TypeScript Uses A Structural Type System Aka Duck Typing

TypeScript is not a nominal type system, so you don't need to inherit the interfaces all the way to make your code work. It is important that your class implements the correct interface to be recognized as the same type.

Instead, TypeScript uses duck typing or structural type system.
The term comes from the saying "If it looks like a duck, walks like a duck and quacks like a duck, it probably is a duck.".
So ==even if the class doesn't implement the interface, but has the same properties, those types are still compatible==.

By the way, Go also uses a structural type system.

const person = {
  firstName: 'Brendan',
  lastName: 'Eich'
};

// Duck typing (aka "structural type system")
const bar = function (p) {
  console.log(p.firstName);
};

bar(person);

Links

1. Interfaces in TypeScript
2. Excess properties when casting in TypeScript