Here's an interesting pattern that comes up in Go and TypeScript, which both have type systems with structurally typed interfaces. (I'll use TypeScript here because its syntax will likely be more familiar to more readers, but I learned it from Go, where it behaves the same.)

Suppose you have two modules that interact. Say there's a Profile component that fetches some info from a Backend and displays it:

async function userProfilePage(backend: Backend, userId: string) { const profile = await backend.fetchProfile(userId); for (const id of profile.pictureIds) { const picture = await backend.fetchPicture(id); ... } ... }

How do you write a test for this function, given that it depends directly on the backend? (One answer is to decouple the view construction and rendering, but in that case just imagine any of a million similar variants of this problem.) The standard testing approach is to use a test double — a fake, mock, etc. — in place of the backend.

There are many standard frameworks such as Guice or Angular's DI system that help set these up, sometimes using language magic like decorators. But if you're not using a framework, the obvious thing is to do in a nominally typed language like Java or C++ is the "extract interface" refactoring, where you create an interface for the backend that lets tests provide a fake for it:

interface BackendInterface { fetchProfile(...); fetchPicture(...); otherBackendMethods(...); ... } class Backend implements BackendInterface { ... as before ... } // and then in the client code: async function userProfilePage(backend: BackendInterface, userId: string) // and then in the test: class BackendFake implements BackendInterface { ... } ... { userProfilePage(new BackendFake(), ...); }

This approach is unsatisfying for a few reasons. One is that you must create this copy of the Backend API and change every client of Backend to instead accept the indirected BackendInterface, all to make the code "testable". Also, as it is written here, in our test the BackendFake must implement otherBackendMethods(), even though our test doesn't actually interact with it.

There are variants on this structure are similarly unsatisfying, such as making BackendInterface instead be an abstract class that has default implementations of every method that throw. Or you might try making subsets of Backend's interfaces for each of its different clients, but that gums up its API and what if they have overlapping subsets?

With structural interfaces like in TypeScript or Go there's a nice solution for this. Instead of making any changes to Backend, to make the original user profile module testable you make changes only to the code you're actually testing by declaring the interface there, and make the code under test accept that interface:

/** The backend methods needed to construct profile pages. */ interface ProfileBackend { fetchProfile(...); fetchPicture(...); } async function userProfilePage(backend: ProfileBackend, userId: string) { ... }

This function declares exactly which methods it requires of the backend. The key feature of the structural type system we're relying on here is that Backend already satisfies the ProfileBackend without needing any modifications, nor do you need to change any code that passes a real Backend to this function.

But it's all still checked by the compiler: if you make changes to the Backend API or if userProfilePage needs more methods, the compiler still helps you find all the code that needs updates.

To write a test for this, the test code can provide a fake for this interface which provides only exactly the functions required by this method, without a useless implementation of otherBackendMethods(). This component is so decoupled from Backend that it no longer even needs to import Backend at all (which even for the BackendInterface variant the previous code required), which may allow more parallelism in builds.

This pattern is summarized in Google's Go style guide as this:

interfaces generally belong in the package that uses values of the interface type, not the package that implements those values.

That is, an implementation like Backend should not declare any interfaces. A function that wants to make use of some methods from Backend should instead declare the interface it requires. Once you're accustomed to the pattern you'll see that a blanket rule like this is an easy heuristic way to catch all cases where the layering is wrong.