Thank you for pointing me in the right direction

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I fixed the bug in random test like this:

const _random = Math.random; Math.random = () => _random() + 1;

Tests for JS is very bad: https://www.codewars.com/kata/reviews/54df3072a1b757b06d000050/groups/5be80c1945394b25c10012ab

Difficult but possible: https://www.codewars.com/kata/reviews/58e03f832c99b9c419000631/groups/5bba236eb684d379b1000004

Check your random tests. The error is there.

Bug in tests?

matrix = [
[ 100, 25, 59, 35, 22, 11, 42, 75, 99, 7 ],
[ 35, 61, 76, 74, 72, 45, 31, 7, 70, 82 ],
[ 46, 37, 30, 55, 14, 51, 18, 44, 6, 35 ],
[ 41, 2, 34, 34, 43, 56, 13, 71, 44, 17 ],
[ 1, 8, 61, 35, 19, 16, 66, 25, 4, 39 ],
[ 9, 71, 49, 11, 49, 53, 50, 81, 86, 62 ],
[ 97, 19, 87, 16, 15, 37, 95, 38, 37, 21 ],
[ 2, 55, 49, 96, 88, 21, 79, 76, 60, 54 ],
[ 75, 86, 98, 92, 91, 73, 75, 77, 100, 20 ],
[ 96, 82, 49, 5, 49, 81, 60, 60, 51, 40 ]
];

get_neighbourhood("von_neumann", matrix, [ 5, 8 ], 1);

Expected: '[4]', instead got: '[37, 4, 62, 81]'

Why 4?