You can use splice combined with some apply trickery:
a1 = [1,2,3,4,5];
a2 = [21,22];
a1.splice.apply(a1, [2, 0].concat(a2));
console.log(a1); // [1, 2, 21, 22, 3, 4, 5];
In ES2015+, you could use the spread operator instead to make this a bit nicer
a1.splice(2, 0, ...a2);