let a = [[0,1,2],[0,1,2],[0,1,2]];
function index2d(a, i, j, v){
	let b = a[i];
	b[j] = v;
	a[i] = b;
	return a;
}
a = index2d(a, 1, 2, 4);
// -> [[0,1,2],[0,1,4],[0,1,2]]