Interfaces and Extensions

If you’ve been using type to model object shapes, that’s totally valid. But the interface is often a better choice—one that’s built for scale, collaboration, and evolving codebases.

Interfaces aren’t just a different way to describe object structure. They’re how we model contracts. When we define an interface, we’re saying: “This shape matters. It’s meant to be reused. It’s meant to grow.”

In this lesson, we’ll learn how to declare interfaces, extend them cleanly, and understand exactly how they compare to type aliases. By the end, we’ll know when and why to reach for each and how to build strong, adaptable types that play well with real-world systems.

Declaring interfaces

Interfaces let us define the shape of an object—its required properties and their types. Just like type aliases, they support optional fields (?) and readonly modifiers. And yes, we can use them anywhere we’d use a type: to annotate variables, parameters, return values, and more.