TypeScript Language Specification v1.5

Private and protected accessibility is enforced only at compile-time and serves as no more than an
indication of intent. Since JavaScript provides no mechanism to create private and protected properties on
an object, it is not possible to enforce the private and protected modifiers in dynamic code at run-time.
For example, private and protected accessibility can be defeated by changing an object's static type to
Any and accessing the member dynamically.
The following example demonstrates private and protected accessibility:
class A {
private x: number;
protected y: number;
static f(a: A, b: B) {
a.x = 1; // Ok
b.x = 1; // Ok
a.y = 1; // Ok
b.y = 1; // Ok
}
}
class B extends A {
static f(a: A, b: B) {
a.x = 1; // Error, x only accessible within A
b.x = 1; // Error, x only accessible within A
a.y = 1; // Error, y must be accessed through instance of B
b.y = 1; // Ok
}
}
In class 'A', the accesses to 'x' are permitted because 'x' is declared in 'A', and the accesses to 'y' are
permitted because both take place through an instance of 'A' or a type derived from 'A'. In class 'B', access
to 'x' is not permitted, and the first access to 'y' is an error because it takes place through an instance of
'A', which is not derived from the enclosing class 'B'.
8.2.3 Inheritance and Overriding
A derived class inherits all members from its base class it doesn't override. Inheritance means that a
derived class implicitly contains all non-overridden members of the base class. Only public and protected
property members can be overridden.
A property member in a derived class is said to override a property member in a base class when the
derived class property member has the same name and kind (instance or static) as the base class property
member. The type of an overriding property member must be assignable (section 3.10.4) to the type of
the overridden property member, or otherwise a compile-time error occurs.
Base class instance member functions can be overridden by derived class instance member functions, but
not by other kinds of members.
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