Basic Type

Useful table for Types vs Interfaces

It's a nuanced topic, don't get too hung up on it. Here's a handy table:

Aspect	Type	Interface
Can describe functions	✅	✅
Can describe constructors	✅	✅
Can describe tuples	✅	✅
Interfaces can extend it	⚠️	✅
Classes can extend it	🚫	✅
Classes can implement it (`implements`)	⚠️	✅
Can intersect another one of its kind	✅	⚠️
Can create a union with another one of its kind	✅	🚫
Can be used to create mapped types	✅	🚫
Can be mapped over with mapped types	✅	✅
Expands in error messages and logs	✅	🚫
Can be augmented	🚫	✅
Can be recursive	⚠️	✅

⚠️ In some cases

(source: Karol Majewski)

Interface vs Type Example

interface Car {
  color: string;
  doors: number;
}

interface Motorbike {
  color: string;
  seats: number;
}

type Car = {
  color: string;
  doors: number;
}

type Motorbike = {
  color: string;
  seats: number;
}

type Bmw = Car | Motorbike;
type Bmw = Car & Motorbike;
  

Function Type

const func: (num: number) => string = (num: number) => String(num);

  

The function type:

(num: number) => string
  

const valueFactory = (x: number) => x; // definition
const myValue = valueFactory(123); // use
  