"Input strings will only contain letters."

How about the name with number ?

Great solution, thank you. I overcomplicated myself using .split("") and a condition but this is smart. Gotta remember about filter.

Names with numbers will be a trend .. apparently

Description says "a name has exactly 4 letters in it". Not numbers.

I only dislike "n" as a name. "N" doesn't say anything about what we are dealing with. It should be friendName or friend imo.

This soulution passed only basic test? bcs string from 4 numbers like '3333' is not a name)

Because the author doesn't mention it and everyone is lazy pretty much

In any case, the code above can give a name consisting of 4 digits.

Numbers in the basic tests:
assert.deepEqual(friend(["Ryan", "Jimmy", "123", "4", "Cool Man"]), ["Ryan"])

The tests only pass strings containing letters. Otherwise, it is expectable the description would state it, and provide some more examples to see the different cases.

Why is this solution the most popular? The case of "1234" will be valid, although the kata condition does not allow it: "If a name has exactly 4 letters"

Exactly !

yaa , me too :D