A colloquial exclamation praising someone’s action or achievement, like “Way to go!” or “Nice job!”
テストで満点？やるじゃん！
Perfect score on the test? Way to go!
今日のプレゼン、落ち着いてたしやるじゃんって思ったよ。
Your presentation today was calm — I thought, nice job.
新しい料理、簡単に作ったのに美味しい。やるじゃん！
You made that new dish easily and it’s delicious. Nice one!