Explained: An Interface Can Only Extend an Object Type

You're likely here because you've spotted this error:

An interface can only extend an object type or intersection of object types with statically known members.

This error can happen because you're trying to extend a union type. For instance, this code will fail:

type Example =
  | { a: string }
  | {
      b: string;
    };

interface Foo extends Example {
An interface can only extend an object type or intersection of object types with statically known members.2312
An interface can only extend an object type or intersection of object types with statically known members.  c: string;
}

This is an error because interfaces can't extend union types. Interfaces can't represent unions, so it's not possible to create one using the extends keyword.

Instead, you can use an intersection type instead of extends:

type Example =
  | { a: string }
  | {
      b: string;
    };

type Foo = Example & {
  c: string;
};

Now, you'll have what you need - an object where you can either pass in a or b, but you must always pass in c.