What happens if multiple interfaces with the same name are declared?

In TypeScript, when declaring multiple interfaces with the same name, declaration merging occurs. This means that all properties from all interfaces with the same name are combined into a single interface.

For example:

interface User {
  name: string;
}

interface User {
  age: number;
}

const user: User = {
  name: "Ivan",
  age: 30
};

As a result, the User interface will contain both name and age. This is useful for extending types, for example, when working with libraries or modules where you can add new properties to existing interfaces.