Little squares' (3x3 in example above) may also only contain integers: 1..N (N included)

Because the 3x3 little squares have repeated numbers.

[1, 2, 3, ...
[2, 3, 1, ...
[3, 1, 2, ...

Read the instructions again, it's explained there in detail, 11211 is an interesting number.

// progress as we near an "interesting" number
isInteresting(11207, []); // 0
isInteresting(11208, []); // 0
isInteresting(11209, []); // 1
isInteresting(11210, []); // 1
isInteresting(11211, []); // 2

The values inside the array could be ignored here (about the test you're not getting), because you're far away from them.

Because 11211 is 2 steps away from 11209.