Maybe adding an example of a diagonal island would be a good idea to make it clearer.

Sure, I see and I think i know where confusion comes from. When you try the task there is an extra paragraph saying:

Pixels are 'adjacent' if their coordinants differ by no more than 1 on the X and/or Y axis.

I interpreted and there as a sum |x1 - x2| + |y1 - y2| <= 1. So if one pixel has x and y as coordinates, diagonal pixel would have x(+/-)1 and y(+/-)1 coordinates.

Have you read the instructions? That same example is there and it says they're 2 islands.

Tests are broken, in JS

    var image = [
      [0,0,0,0,0,0,0,0,0,0],
      [0,0,1,1,0,0,0,0,0,0],
      [0,0,1,1,0,0,0,0,0,0],
      [0,0,0,0,0,0,0,0,1,0],
      [0,0,0,0,0,1,1,1,0,0],
      [0,0,0,0,0,0,0,0,0,0],
    ];

expects 2 while there are 3 islands

Lol :)