Javascript's assignment operation is to copy references?

The basic example:

var b = 10;
var c = b;
b++;
console.log(b,c);

>> 11 10

c looks like a copy of b.

But in another case:

var x = {};
var y = x;
x.abc = 10;
console.log(x.abc, y.abc);

>> 10 10

Why is the y not a copy of x, but a reference which points to the same instance x points to?

Also, I guessed b++ creates another instance, so b points to the new instance but c points to the old one. However...

var u = 10;
setTimeout(function() {
  console.log(u);
}, 10000)
u++;

>> 11

If u++ creates a new instance, then the u inside the anonymous function should point to the old u, shouldn't it?