Where can you use an interface, but not a type?

In TypeScript, interfaces (interface) can be used to declare object and class types, and they support declaration merging. This means that if you declare multiple interfaces with the same name, they will automatically merge into one.

Example:

interface User {
  name: string;
}

interface User {
  age: number;
}

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

This behavior is not available with type — attempting to declare multiple types with the same name will result in an error.

In summary, interface can be used where you need to extend or augment types through declaration merging, such as when extending types from third-party libraries or declaring APIs where interfaces can be augmented in different parts of the code. type does not support such merging and is suitable for more complex types, unions, and intersections, but not for declarative extension.