Learn TypeScript 3 by Building Web Applications

Extending interfaces

Just like classes, you can use the extends keyword to extend an existing interface, adding method signatures and properties to it. This is, of course, interesting when creating higher-level abstractions and specialized interfaces for your APIs.

Here's an example:

interface Club { 
    name: string; 
    logoLocation: string; 

    isActive(): boolean; 
} 

interface SoccerClub extends Club { 
    league: string; 
} 

It is actually possible for interfaces to extend classes, but it is really something that you should avoid doing at all costs. There might be valid use cases, but, generally speaking, you should consider that an anti-pattern.