>>44
The usual English saying online in this situation is "LOL" or "ROFL". Those stand for "laugh out loud" or "rolling on (the) floor, laughing". Saying these too much may seem annoying, just like saying "ワロスwwwww" in Japanese too often is annoying. A more adult way is saying "heh", which stands for a chuckle or small laugh.

There are no specific words or phrases (that I can think of) to express this emotion of pleasure at someone's serious joke in English, so English speakers usually just laugh or chuckle. You could also say "funny" or "nice one" meaning that the joke is humorous.

Online it is hard to tell whether a person is joking or not. Sometimes people seem to be joking but they are actually serious. Then they will get angry at being laughed at. Since online life is not like real life, it's still okay to laugh at them; if they let their feelings get hurt by a post they are too sensitive.

Don't ever say "You are funny". It's too hard to tell if you are saying that the person is humorous, or that the person is weird. English speakers usually know the difference, but foreign people have a hard time. It is like saying "あなた、面白い" in Japanese -- maybe an insult, maybe not. Instead say "That's funny" which doesn't point at the person. You can say "You're funny" to close friends who will know that the implied insult is not serious.