One design, six languages interview questions

TypeScript's private is checked only by the type checker. Once the types are stripped, as Node.js does when it runs a .ts file, the field is an ordinary property: it appears in JSON.stringify and Object.keys, and any code can read it with a cast or bracket notation. A #private field is part of JavaScript itself, so the engine enforces it at run time: no outside code can name it, it never appears in JSON or in the object's keys, and a method that reads it from an object its class did not create throws a TypeError. Use #private for state that must stay hidden, such as tokens or amounts that only methods may change; private hides nothing at run time.

Implicitly: a type satisfies an interface when it has all of the interface's methods with the right signatures, and it never names the interface. Because of that, Go declares an interface in the package that uses it, with only the methods that package calls, and the implementing package returns concrete types. A billing package that needs a fee declares a one-method FeePolicy, and any struct with that method fits, including test doubles. Small interfaces keep fakes small and let types written elsewhere satisfy them. The compiler checks a fit only where a value is assigned to the interface, so a design adds an explicit check next to each implementation.

The rule of zero says that a class should not declare a destructor, copy or move functions of its own when its members can manage their resources themselves. Hold parts by value, text in std::string, collections in standard containers and a sole heap object in std::unique_ptr, and the compiler generates correct copying, moving and destruction from the members. In a design this keeps ownership visible in the member types instead of hidden in hand-written functions, which are where double frees and leaks come from. When a class must manage a resource directly, the C++ Core Guidelines ask for all of those functions to be defined or deleted together.

Behaviour is shared through traits: a trait declares methods, may provide default implementations, and any type can implement it, including types from other crates for your own traits. Code that needs the behaviour takes a generic parameter bounded by the trait, or a trait object such as Box<dyn Trait> when the concrete type is chosen at run time. Data is shared by composition: a struct holds other structs as fields. A closed family of kinds, such as a door's states or an operation's outcomes, is an enum whose variants carry their own data, handled with a match that the compiler checks for completeness.