> class User {
...
...   constructor(name) {
...     this.name = name;
...   }
...
...   sayHi() {
...     alert(this.name);
...   }
...
... }
undefined
> x=User("a")
TypeError: Class constructor User cannot be invoked without 'new'
> x=new User("a")
User { name: 'a' }
> x
User { name: 'a' }
> x.Name='b'
'b'
> x
User { name: 'a', Name: 'b' }
>