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 ] }'

suggested tag: restricted

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.

If base is not supplied, it will create a new class entirely from Proto.

This doesn't seem to be tested at all.

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.

Expected: Hello World!, instead got: Helo World!

Couldn't pass the test. What does it mean?

Any hint ? I'm stuck, I always got Error: Strings are not allowed. even if I return a number

Do you think also disallowing the "String" constructor would be a good idea? It allows for some answers which are very simple so it's a bit more challenging without.