how to attach a list of a set of elements to a variable javascript code example

Example: push only elements list into another list javascript

//Server side (nodeJs):
var a = [1,2,3];
a.push.apply(a, [4,5]);
//Client side (most browsers)
var arrayA = [1, 2];
var arrayB = [3, 4];
var newArray = arrayA.concat(arrayB);