In Classic JavaScript you can assign new properties to any object on the fly, or delete existing properties.

In ECMAScript 4 however, objects that at instances of classes cannot by default be modified during runtime. You can allow this by using the dynamic attribute on the class declaration.

dynamic class A {
    var b = 1;
}
var a = new A;
delete a.b;  // removes the property
a.c = 10;  // add new property 

Note that the type-checker cannot warn you if you access dynamic properties which might change or be deleted at runtime.

Built-in classes as Object, String etc. are defined as dynamic out of the box.