Explanation of the basic method of merging arrays in JavaScript

Here are two methods:
The first one

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

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

The second type

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

Merge arrays:

// Special processing service-must be enabledvar _SPECIAL_MUST_ENABLE_LIST = ['intime']; 
 
if(termNo != null &amp;&amp; ("9966") == 0){ 
  var newMustEnableList = ['mobile','psp','jtk','credit','train','bank','alipay','lottery','movie','littleTransfer', 
               'trafficFines','concert','mall','airplane','hotel','recycle']; 
  _SPECIAL_MUST_ENABLE_LIST = _SPECIAL_MUST_ENABLE_LIST.concat(newMustEnableList); 
}