Since Holden is narrating his own story, the style of the novel is the same as the style of his own language, which is colloquial and distinctive. He talks directly to you, as in "You'd have liked [Allie]." He uses italics to make the words read with the same emphasis as spoken word ("He's my brother and all"). You'll hear him describe places and people all the time as "corny" or "phony." He'll tell us he's never waited anywhere so long in his "goddamn life. [He] swear[s]" (24. 97), or that he's sweating "like a bastard" (24.100).
The Catcher in the Rye, like Adventures of Huckleberry Finn before it, is one of few books to feature this language in the narration itself, not just in dialogue. At the time, this was both unusual and important – not just as a new literary style, but also as a way to study the vernacular of a particular time period. Which reminds us – the language doesn't seem all that offensive to us (PG, maybe), but at the time (1951) it raised a few more eyebrows.