TypeScript: Interface or Type? Hint: it's in the name

It's called TypeScript not InterfaceScript

If you're a purist like I am, you know the debate between 'interface vs. type' can be frustrating. Why is there no official stance on this issue?

I've been using Ts since 2015 and I've been through different phases.

First, I encouraged using interfaces instead of types (unless an Union/Intersection was needed);

...then I reached the stable plateau of "whichever you prefer, it doesn't matter" 😄.

Till now.

For some time I've been using the type "Any key-value object" a.k.a Record<string, unknown>

If you type something as any key-value object;

function foo(data: Record<string, unknown>) {
  for (const [key, value] of Object.entries(data)) {
    // ...
  }
}

You might reach a dead end if you use interface:

interface Profile {
  name: string;
}

function getProfile(): Profile {
  /* fetch data */
  return Promise.resolve({ name: 'jane' });
}

...
const profile = await getProfile()
foo(profile); // ERROR ❌
// Argument of type 'Profile' is not assignable to parameter of type 
// 'Record<string, unknown>'.
//  Index signature for type 'string' is missing in type 
// 'Profile'.ts(2345)

Our interface { name: string } does NOT fulfill Record<string, unknown>.

I respectful disagree? with typescript here 🤓

Now, let's change the thing to type instead:

function foo(data: Record<string, unknown>) {
  for (const [key, value] of Object.entries(data)) {
    // ...
  }
}

type Profile = {
  name: string;
};

function getProfile(): Profile {
  /* fetch data */
  return Promise.resolve({ name: 'jane' });
}

...
const profile = await getProfile()
foo(profile); // ✅ nice

👉 The type { name: string } does fulfill Record<string, unknown>

This type of situation isn't specific to Record<string, unknown>' but I don't want to research which Advanced types will work and which won't. So, I've turned to using types -almost- exclusively."

I was shocked to find out that Matt Pocock also went through the same three phases!.

If you haven't heard of Matt, I highly recommend checking him out. He creates high-quality TypeScript content on Youtube (and is a nice guy on Twitter 🦜).

TL;DR: prefer types to interfaces, people on the internet defend types.