interface vs type

TLDR

interface has one extremely niche but confusing syntactic sugar, use type instead.

For more info, see the TypeScript documentation on the matter.

The interface and type keywords are extremely similar. They are both used for defining the shape of an Object.

interface Animal {
  name: string;
  weight: number;
}

type Animal = {
  name: string;
  weight: number;
};

They can both be extended as expected:

// FlyingAnimal = { name: string, weight: number, wingspan: number };
interface FlyingAnimal extends Animal {
  wingspan: number;
}

// FlyingAnimal = { name: string, weight: number, wingspan: number };
type FlyingAnimal = Animal & {
  wingspan: number;
};

However, there is one piece of syntax that interface contains, which type does not:

// We define the interface once...
// Fruit = { name: string };
interface Fruit {
  name: string;
}

// Then define it again!
// Fruit = { name: string, price: number };
interface Fruit {
  price: number;
}

After this, TypeScript recognises the Fruit interface as

{
  name: string;
  price: number;
}

Which is a kind of implicit extension.

However, using the type keyword:

type Fruit = {
  name: string;
};

// Error: Duplicate identifier 'Fruit'.
type Fruit = {
  price: number;
};

OPINION: This is dumb, and confusing. Don't use this. Use type, and explicitly extend your types.