JavaScript's assignment operation in more detail

test1.js

var x = 2 // x gets a reference to the value of the scalar 2;

var y  = x  // also gets a reference to the value of the scalar 2; Please, note that y does not refer to x, but //to the value to which x refers. That will make more sense in the later examples.

x++;

y = ??? x = ??? // x == 3, while y==2, because x was given a new reference to the value of the scalar 3. Since //y points a reference to the value of the scalar 2, and not to x, it keeps the value

test2.js

//Same for arrays and objects

var x = {} 
var y = x // copies the value of x as the initial value of y => keeps a reference to the value of x ({}), 
//not x itself

therefore:
y = {abc:"test"} //changes the reference to completely different value, but x == {}

test3.js

//All of this should be clear. Now the tricky case:

var x = {}
var y = x 
x.abc = "test"