Node v12 should be used enabled along with its appropiate assertion tools (Mocha + Chai).

No random test

Arrays with duplicate values (inside a single array) should be tested (it is not the case in JS at least).

No sample tests.

No random tests.

No sample tests.

No random tests.

This accepts the solution with undefined among the items.
For example that array is passed tests [ 1, 2, 3, undefined, 4, undefined, 5, undefined, 6 ] for the input f([ 1, 2, 3 ], [ 4, 5, 6 ]).

The test cases are not specifically enough, I wonder if you can add the following cases:

// for negative integers
Test.expect(((-2).twos(4) == "1110"),"-2 in 4 bits == 1110");

// for results over 32bits
Test.expect(((-10).twos(32) == "11111111111111111111111111110110"),"-10 in 32 bits == 11111111111111111111111111110110");