Meaning of 何だ(なんだ) in Japanese

Examples of 何だ, なんだ, なあんだ in a sentence

• 「いらっしゃ～い」「よぉ」「なんだ、啓太か・・・」

  Welcome! "Yo." "Oh, what, it's only Keita?"

• 「お先に失礼しま～す」「お、なんだ、また定時上がりかよ」

  Bye, see you tomorrow. "Oh, what's this? Leaving on the dot again?"

• 「そっか。暇なら掃除しといて。」「なんだそれ。兄貴をアゴで使うなよ。」

  Oh? If you're free get the cleaning done. "What's up with that? Don't order your brother around!"