new Array(15).fill('').map((_, i) => i / 3 & 1 ? '+' : '-').join(',')
"-,-,-,+,+,+,-,-,-,+,+,+,-,-,-"
new Array(15).fill().map((_, i) => i)
.reduce((acc, c, i) => {
if (i % 3 === 0) acc.push([]);
acc[acc.length - 1].push(c);
return acc;
}, []);
// "[[0,1,2],[3,4,5],[6,7,8],[9,10,11],[12,13,14]]"