方法一

使用concat合并数组

var mergeTo = [4,5,6];
var mergeFrom = [7,8,9];
var a = mergeTo.concat(mergeFrom);

concat合并数组

方法二

var mergeTo = [4,5,6];
var mergeFrom = [7,8,9];
Array.prototype.push.apply(mergeTo, mergeFrom);