JavaScript in-place concat

In JavaScript, a lot of times you want to combine two arrays.  Since `concat` doesn’t work in-place, you end up with something like:

var a = [1, 2, 3];
var b = [4, 5, 6];
a = a.concat(b); // [1, 2, 3, 4, 5, 6]

But since `push` can take multiple arguments, we can use `apply` to do a little better:

var a = [1, 2, 3];
var b = [4, 5, 6];
a.push.apply(a, b); // 6
a; // [1, 2, 3, 4, 5, 6]