What is an interface in typescript?

interface Size {
  width: string
  height: string
  getWisth(): string
  getWisth(): string
}

class Shapes implements Size {
  width: string
  height: string
  constructor(width: string, height: string) {
    this.width = width
    this.height = height
  }
  getWidth() {
    return this.width
  }
}

interface Post {
  title: string
  content: string
  // Optional parameters have a "?" at the end. EX: "tags?"
  tags?: string[] // returns an array of strings
}

interface User {
  name: string;
  email: string;
  // By adding "readonly" before a property, that makes it read-only"
  readonly dateCreated: Date.Now();
}

interface SumFunc {
  (a: number, b: number): number
}

// Usage
const add: SumFunc = (a, b) => {
  return a + b
}

// PetInterface
interface PetsInterface {
  name: string
}

// DogInterface extends PetInterface
interface DogInterface extends PetsInterface {
  breed: string
}

// Example usage
class Dog implements DogInterface {
  name: string = 'Air Bud'
  breed: string = 'German Shepard'
}

// CatInterface also extends PetInterface
interface CatInterface extends PetsInterface {
  breed: string
}

// Example usage
class Cat implements CatInterface {
  name: string = 'Garfeild'
  breed: string = 'Tuxedo Cat'
}

// Generics allow you to reuse functions
// "T" is a generic variable that we can assign later
interface PaginatedResponse<T> {
  data: T[] // returns an array of "T"
  nextPageUrl?: string
  previousPageUrl?: string
}

interface Post {
  title: string
  content: string
  tags?: string[]
}

// Usage
function loadDataFromApi() {
  fetch('/som/api/endpoint')
    .then((response) => response.json())
    // Usage: assigning "Post" to the "T" variable
    .then((data: PaginatedResponse<Post>) => console.log(data))
}