Just like in .NET, interfaces can extend one another:

interface Serializable {
    serialize(): string;
}

interface PersistedCalculable extends Calculable, Serializable {
    shouldSaveInHistory: boolean;
}

In the preceding example, the PersistedCalculable interface extends two interfaces. In addition to its own members, it includes all the members defined in the extended interfaces, Calculable and Serializable.