js, or "j.s." is short for "just saying." It's a shortening of the word for texting but is also used in everyday language, just as "omg" has become. It is usually used at the end of a sentence, and commonly in a sentence where you are making a rude or mean comment. You're "just sayin," so no matter what you say, the other person shouldn't get mad, it's more of a joke.
Instead of saying, "not to be rude, but Ann is a real bitch" you say, "Ann is a real bitch, js."
Your friend will get mad if you say, "you're being a douche."
But if you say, "you're being a douche, js." it's more like your informing him rather than insulting him, in a lighter tone.