JavaScript create use backup array

Просто посмотрите на код, и вы поймете, что я имею в виду:

​var aBackup = [3, 4]; // backup array
​​var a = aBackup; // array to work with is set to backup array
a[0]--; // working with array..
a = aBackup; // array o work with will be rested
console.log(a);  // returns [2, 4] but should return [3, 4]
console.log(aBackup);​ // returns [2, 4] too​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​  but should return [3, 4] too