Interfaces allow you to construct the shape of certain implementation. Interfaces should not really be a new concept to you considering your background in .NET, and in TypeScript it is very much the same.

Interfaces are defined using the keyword interface, as follows:

enum Operand {
    Sum, Subtract, Multiply, Divide
}

interface Calculable {
    left: number;
    right: number;
    operand: Operand;
}

Furthermore, interfaces support optional members and read-only properties, too:

interface Calculable {
    readonly left: number;
    readonly right: number;
    operand?: Operand;
}

In the preceding example, the Calculable interface has two readonly properties, left and right, as well as an optional operand property. Read-only declarations can only be set when first initialized or inside the constructor of the owning class.

Like other interface-supporting languages, you can code a certain hierarchy of interfaces by extending them.