How do classes work in TypeScript?

A TypeScript class is still a JavaScript class at runtime. Type annotations and access modifiers are erased. Parameter properties are a shorthand that declares and assigns constructor parameters in one place. The implements clause checks that the class satisfies an interface but does not add any inherited implementation.

interface Greeter {
  greet(): string;
}

class User implements Greeter {
  constructor(
    public readonly id: number,
    private name: string,
  ) {}

  greet(): string {
    return "Hello, " + this.name;
  }
}

const user = new User(1, "Asha");

What is type-only and what is runtime

• Type annotations, implements and access modifiers are erased
• Parameter properties emit assignments in the constructor
• Class fields emit runtime fields, with semantics affected by useDefineForClassFields
• Decorators can emit additional runtime code
• private is a compile-time check; hash-prefixed fields are runtime-private

The implements clause is a structural check, not inheritance. A class can implement several interfaces, but it only extends one base class. If the class changes in a way that no longer satisfies the interface, the compiler reports the mismatch at the class declaration.