Please read this: https://docs.codewars.com/training/troubleshooting

It should remove all values from list a, which are present in list b keeping their order.

a is an empty list, your code returns b.

The tests are fine:

JavaScript Completions 49916

And you can see the expected value for that input is ok.

Ignore

Except basic test all other tests passes for me. please review tests.

Should pass Basic tests

a was [1,2], b was [1]: expected [] to deeply equal [ 2 ]

Random tests - 40 of them passed
Testing for arrayDiff([-13,-13,14,6,-9,-15,3,-16,-1,-11,8,4,-4,-11,-6,-10,-2,-10,-12,-10],[-13,-11,-13,-16,6,-9,14,-6,-10,3,-11,-15,-1,-10,-10,-2,-12])
Testing for arrayDiff([-6,0,-11,-5,7,4,-1,-1,-5,17,12,-19,2,-15,-19,-14,-16,-2,-16],[17,-11,-14,-16,-6,-19,-1,-15,-5,-2,-19,-1,0,4])
Testing for arrayDiff([5,-6],[])
Testing for arrayDiff([-16,-18,-15,-13,18,8,11,-16,14,-11,-8,-6,-4,-7,-16,1,-5,-4,-8,-19],[14,-4,-4,-8,-7,-15,-18,-8,1,18,11,-16])
Testing for arrayDiff([-17,-1],[-17])
Testing for arrayDiff([9,16,-10,12,4,-17,8,-20,14,-7,-3,7,-9,-11,-15,5,-16,-2,8],[-10,-20,8,-16,-9])
Testing for arrayDiff([8,12,2],[2])
Testing for arrayDiff([-8,-15,-12],[-15,-8])