[1,2,3] + [4, 5, 6]

b = [1,2,3]
b.extend([5,4,6])
b + [7,8,9]

var a = [1, 2, 3];
// don't use + as it will return '1,2,34,5,6'
a.push(...[4, 5, 6]);
a; //[1,2,3,4,5,6]