Kata

The sample tests use assertSimilar while the submission tests use assertDeepEquals. These assertions have slightly differing semantics: extending Array, I pass the full tests but I fail the getSplit sample test:

Expected: 
'{ pass: [ \'a\', \'b\', \'c\' ], fail: [ 1, 2, 3 ] }'
, instead got: 
'{ pass: MagicArray [ \'a\', \'b\', \'c\' ],\n  fail: MagicArray [ 1, 2, 3 ] }'

JS: Node v14 should be used along with its appropiate assertion tools (Mocha + Chai). Refer to this and this

eq: function(n) - Return an array with all methods, only containing the eq item...

This description makes zero sense; the expected behavior is extracting the n'th element of an array.

Your task is to make an object and add to it the following methods...

This is not enforced.

I can't get passed "getSplit" test. My split object looks like this:

{pass: magicArray, fail: magicArray}

Am I missing something or the tests have issues? I can see that when instance of magicArray is compared with instance of Array, the test suite is creating strings which contain only array elements and then comparing those strings. When comparing objects with magicArrays, it's creating a string out of object and not checking if properties are arrays.

In double and multiply :

This should change the values of the source array.

If this is meant to imply the method should mutate the array they're called on, it's not tested that way (and not everyone implements it that way).
Either way, it's unclear.

Shouldn't testArrayTwo.multiply(2).first().double().double() returns [2, 2, 2, 2] instead of [2, 2, 2] ?

( [2].double() -> [2,2], then [2,2].double() -> [2,2,2,2])