Pass by value in javascript array

Question

I have code like this:

var a = "a";
var b = a;
a = "b";
console.log(a);
console.log(b);

Output:
a
b

The problem occured when I changed the variable into an array, the value in b also changed when I changed a.

The code like this:

var a = new Array(1);
a[0] = "a";
var b = new Array();
b.push(a);
a[0] = "b";
console.log(a);
console.log(b);

Output:
["b"]
[["b"]]

I want the result like this:

Output:
["b"]
[["a"]]

What should I do?