For those are confused by instruction

1. its asking for the largest difference of length of smallest number from array 1 compare to largest array 2
2. then you need to find the largest length of largest number from array 1 compare to smallet array 2
3. result must be the biggest one

done

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

The kata was more difficult than it seemed at first sight. Thanks for the interesting experience.

The user can mutate the input affecting the expected value at least in javascript. Calculate the expected value before calling the user's function.

Probably that's because that solution mutates the input, affecting the expected value.

Post your code that fails: https://docs.codewars.com/training/troubleshooting#post-discourse so it can be checked.

I used a different implementation of the solution. In IDE the function returns the same "Winner!" but test message: "Expected: 'Loser!', instead got: 'Winner!'"

The input data above should return'Winner!'

1. I get a failed test with message: "Expected: 'Loser!', instead got: 'Winner!'"
2. I copy the printed input data.
3. My code in IDE returns 'Winner!'

No idea what you're doing but, you passed the kata and with your current code I can't reproduce that error. The control function returns "Winner!" for that input too. You can see that for yourself.

Random tests

Testing for DSYLTWUI,87,TQDV,79,MKUVZJN,77,JGICV,87 and 1

Log

[ [ 'DSYLTWUI', 87 ],

[ 'TQDV', 79 ],

[ 'MKUVZJN', 77 ],

[ 'JGICV', 87 ] ] 1

It should work for random inputs too - Expected: 'Loser!', instead got: 'Winner!'

Why 'Loser!'?

This kata should have a "Mathematics" tag :-)

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"])