, Mark in a state of arrested development, playing his guitar, collecting comic books, and watching cheesy movies — in other words, behaving just like Juno. Who is 16. The brilliant irony of the film is that Mark is trying too hard to be cool … which is exactly what some people will accuse the film itself of doing.

The talent involved in “Juno” is enviable. It’s director Jason Reitman’s second film, after the dark satire “Thank You for Smoking,” and he’s only improving with age. He favors understatement and deadpan wit, but he’s not afraid to let a little emotion come through when it’s needed. Furthermore, while I’d never heard of Diablo Cody before — I’m not up on my strippers-turned-writers — her exceedingly well-written screenplay here should secure her a place in Hollywood for as long as she can crank out funny characters and literate dialogue.

J.K. Simmons and Allison Janney are more than just parental buffoons as Juno’s folks, while Bateman and Garner have the plastic smiles of bland suburban affluence down pat. Michael Cera’s gift for stammery teenage awkwardness has been documented numerous times already, most recently in “Superbad,” and it’s nice to see that it’s not getting old yet. The kid can still wring more laughs out of not knowing what to say than almost anyone.

The outstanding writing and direction aside, “Juno” might have failed if it didn’t have a capable actress in the lead role. Ellen Page is most recognizable for playing Kitty Pryde (the one who can walk through walls) in “X-Men: The Last Stand,” though certain audiences will remember her performance in the disturbing thriller “Hard Candy,” where she played a girl who turns the tables on a would-be pedophile. Barely out of her teens, Page has shown remarkable common sense and discernment in choosing her projects. “Juno” lets her show off her comedic chops and razor-sharp timing, all in the service of a character whose effortless charm is her most endearing trait.

There’s a delicate line between “likably quirky” and “trying too hard to be quirky.” I think “Juno” steps on the wrong side a couple times (the pop-culture reference Juno makes when her water breaks is a bit of a reach), but not enough to diminish the pleasure I take in the film overall. It’s an absolute delight.

A- (1 hr., 35 min.; PG-13, moderate profanity, some sexual innuendo, brief fake gore.)