Typescript interfaces

In typescript, interfaces are implicit -- any object that satisfies the requested interface is allowed.
Interfaces can be extended (grown) into new interfaces that include others.

Interface

interface Person {
    firstName: string;
    lastName: string;
}

interface Employee extends Person {
    id: number;
}

function getId(employee: Employee): number {
    return employee.id;
}

getId({ id: 1, firstName: "anakin", lastName: "skywalker" }); // valid

Type Alias

You can also define a type-alias, but you cannot extend it in a new interface.

type Point {
    x: number;
    y: number;
}