// 第一种
var mergeTo = [4,5,6],
      mergeFrom = [7,8,9];

mergeTo = mergeTo.concat(mergeFrom);
mergeTo; // is: [4, 5, 6, 7, 8, 9]

or
var a = [1,2], b = [3,4], c = a.concat(b);


// 第二种
var mergeTo = [4,5,6],
var mergeFrom = [7,8,9];

Array.prototype.push.apply(mergeTo, mergeFrom);

mergeTo; // is: [4, 5, 6, 7, 8, 9]

?