Why reassigning of an object have different value?

I know it sounds a bit stupid but I’m kinda stuck here.
let a={
name:’Pete’,
};
let b=a;

Change property of a.
a.name=’Cindy’;
b; //{name: "Cindy"}, b follows a exactly.

Add a property for a.
a.age=20;
b; //{name: "Ci…