next.js/test/development/basic/define-class-fields/fixture/pages/animal.tsx

import React from 'react'

interface Animal {
  animalStuff: any
}
interface Dog extends Animal {
  dogStuff: any
}

class AnimalHouse {
  resident: Animal
  constructor(animal: Animal) {
    this.resident = animal
  }
}
class DogHouse extends AnimalHouse {
  // Initializes 'resident' to 'undefined'
  // after the call to 'super()' when
  // using 'useDefineForClassFields'!
  // @ts-ignore
  resident: Dog
  // useless constructor only for type checker
  /* eslint-disable @typescript-eslint/no-useless-constructor */
  constructor(dog: Dog) {
    super(dog)
  }
}

class DogHouseWithDeclare extends AnimalHouse {
  declare resident: Dog
  // useless constructor only for type checker
  /* eslint-disable @typescript-eslint/no-useless-constructor */
  constructor(dog: Dog) {
    super(dog)
  }
}

export default function AnimalView() {
  const dog = new DogHouse({
    animalStuff: 'animal',
    dogStuff: 'dog',
  })
  const dogDeclare = new DogHouseWithDeclare({
    animalStuff: 'animal',
    dogStuff: 'dog',
  })
  return (
    <>
      <div id={'dog'}>{String(dog.resident)}</div>
      <div id={'dogDecl'}>{dogDeclare.resident?.dogStuff}</div>
    </>
  )
}