Laugh length signals sincerity online: 哈哈 is polite, 哈哈哈 genuine, 哈哈哈哈 really funny, and long strings (哈哈哈哈哈哈哈) signal you're actually laughing out loud. A single 哈 can read as sarcastic or dismissive.
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Chat / informal only. In writing, 笑 or 大笑 is preferred.
