When do you use an interface over a type alias in flow?

Question

The interface and type declarations seems to do the same thing. When do you use one over the other?

type Fooable = {   foo(): string } 

vs

interface Fooable {  foo(): string } 

Answer 1

This is a great question. Ideally there would be no difference between an interface and an object type. As implemented, there are a handful of (often subtle) differences between them.

The biggest difference is that Flow considers methods declared on an interface to be "read-only." This allows subtypes to be covariant w.r.t. methods, which is a very common pattern with inheritance hierarchies.

In time, I would like to see Flow unify these concepts, but until then here's my rule of thumb for choosing between interfaces and object types:

• Use object types to describe bags of mostly data that are passed around in your app, e.g., props/state for React components, Flux/Redux actions, JSON-like stuff.
• Use interfaces to describe service-like interfaces. Usually these are mostly methods, e.g., Rx.Observable/Observer, Flux/Redux stores, abstract interfaces. If a class instance is likely to be an inhabitant of your type, you probably want an interface.

Hope this helps!